Urinary Tract Infection
A urinary tract infection is an infection of the urinary system, most commonly caused by bacteria ascending through the urethra to the bladder or kidneys, presenting with dysuria, frequency, and urgency.
Urinary Tract Infection (UTI)
Urinary tract infection (UTI): inflammatory response of the urothelium to invasion by bacteria or other pathogens, usually associated with bacteriuria and pyuria. [1]
Let's break down what this means from first principles:
- Urothelium = the specialised transitional epithelium lining the urinary tract from the renal pelvis down to the urethra. When bacteria invade this epithelium, the host mounts an inflammatory response — that is the UTI.
- The definition deliberately says "usually associated with" bacteriuria and pyuria because these terms are not synonymous with UTI:
| Term | Definition | Equals UTI? | Other Causes |
|---|---|---|---|
| Bacteriuria | Presence of bacteria in urine | No — does NOT equate infection | Colonisation, contamination of urine sample [1][2] |
| Pyuria | Presence of WBCs in urine | No — indicative of infective or inflammatory response but does NOT equate infection | Stones, cancer, TB, interstitial nephritis [1][2] |
UTI = significant bacteriuria + pyuria + clinical symptoms/signs [3]. You need all three to clinch the diagnosis. A positive culture alone (bacteriuria) without symptoms may just be asymptomatic bacteriuria — a completely different entity that usually does NOT require treatment (with specific exceptions we will cover later).
Common Exam Pitfall
Students frequently equate a positive urine culture with UTI. Remember: bacteriuria ≠ UTI. You must have clinical evidence of infection (symptoms/signs + pyuria) alongside significant bacteriuria to diagnose UTI. Treating asymptomatic bacteriuria unnecessarily drives antimicrobial resistance.
2. Epidemiology
UTI is one of the commonest bacterial infections, accounting for 1–3% of GP consultations. [4]
- 30% of females have symptomatic UTI by age 24 [4]
- 50% of females will have at least one UTI during their lifetime [4]
- UTI is the most common nosocomial infection worldwide, largely driven by catheter-associated UTI (CAUTI)
- In Hong Kong, UTI is a leading cause of bacteraemia in elderly patients, particularly in residential care homes
Demographics: F >>> M (rare in M except at extremes of age: 0–1 year or > 60 years with BPH). Bacteriuria is 30× more common in young females than males. Incidence increases with age, with a sharp increase after onset of sexual activity. [4]
| Age Group | Sex Predilection | Explanation |
|---|---|---|
| Neonates (< 1 year) | M > F | Uncircumcised boys have a 10-fold higher risk of UTI in the first year due to bacterial colonisation of the glans and foreskin [1]. Also higher rates of congenital urological anomalies (e.g. posterior urethral valves) in males |
| Neonates and infants | Both | Increased bacterial colonisation of the periurethral area and an immature immune system, especially in the first few weeks [1] |
| Children (1–5 years) | F > M | Girls – 5%, Boys – 1% [1]. Female urethra is shorter (~4 cm vs ~20 cm in adult males), facilitating ascending infection |
| Reproductive age | F >>> M | Short female urethra, proximity to anus, sexual intercourse ("honeymoon cystitis"), hormonal effects on vaginal flora |
| Post-menopausal | F >> M | Oestrogen deficiency → loss of protective vaginal lactobacilli → ↑vaginal pH → ↑periurethral colonisation |
| Elderly (> 60 years) | Gap narrows | Males: BPH → incomplete bladder emptying → stasis. Both sexes: ↑comorbidities (DM, catheterisation, institutionalisation) |
- Ageing population: Hong Kong has one of the highest proportions of elderly globally → high burden of complicated UTI, CAUTI, and UTI-related bacteraemia
- Antimicrobial resistance: Hong Kong outpatient urinary E. coli isolates show high resistance to ampicillin (64–67%), levofloxacin (36–46%), and co-trimoxazole (31–32%), but low resistance to nitrofurantoin (1–2%) and fosfomycin (~2%). This is why empirical cystitis treatment locally favours nitrofurantoin or amoxicillin-clavulanate over fluoroquinolones/co-trimoxazole.
- DM prevalence: ~10% of HK adults have DM → increased UTI risk (impaired immunity, glycosuria, neurogenic bladder, diabetic cystopathy)
3. Risk Factors
UTI is a result of the interaction between the host and the uropathogens. [1]
| Risk Factor | Mechanism / Pathophysiology |
|---|---|
| Female sex [2] | Shorter urethra (~4 cm), proximity of urethral meatus to vaginal introitus and anus → easier ascending bacterial migration |
| Lack of circumcision [2] | Mucosal surface of uncircumcised foreskin is more likely to bind uropathogenic bacterial species than keratinised skin on a circumcised penis; partial obstruction of urethral meatus by tight foreskin |
| Sexual activity [2] | Mechanical introduction of periurethral bacteria into the bladder during intercourse ("urethral milking"); classically occurs after first sexual encounter or after a period of abstinence — "honeymoon cystitis" [4] |
| Vesicoureteral reflux (VUR) [2] | Retrograde passage of urine from bladder into upper urinary tract → important risk factor for renal scarring |
| Urinary obstruction [2] | Stagnancy of urine increases risk of UTI. Anatomical causes: posterior urethral valves, ureteropelvic junction obstruction. Neurological causes: myelomeningocele with neurogenic bladder. Functional causes: bladder and bowel dysfunction |
| Bladder and bowel dysfunction [2] | Characterised by abnormal elimination pattern (frequent or infrequent voids, daytime wetting, urgency, infrequent stool), bladder or bowel incontinence, withholding manoeuvre → incomplete bladder emptying → urine stasis |
| Constipation [1] | A full rectum compresses the bladder base → incomplete emptying → residual urine → bacterial growth. Particularly important in paediatric UTI |
| Urinary instrumentation / indwelling catheter [2] | Bypasses normal defence mechanisms; biofilm formation on catheter surface; 3–7% risk of bacteriuria per day of catheterisation |
| Genitourinary abnormalities [1] | Pelviureteric or vesicoureteric obstruction; ureterocele; posterior urethral valves |
| Voiding dysfunction [1] | Abnormal bladder activity, compliance, or emptying → residual urine → bacterial proliferation |
| Diabetes mellitus | Glycosuria provides a substrate for bacterial growth; impaired neutrophil function; autonomic neuropathy → diabetic cystopathy (incomplete emptying, overflow incontinence, recurrent UTI) [5] |
| Pregnancy | Progesterone-mediated ureteral dilation + mechanical compression by gravid uterus → physiological hydronephrosis + urine stasis |
| Immunosuppression | ↓Host immune response → ↓bacterial clearance |
| Post-menopausal oestrogen deficiency | ↓Oestrogen → ↓vaginal glycogen → ↓Lactobacilli → ↑vaginal pH → ↑periurethral colonisation by uropathogens |
3.2 Risk Factors for Recurrent UTI
Recurrent UTI is extremely common — 20% of females with UTI will recur within 6 months [4].
| Risk Factor | Explanation |
|---|---|
| Sexual intercourse (frequency) | Mechanical inoculation of bacteria into bladder |
| New sexual partner | Exposure to new bacterial flora |
| Use of spermicide and diaphragm | Spermicides alter vaginal flora (↓Lactobacilli, ↑E. coli colonisation); diaphragm causes mechanical obstruction |
| Pelvic anatomy (↓urethra-to-anus distance) | Shorter distance = easier bacterial migration from perianal area |
| Age of first UTI | Earlier first UTI suggests greater inherent susceptibility |
| Family history (genetic factors) | Genetically determined susceptibility of vaginal epithelial cells to uropathogen adherence (e.g. non-secretor status of blood group antigens → ↑bacterial binding) |
| Prior antimicrobial use | Disrupts normal vaginal and periurethral flora → ↓protective Lactobacilli → ↑colonisation by uropathogens |
| Risk Factor | Explanation |
|---|---|
| Oestrogen deficiency | ↓Glycogen in vaginal epithelium → ↓Lactobacilli → ↑pH → ↑colonisation |
| Urinary incontinence | Moisture and bacterial contamination of perineum |
| Presence of cystocele | Bladder prolapse → incomplete emptying → residual urine |
| Large post-void residual urine | Static urine = culture medium for bacteria |
| History of UTI before menopause | Indicates inherent susceptibility |
| Genetic factors | As above — cell surface receptor expression |
High Yield: Recurrent UTI Risk Factors
For exams, the risk factor lists for premenopausal vs postmenopausal women come up repeatedly. The key difference: premenopausal factors relate to sexual activity and anatomy, while postmenopausal factors relate to oestrogen deficiency and bladder mechanics.
4. Anatomy and Functional Considerations
Understanding UTI requires understanding the normal defences of the urinary tract, because UTI occurs when these defences are breached.
| Feature | Female | Male |
|---|---|---|
| Urethral length | ~4 cm | ~20 cm |
| Proximity to anus | Close (short perineal body) | Distant |
| Prostate | Absent | Present — prostatic secretions contain zinc and other antimicrobial factors |
| Vaginal flora | Provides protective Lactobacilli (oestrogen-dependent) | N/A |
Why are females more susceptible? The shorter female urethra means bacteria need to travel a much shorter distance to reach the bladder. The close proximity of the urethral meatus to the vaginal introitus and anus creates a "highway" for faecal flora to colonise the periurethral area and ascend.
4.2 Natural Defence Mechanisms of the Urinary Tract
UTI is a result of the interaction between the host and the uropathogens — specifically the host's inherent susceptibility, defence mechanisms vs the uropathogen's virulence and inoculum size. [1]
| Defence Component | Mechanism | Disrupted By |
|---|---|---|
| Lactobacilli in introitus, vagina, and periurethral area | Produce low vaginal pH (convert glycogen → lactic acid) → inhibits uropathogen colonisation | Antimicrobial use, spermicidal agents, hypo-oestrogenic state [4] |
| Cervical IgA | Secretory IgA provides mucosal immunity against uropathogens | Immunodeficiency |
| Vaginal environment related to oestrogen | Oestrogen stimulates glycogen deposition in vaginal epithelium → substrate for Lactobacilli | Menopause, oophorectomy |
| Defence Component | Mechanism | Disrupted By |
|---|---|---|
| Antegrade flow of urine | "Washout" effect — constant flushing of bacteria from the bladder | Urinary tract obstruction [4] |
| Normal pH with high osmolality | Unfavourable environment for bacterial growth | Alkalinisation, dilution |
| Tamm-Horsfall protein (uromodulin) | Saturates mannose-binding sites of type 1 pili → prevents bacterial adhesion to urothelium [4] | ↓Production in CKD |
| Defence Component | Mechanism | Disrupted By |
|---|---|---|
| Normal emptying of bladder | Removes bacteria before they can establish infection (residual urine < 50 mL normally) | BOO, neurogenic bladder, bladder diverticulum, indwelling catheter, VUR [4] |
| Normal exfoliation of urothelial cells | Infected superficial umbrella cells shed into urine, carrying adherent bacteria with them | Chronic inflammation |
| Urothelial innate immunity receptors | Receptors on superficial urothelial cells recognise LPS → activate innate immunity → ↑local PMN, macrophages → local inflammation [4] | Immunosuppression |
Intrinsic susceptibility of vaginal cells to bacterial adherence is genetically determined and directly affects rates of recurrent UTI. [4]
- Non-secretors of ABO blood group antigens have ↑expression of specific glycolipid receptors on uroepithelial cells → ↑bacterial binding → ↑UTI risk
- This is why some women get recurrent UTI despite no identifiable anatomical or behavioural risk factor — it's in their genes
5. Aetiology and Microbiology
| Route | Mechanism | Examples |
|---|---|---|
| Ascending (commonest) | Rectum → periurethral area → urethra → bladder ± kidney | Most community-acquired UTI. Facilitated by: indwelling catheter, vesicoureteral reflux, ureteral obstruction [4] |
| Haematogenous (uncommon) | Extrarenal source of bacteraemia → secondary infection of kidney | Renal abscess from septic emboli (e.g. S. aureus endocarditis), genitourinary TB [4] |
| Direct extension (unusual) | Spread from neighbouring suppurative infections | Diverticulitis, appendicitis, Crohn's disease forming fistula to bladder. Uncommon because of thick Gerota's fascia [4] |
5.2 Microbiology
Majority caused by facultative anaerobes, mostly from bowel flora. [4]
The table below shows the microbiological spectrum by UTI category:
| Organism | Uncomplicated UTI | Complicated UTI | Nosocomial UTI |
|---|---|---|---|
| E. coli | 75% | 65% | 50% |
| S. saprophyticus | 6% | — | — |
| Klebsiella pneumoniae | 6% | 8% | + |
| Enterococcus spp | 5% | 11% | + |
| S. agalactiae (GBS) | 3% | 2% | — |
| Proteus mirabilis | 2% | 2% | + |
| P. aeruginosa | Rare | 2% | + |
| S. aureus | Rare | 3% | + |
| Candida spp | Rare | 7% | + |
| Citrobacter, Serratia, Providencia, Enterobacter | Rare | Rare | Can be polymicrobial |
E. coli — the dominant pathogen across all categories:
- Uropathogenic E. coli (UPEC) possesses specific virulence factors (see below)
- Community-acquired uncomplicated UTI: 75–95% caused by E. coli
- In Hong Kong: high rates of fluoroquinolone- and co-trimoxazole-resistant E. coli — this impacts empirical therapy
S. saprophyticus — "staph. sapro":
- Second commonest cause of uncomplicated UTI in young sexually active women
- Gram-positive coccus; characteristically novobiocin-resistant (distinguishes it from S. epidermidis)
Proteus mirabilis:
- Produces urease → breaks down urea into ammonia → alkalinises urine → promotes struvite (magnesium ammonium phosphate) stone formation
- Think of Proteus when you see recurrent UTI + alkaline urine + staghorn calculi
Klebsiella pneumoniae:
- Second commonest Gram-negative cause after E. coli
- Important in complicated/nosocomial UTI
- In HK: rising carbapenem-resistant Klebsiella is a concern
Pseudomonas aeruginosa:
- Important in catheter-associated and nosocomial UTI
- Intrinsically resistant to many antibiotics
- Think of Pseudomonas in recurrent UTI with instrumentation/catheter history
Enterococcus spp:
- Intrinsically resistant to cephalosporins
- Important in complicated and catheter-associated UTI
- Treat with amoxicillin or vancomycin (if VRE, use linezolid or daptomycin)
Candida spp:
- Think fungal UTI in: ICU patients, prolonged antibiotics, immunosuppressed, indwelling catheter, diabetics
- Adenovirus (types 11, 21): haemorrhagic cystitis in children and immunocompromised
- Schistosoma haematobium: granulomatous cystitis → squamous cell carcinoma of bladder (rare in HK but important for travel history)
- Mycobacterium tuberculosis: sterile pyuria (WBCs but culture-negative on routine media) — always consider in HK given TB prevalence
Bacterial virulence: characteristics of uropathogens that allow them to colonise and invade the urinary tract. [1]
Can be divided into:
Virulence Factors Against the Host (Focus on UPEC)
| Virulence Factor | Function | Clinical Relevance |
|---|---|---|
| Type 1 (mannose-sensitive) pili (FimH, FimA) | Commonly expressed in E. coli; found in majority of isolates from acute cystitis [4] | Binds to uroplakin on bladder epithelium → mediates colonisation of the lower tract |
| Type P (mannose-resistant) pili (PapG, X-adhesin) | Binds preferentially to upper tract urothelium; found in 80% of isolates from acute pyelonephritis [4] | The "P" stands for pyelonephritis — these pili allow bacteria to ascend and infect the kidney |
| Type S pili | Both bladder and kidney infection [4] | — |
| Afimbrial adhesins (e.g. Dr adhesin family) | Mediates binding independent of pili | Important in recurrent/complicated UTI |
| Haemolysin (HlyA) | Toxin → forms pores in host cell membrane and RBC lysis [4] | Tissue damage → facilitates invasion and nutrient acquisition |
| Urease | Breaks down urea into ammonia [4] | Alkalinises urine → struvite stone formation (Proteus, Klebsiella) |
| IgA inactivating protein | Degrades secretory IgA | Evades mucosal immunity |
| Phasic variation of piliated state | Bacteria can switch pili on/off | Evades immune recognition; adapts to different niches |
| Siderophores (e.g. aerobactin) | Iron acquisition systems | Scavenges iron from host → essential for bacterial growth |
| Capsular polysaccharide (K antigen) | Resists phagocytosis and complement-mediated killing | ↑Survival in bloodstream → important in urosepsis |
| Lipopolysaccharide (LPS) | Endotoxin → activates TLR4 → systemic inflammatory response | Triggers sepsis cascade if bacteria enter bloodstream |
6. Pathophysiology
This is the overwhelmingly commonest route:
1. Colonisation of periurethral area by uropathogenic enteric pathogen
(usually from faecal flora, especially E. coli)
↓
2. Bacterial ascent through the urethra into the bladder
(facilitated by short female urethra, sexual intercourse, catheterisation)
↓
3. Attachment to uroepithelial cells via active receptor-mediated process
- Mediated by glycosphingolipid receptors on epithelial cell surface
- Type 1 pili (FimH) bind to uroplakin Ia on umbrella cells
↓
4. Toll-like receptor (TLR) activation
- Bacterial attachment recruits TLRs (transmembrane co-receptors)
- TLR4 recognises LPS → triggers cytokine cascade (IL-6, IL-8, TNF-α)
- Generates local inflammatory response → PMN recruitment → pyuria
↓
5. Bacterial proliferation in bladder → CYSTITIS
(if host defences overwhelmed or impaired)
↓
6. ± Ascent into ureters and kidney → PYELONEPHRITIS
- Bacteria with P-pili adhere to upper tract urothelium
- Facilitated by VUR, ureteral obstruction, or catheterisation
↓
7. ± Invasion into bloodstream → UROSEPSIS
(if renal parenchymal barrier breached)Understanding why UTI produces specific symptoms connects directly to the inflammatory response:
- Dysuria (painful urination): Inflammation of the bladder/urethral mucosa → exposure of sensory nerve endings to inflammatory mediators + urine acidity
- Frequency and urgency: Inflamed bladder wall has ↓compliance and ↑irritability → the detrusor muscle contracts at lower volumes → premature sense of fullness
- Suprapubic pain: Peritoneal irritation from inflamed bladder dome (the dome is covered by peritoneum)
- Haematuria: Mucosal inflammation → capillary damage → blood in urine
- Fever: Systemic response to infection — more prominent in upper tract infection where bacteria have access to the bloodstream through the renal parenchyma's rich blood supply
- Loin/flank pain: Renal capsular distension from oedema and inflammation (pyelonephritis), or obstruction
- Uncommon for typical UTI organisms
- S. aureus is the classic organism that seeds the kidney via the bloodstream (e.g., from endocarditis or infected IV lines)
- Genitourinary TB: Mycobacterium tuberculosis spreads haematogenously from a primary pulmonary focus to the highly vascular renal cortex → caseous granulomas → "autonephrectomy" in severe cases
6.4 Pathophysiology of Special UTI Entities
EPN pathogenesis [1]:
- Factors: high glucose level, gas-forming microbes, impaired vascular blood supply, reduced host immunity, urinary tract obstruction [1]
- Mechanism: G-negative facultative anaerobes e.g. E. coli produce gas via fermentation of glucose → high levels of nitrogen, oxygen, CO₂ and H₂ accumulating at inflammatory site → gas may extend from the inflammatory site to subcapsular, perinephric and pararenal spaces [1]
EPN associated factors [1]:
- DM — single most common factor
- Female > Male
- Urinary tract obstruction
- Stones
- Immunocompromised
EC [1]:
- DDx: instrumentation, fistula to hollow viscus, tissue infarct with necrosis, infection
- EC more common in middle-aged diabetic women (M:F = 1:6)
- Predisposing factors: DM (66%), chronic UTI, indwelling urethral catheter, urinary stasis due to BOO, neurogenic bladder
- Various S/S: asymptomatic, pneumaturia, irritative voiding, acute abdomen to severe sepsis
- Pathogens: similar to EPN
- Pathogenesis: like EPN; in non-diabetic patients: urinary albumin as substrate (rather than glucose)
7. Classification
UTI can be classified along multiple axes:
| Classification | Site | Definition |
|---|---|---|
| Upper UTI | Kidney (± ureter) | Acute pyelonephritis is defined as bacteriuria with (i) fever ≥ 38.0°C or (ii) loin pain or tenderness [2] |
| Lower UTI | Bladder / Prostate / Urethra | Cystitis, prostatitis, urethritis |
- In anatomically, physiologically normal urinary tract with normal host defence mechanisms (i.e. otherwise normal individuals)
- Usual uropathogens such as E. coli (70–95%)
- Outpatient treatment
- Shorter duration of treatment
- No increased risk of failing standard therapy
- E.g. neuropathic bladder, urinary tract obstruction, bladder diverticulum, presence of stones
- ↑Risk of acquiring infection and failing treatment
- Broader spectrum of pathogens, often drug-resistant
- Admission may be required
- Longer duration of treatment
- Measures should be made to eradicate the complicating feature
Special groups: pregnancy, children [1]
Complicated UTI - What Makes It 'Complicated'?
Any structural abnormality (stones, obstruction, diverticulum), functional abnormality (neurogenic bladder, VUR), foreign body (catheter, stent), or immunocompromised state (DM, transplant, neutropenia) that either increases the risk of infection or the risk of treatment failure makes a UTI "complicated." The practical implication: you need a broader antibiotic spectrum, longer treatment duration, and investigation for the underlying cause.
| Classification | Definition | Key Points |
|---|---|---|
| Isolated UTI | UTI isolated from the last episode by > 6 months [4] | First episode or infrequent occurrence |
| Recurrent UTI | UTI after resolution of previous one, occurring ≥ 2×/6 months or ≥ 3×/12 months [4] | Documented by negative culture in between episodes |
Recurrent UTI is further subdivided:
| Type | Definition | Cause | Frequency |
|---|---|---|---|
| Bacterial reinfection | Recurrent infection from a source outside the urinary tract, caused by different organisms (although it may be the same genera and species) | Due to underlying susceptibility to UTI (genetically determined → not amenable to correction) | > 95% of recurrent UTIs in women [4] |
| Bacterial persistence / relapse | Recurrent infection due to a focus within the urinary tract, caused by the same organism (exactly the same strain) | E.g. stones, urethral diverticulum | Potentially correctable by removal of infective focus [4] |
Classification by clinical syndromes [4]:
- Asymptomatic bacteriuria
- Acute bacterial infections:
- Upper tract: pyelonephritis, pyonephrosis, renal abscess
- Lower tract: cystitis, prostatitis, epididymo-orchitis, urethritis and STDs
- Other infections: mycobacterial, parasitic, fungal infections
The table from the lecture slides summarises the categories:
| Category | Clinical Features | Laboratory Criteria |
|---|---|---|
| 1. Acute uncomplicated cystitis in women | Dysuria, urgency, frequency, suprapubic pain, no urinary symptoms in 4 weeks before | > 10 WBC/mm³; > 10³ CFU/mL |
| 2. Acute uncomplicated pyelonephritis | Fever, chills, flank pain; other diagnoses excluded; no history or clinical evidence of urological abnormalities (ultrasonography, radiography) | > 10 WBC/mm³; > 10⁴ CFU/mL |
| 3. Complicated UTI | Any combination of symptoms from categories 1 and 2; one or more complicating factors | > 10 WBC/mm³; > 10⁵ CFU/mL in women, > 10⁴ CFU/mL in men or in straight catheter urine in women |
| 4. Asymptomatic bacteriuria | No urinary symptoms | > 10 WBC/mm³; > 10⁵ CFU/mL in two consecutive MSU cultures > 24 hours apart |
| 5. Recurrent UTI (antimicrobial prophylaxis) | At least 3 episodes of uncomplicated infection documented by culture in last 12 months; women only; no structural/functional abnormalities | < 10³ CFU/mL (between episodes) |
8. Clinical Features
8.1 Symptoms
| Symptom | Pathophysiological Basis |
|---|---|
| Dysuria (painful/burning urination) | Inflammation of bladder/urethral mucosa → inflammatory mediators (prostaglandins, bradykinin) sensitise nociceptors → pain on contact with urine (acidic) |
| Frequency (voiding more often than usual) | Inflamed detrusor muscle has ↓compliance and ↑sensitivity → contracts at lower bladder volumes → reduced functional capacity |
| Urgency (sudden compelling desire to void that is difficult to defer) | Same mechanism as frequency — ↑afferent signalling from inflamed mucosa → overwhelming urge |
| Suprapubic pain/discomfort | Inflammation of the bladder dome (which is covered by peritoneum) → visceral pain referred to suprapubic region via T10-L1 sympathetic afferents |
| Nocturia (waking from sleep to void) | Reduced functional bladder capacity from inflammation → even normal nocturnal urine production causes multiple awakenings |
| Haematuria (blood in urine) | Mucosal inflammation and capillary damage in the bladder wall → bleeding. Can be gross (visible) or microscopic |
| Turbid/cloudy urine | Pyuria (WBCs) + bacteria + cellular debris → urine appears cloudy |
| Malodorous urine | Bacterial metabolism of urea and other urinary constituents → volatile amines and ammonia |
Tip: Lower UTI presents with storage LUTS (dysuria, frequency, urgency, nocturia) because the problem is an inflamed bladder that cannot store urine comfortably. There should be no systemic symptoms (fever, rigors) — if present, think upper tract involvement.
| Symptom | Pathophysiological Basis |
|---|---|
| Fever (≥ 38°C) with rigors/chills | Bacteria invade renal parenchyma → access the highly vascular renal bed → bacterial products (LPS) and inflammatory cytokines (IL-1, IL-6, TNF-α) enter systemic circulation → hypothalamic set-point elevation |
| Loin/flank pain (unilateral or bilateral) | Infection → renal parenchymal oedema → distension of the renal capsule (which is richly innervated) → somatic pain localised to the flank. May radiate to groin via T10-L1 dermatomes |
| Nausea and vomiting | (i) Visceral pain → vagal stimulation → emesis. (ii) Systemic toxaemia from bacteraemia |
| Malaise and fatigue | Systemic inflammatory response → cytokine-mediated sickness behaviour |
| Lower tract symptoms | Often present concurrently as infection ascended from below |
| Symptom | Pathophysiological Basis |
|---|---|
| Systemic disturbance (high fever, chills, malaise) | Prostatic infection with rich blood supply → easy systemic dissemination |
| Deep pelvic/perineal pain | Inflammation of the prostate → capsular distension → pain felt deep in pelvis/perineum (innervation via S2-S4 pudendal nerve) |
| Obstructive voiding symptoms | Acutely swollen, oedematous prostate compresses the prostatic urethra → poor flow, hesitancy, incomplete emptying |
| Dysuria | Inflammation at the prostatic urethra |
| Symptom | Pathophysiological Basis |
|---|---|
| Subtle/recurrent UTI symptoms | Low-grade persistent prostatic infection serving as a focus of bacterial persistence |
| Low-grade fever | Chronic inflammatory response |
| Deep pain (pelvis, perineum, scrotum, especially during ejaculation) | Chronic prostatic inflammation → irritation of periprostatic nerves |
| Haematospermia | Inflammation → capillary damage in prostatic/seminal vesicle epithelium |
| ± Obstructive symptoms | Chronic inflammation → prostatic fibrosis/oedema |
| Symptom | Pathophysiological Basis |
|---|---|
| Unilateral testicular pain | Inflammation of epididymis → stretching of the tunica vaginalis → localised pain. Typically starts posterolaterally (where epididymis sits) |
| Storage LUTS | Often concurrent lower UTI (ascending infection via vas deferens) |
| High fever/rigors | Systemic response; epididymis has good blood supply |
Children (especially neonates and infants) present atypically [1]:
| Age Group | Presentation |
|---|---|
| Neonates | Non-specific: fever, irritability, poor feeding, vomiting, jaundice, failure to thrive, sepsis. UTI is a common cause of neonatal sepsis |
| Infants (< 2 years) | Fever (often the only sign), irritability, poor feeding, vomiting, malodorous urine, failure to thrive |
| Toddlers/preschool | Fever, abdominal pain, vomiting, new-onset enuresis, frequency, dysuria |
| Older children | More "classic" — dysuria, frequency, urgency, suprapubic pain, haematuria, fever (if upper tract) |
Key Clinical Point
In neonates and infants, the classic triad of dysuria-frequency-urgency is absent. Unexplained fever in a child < 2 years old mandates urine testing to rule out UTI. This is a frequently tested clinical scenario.
- Presentation can be atypical and non-specific: confusion, falls, functional decline, incontinence, anorexia
- Beware: Asymptomatic bacteriuria is extremely common in the elderly (up to 50% in institutionalised patients) — do NOT treat positive urine cultures without genuine UTI symptoms
- Fever may be blunted due to impaired thermoregulatory response
8.2 Signs
| Sign | Pathophysiological Basis |
|---|---|
| Suprapubic tenderness | Palpation compresses the inflamed bladder → pain. Elicit by firm pressure over the suprapubic region |
| Low-grade fever or afebrile | Lower tract infection is generally mucosal/superficial → limited systemic inflammatory response |
| Turbid urine on inspection | Pyuria, bacteria, debris |
| Sign | Pathophysiological Basis |
|---|---|
| High fever (≥ 38°C), often spiking | Systemic bacteraemia from infected renal parenchyma |
| Costovertebral angle (CVA) tenderness / Renal angle tenderness | Fist percussion over the 12th rib posteriorly (Murphy's kidney punch) stretches the inflamed, oedematous renal capsule → pain. This is the hallmark physical sign of pyelonephritis |
| Tachycardia | Fever-related (↑HR by ~10 bpm per °C above 37°C) + systemic inflammatory response |
| ± Hypotension | If progressed to urosepsis → vasodilation and ↓SVR from septic mediators |
| ± Septic appearance (toxic, rigors) | Bacteraemia → cytokine storm |
| Sign | Pathophysiological Basis |
|---|---|
| DRE: exquisitely tender, swollen, "boggy" prostate | Acute inflammation → oedema and abscess formation → extremely painful on palpation. Important: be GENTLE — vigorous DRE can cause bacteraemia |
| Fever | Systemic response |
| Sign | Pathophysiological Basis |
|---|---|
| Tender, swollen epididymis ± testis | Inflammatory oedema |
| Positive Prehn's sign (relief of pain with scrotal elevation) | Elevating the scrotum ↓venous congestion and ↓stretch on the inflamed cord → pain relief. (Helps distinguish from testicular torsion where elevation does NOT relieve pain) |
| Erythematous, warm scrotal skin | Local inflammation → vasodilation |
| Cremasteric reflex present | Intact in epididymitis (absent in torsion) |
| Sign | Pathophysiological Basis |
|---|---|
| Fever | Often the ONLY sign in infants |
| Abdominal/suprapubic tenderness | Bladder inflammation |
| Palpable kidneys or bladder | If there is associated obstructive uropathy (e.g. posterior urethral valves) |
| Examine for phimosis (boys) | Tight foreskin → ↑UTI risk |
| Examine external genitalia | Rule out labial adhesion (girls), hypospadias, abnormal urethral meatus |
| Check spine for midline defects | Sacral dimple, hairy patch → occult spinal dysraphism → neurogenic bladder |
9. Special Considerations in Paediatric Urology and UTI
(From GC 213 and Pediatric urology lecture slides)
Risk factors of UTI in children [1]:
- Age: neonates and infants have increased bacterial colonisation of the periurethral area and an immature immune system, especially first few weeks
- Vesicoureteric reflux
- Genitourinary abnormalities (pelviureteric or vesicoureteric obstruction; ureterocele; posterior urethral valves)
- Voiding dysfunction (abnormal bladder activity, compliance, or emptying)
- Foreskin: uncircumcised boys have a 10-fold higher risk of UTI in the first year due to bacterial colonisation of the glans and foreskin
- Constipation
Why is VUR important?
- VUR allows infected urine to travel retrogradely from the bladder to the kidney → reflux nephropathy → renal scarring → CKD and hypertension
- VUR is found in 30–50% of children presenting with UTI
- Most cases are primary (congenital short intramural ureter → incompetent flap valve mechanism at the ureterovesical junction)
- VUR is graded I–V (International Reflux Study):
- I–II: into ureter only or non-dilated collecting system
- III: moderate ureteral/pelvic dilation
- IV–V: gross dilation with tortuous ureters and loss of papillary impression
- Most common cause of congenital bladder outlet obstruction in males
- Obstructing membrane in the posterior urethra → bilateral hydroureteronephrosis → renal dysplasia if severe
- Presents: antenatal hydronephrosis, poor urinary stream, UTI, failure to thrive, renal failure
- Diagnosis: MCUG (micturating cystourethrogram)
- Management: valve ablation (endoscopic) ± temporary vesicostomy if severe
Bladder and bowel dysfunction is characterised by: [2]
- Abnormal elimination pattern (frequent or infrequent voids, daytime wetting, urgency, infrequent stool)
- Bladder or bowel incontinence
- Withholding manoeuvre
Why does this matter for UTI?
- Incomplete bladder emptying → residual urine → bacterial proliferation
- Constipation → rectal distension compresses bladder → ↓bladder capacity and ↑residual volume
- BBD perpetuates UTI and VUR → must treat both to break the cycle
(From GC 209 lecture slides)
While urinary incontinence (UI) and overactive bladder (OAB) are separate conditions from UTI, they are closely interrelated:
- UTI → OAB symptoms: Bladder inflammation → detrusor irritability → urgency, frequency, urge incontinence. This is why you must always rule out UTI before diagnosing OAB
- UI → ↑UTI risk: Incontinence → perineal moisture → ↑periurethral bacterial colonisation → ↑ascending infection risk (especially in postmenopausal women and elderly)
- Overflow incontinence → UTI: Chronic retention (e.g., neurogenic bladder from DM, BPH) → large post-void residual → bacterial stasis → recurrent UTI [5]
Clinical Pearl
In a patient presenting with new-onset urgency and frequency, always do a urinalysis and urine culture FIRST to rule out UTI before labelling them as "overactive bladder." UTI is a reversible cause of OAB symptoms.
From the endocrine notes [5]:
Diabetic autonomic neuropathy — genitourinary manifestations:
- Bladder dysfunction: ↓ability to sense full bladder, incomplete emptying, recurrent UTI, overflow incontinence
Pathophysiology:
- Chronic hyperglycaemia → metabolic/osmotic damage to autonomic nerves supplying the detrusor and bladder sensory afferents → ↓bladder sensation (patient doesn't feel the urge to void) → ↑bladder capacity → ↓detrusor contractility → ↑post-void residual → stagnant urine → recurrent UTI
- Additionally: glycosuria provides excellent substrate for bacterial growth
High Yield Summary
Definition: UTI = inflammatory response of urothelium to bacterial invasion, associated with bacteriuria AND pyuria. Bacteriuria alone ≠ UTI. Pyuria alone ≠ UTI.
Epidemiology: Commonest bacterial infection; 50% of women will have UTI in lifetime; F >>> M (except neonates where M > F); incidence ↑ with age and sexual activity.
Risk Factors:
- Female sex (short urethra), sexual activity, lack of circumcision, VUR, urinary obstruction, BBD, constipation, catheterisation, DM, oestrogen deficiency
- Recurrent UTI: premenopausal = sexual activity + anatomy + genetics; postmenopausal = oestrogen deficiency + bladder mechanics
Microbiology: E. coli dominates (75% uncomplicated, 65% complicated, 50% nosocomial). S. saprophyticus in young sexually active women. Proteus = struvite stones. Broader spectrum + resistance in complicated/nosocomial UTI.
Pathogenesis: Ascending route commonest → faecal flora colonise periurethral area → ascend via urethra → adhere via pili (Type 1 for cystitis, Type P for pyelonephritis) → TLR activation → inflammatory cascade → symptoms.
Classification: Upper vs Lower; Uncomplicated vs Complicated; Isolated vs Recurrent (reinfection vs persistence); Clinical syndromes (ABU, cystitis, pyelonephritis, prostatitis, etc.).
Clinical Features:
- Lower UTI: dysuria, frequency, urgency, suprapubic pain, haematuria, NO systemic symptoms
- Upper UTI: fever ≥ 38°C, loin pain/CVA tenderness, rigors, ± lower tract symptoms
- Children: atypical — fever, irritability, poor feeding (always test urine in febrile child < 2 years)
- Elderly: atypical — confusion, falls, functional decline (but don't over-treat asymptomatic bacteriuria)
Host Defences: Vaginal flora (Lactobacilli, low pH), antegrade urine flow, Tamm-Horsfall protein, normal bladder emptying, urothelial innate immunity (TLR recognition of LPS), genetic susceptibility of epithelial cells.
Virulence Factors: Pili (Type 1 = cystitis, Type P = pyelonephritis), haemolysin, urease, capsular K antigen, LPS, siderophores, antimicrobial resistance.
Active Recall - Urinary Tract Infection (Definition to Clinical Features)
[1] Lecture slides: GC 210. Urinary tract infection.pdf [2] Senior notes: felixlai.md (UTI section) [3] Senior notes: Ryan Ho Fundamentals.pdf (p478, Urinalysis section) [4] Senior notes: Ryan Ho Urogenital.pdf (p122–126, UTI section) [5] Senior notes: Ryan Ho Endocrine.pdf (p98, Diabetic autonomic neuropathy)
Differential Diagnosis of UTI
The differential diagnosis of UTI is really about answering one clinical question: "This patient has urinary symptoms — is it truly a UTI, or is something else mimicking it?" The approach differs depending on whether the presenting complaint is predominantly dysuria, storage LUTS (frequency, urgency, nocturia), haematuria, loin/flank pain, or fever with urinary symptoms. Let's work through this systematically.
UTI is extremely common, and there is a temptation to reflexively treat every patient with dysuria or positive dipstick with antibiotics. This is dangerous for two reasons:
- Over-diagnosis: Asymptomatic bacteriuria (especially in elderly, catheterised patients) is NOT UTI and should not be treated in most cases — doing so drives antimicrobial resistance [1]
- Under-diagnosis of serious mimics: Conditions like bladder cancer, renal TB, interstitial cystitis, and sexually transmitted infections can present identically to UTI. Missing them has serious consequences
The workup of suspected UTI should include history about: associated gross haematuria, previous antibiotic treatment, presence of "complicated" features (e.g. neuropathic bladder, renal stone, previous surgery to urinary tract), recurrent attacks, and social & drug history — specifically ketamine use. [1]
Why Ask About Ketamine?
Ketamine cystitis is an increasingly important differential in Hong Kong, particularly in younger patients. Ketamine is directly toxic to the urothelium, causing a contracted, fibrotic bladder with severe storage LUTS and haematuria that mimics recurrent UTI. The urine culture is typically negative. Always ask about recreational drug use in young patients with refractory "UTI" symptoms.
2. Differential Diagnosis by Presenting Complaint
Dysuria = "dys" (difficult/painful) + "uria" (urination). It is the most common symptom that brings a patient to consider UTI. But dysuria has a differential that varies by sex [3][4]:
| Female | Male |
|---|---|
| Urinary tract infection | Urinary tract infection |
| Sexually transmitted diseases: vaginitis, urethritis | Urethritis: due to STD |
| Pelvic inflammatory disease | Acute/chronic prostatitis |
| Interstitial cystitis (painful bladder syndrome) | Bladder stones |
| Bladder stones | Epididymitis |
Clinical pointers to distinguish these [3][4]:
| Condition | Key Distinguishing Features | Why These Features Occur |
|---|---|---|
| UTI | Storage LUTS, turbid/bloody urine, suprapubic pain ± loin pain, fever, chills (if upper tract) | Mucosal inflammation → irritative symptoms; pyuria → turbid urine; upper tract invasion → systemic response |
| STDs | +ve sexual history, urethral/vaginal discharge (especially during morning void) | Urethral/vaginal mucosal infection → discharge accumulates overnight → most visible on first morning void |
| Acute prostatitis | Systemic disturbance (fever, chills, malaise), deep pelvic/perineal pain ± obstructive symptoms (with acute swollen prostate) | Prostatic inflammation and oedema → compresses prostatic urethra → obstruction; rich blood supply → easy systemic dissemination |
| Chronic prostatitis | Can be subtle, a/w recurrent UTI symptoms, low-grade fever, deep pain (in pelvis, perineum, scrotum, and especially if during ejaculation), haematospermia ± obstructive symptoms | Chronic prostatic inflammation → serves as a focus of bacterial persistence; inflamed seminal vesicles → blood in ejaculate |
| Acute epididymitis | Storage LUTS + unilateral testicular pain + high fever/rigors | Ascending infection via vas deferens from bladder/urethra → epididymal inflammation |
| Interstitial cystitis | Diagnosis of exclusion → chronic, refractory bladder symptoms and pain | Poorly understood; likely defective GAG layer on urothelium → exposure of suburothelial nerves to urinary irritants → chronic pain and urgency without infection |
When a patient presents with classic lower UTI symptoms, the differential is narrower [4]:
| S/S suggestive of cystitis | S/S incompatible with cystitis |
|---|---|
| Dysuria | Fever and systemic upset (should have none) |
| Storage LUTS, e.g. frequency, urgency, nocturia | Flank pain and renal angle tenderness |
| Suprapubic pain/discomfort | Vaginal discharge |
| Haematuria | |
| Cloudy, foul-smelling urine |
D/dx of acute cystitis [4]:
- Vaginitis (e.g. STDs, BV) → vaginal pruritus, discharge
- Pyelonephritis → systemic upset, flank pain/renal angle tenderness, Murphy's kidney punch positive
- Non-infective lower urinary tract pathologies, e.g. interstitial cystitis, bladder stones
Key principle: If there is fever or flank pain, it is NOT just cystitis — you must consider pyelonephritis (upper tract infection). If there is vaginal discharge, you must consider a gynaecological/STD cause rather than true cystitis.
Haematuria is the most common presentation of UTI (60% of haematuria cases) but the most worrying cause is malignancy (until proven otherwise) [6].
When haematuria accompanies UTI symptoms, you must ensure it resolves after treatment. If it persists, the differential broadens significantly [3][7]:
| Category | Cause | Distinguishing Features |
|---|---|---|
| Glomerular | Glomerulonephritis | Smoky brown "Coca-Cola" urine WITHOUT clots; concomitant proteinuria; features of nephritic syndrome (HTN, oedema, oliguria); dysmorphic RBCs/RBC casts on microscopy [7] |
| Renal parenchymal | Pyelonephritis | High fever, vomiting, loin pain [7] |
| Renal cell carcinoma | Traditional triad: flank pain, painless haematuria, palpable flank mass (rare); constitutional symptoms; paraneoplastic features [7] | |
| Polycystic kidney disease | Insidious HTN, bilateral flank masses, family history [7] | |
| Stones | Urolithiasis | Unilateral flank colic radiating to groin; irritative symptoms if bladder stone [7] |
| Infection | UTI | Irritative symptoms, dysuria, foul-smelling urine [7] |
| Tumours | CA bladder | Painless gross haematuria (classically); irritative symptoms; constitutional symptoms [7] |
| CA prostate | Obstructive symptoms: hesitancy, weak stream, straining, dribbling [7] | |
| Other | BPH | Advanced age, obstructive symptoms, diagnosis by exclusion [7] |
| Non-infectious cystitis | History of pelvic irradiation or cyclophosphamide chemotherapy [7] | |
| Trauma | History of urological procedures or trauma [7] |
Crucial anatomical localisation by timing of haematuria in the stream [7]:
- Initial stream → anterior urethra (distal to urogenital diaphragm)
- Terminal stream → bladder neck or posterior urethra
- Throughout → bladder and upper urinary tract
Exam Pearl: Painless Haematuria
Painless gross haematuria in a patient > 35 years old = malignancy until proven otherwise. Do not assume it is "just a UTI." This patient needs cystoscopy and upper tract imaging even if urine culture is positive — they could have concurrent UTI and bladder cancer.
UTI presents with predominantly storage (irritative) symptoms. The full differential of LUTS includes [5]:
| Pattern | Differential Diagnoses |
|---|---|
| Bladder outlet obstruction (predominantly voiding symptoms) | Bladder: bladder stones, bladder cancer, bladder neck contracture (scarring from surgery e.g. radical prostatectomy, radiotherapy for CA prostate), interstitial cystitis, ketamine cystitis. Prostate: BPH, prostatic cancer. Urethra: urethral stricture (urinary instrumentation) |
| Overactive bladder / detrusor overactivity (predominantly storage symptoms) | Neurogenic: stroke, spinal cord injury, multiple sclerosis, Parkinson's disease. Non-neurogenic (idiopathic): post-operative pelvic surgery, bladder outlet obstruction (OAB can be secondary to BOO) |
Why does BOO cause OAB? When the bladder has to contract against a chronically obstructed outlet (e.g. BPH), the detrusor muscle undergoes hypertrophy and develops unstable, involuntary contractions → secondary detrusor overactivity → storage symptoms superimposed on voiding symptoms.
This is an important concept. Pyuria does not equate infection. [1] When you find WBCs in the urine but the standard culture is negative, the differential is:
| Cause | Explanation | Key Clinical Clue |
|---|---|---|
| Urolithiasis | Stones irritate the urothelium → inflammatory response → WBCs without bacteria | Colicky flank pain, history of stones |
| Malignancy (bladder cancer) | Tumour-associated inflammation | Painless haematuria, risk factors (smoking, age > 35) |
| Tuberculous infection | M. tuberculosis does not grow on standard culture media → culture-negative pyuria is the hallmark | Sterile pyuria, chronic symptoms, TB exposure history, emigration from endemic area; EMU (early morning urine) AFB culture or TB-PCR needed [8] |
| Partially treated UTI | Prior antibiotics sterilised culture but inflammation persists | Recent antibiotic use |
| Interstitial cystitis | Chronic non-infective bladder inflammation | Chronic pain, frequency, urgency; diagnosis of exclusion |
| Ketamine cystitis | Direct urothelial toxicity from ketamine metabolites | Social & drug history: ketamine use [1]; young patient, contracted bladder on imaging |
| Interstitial nephritis | Drug-induced (e.g. NSAIDs, antibiotics) or autoimmune tubulointerstitial inflammation | Eosinophiluria, rash, fever, ↑creatinine |
| Appendicitis | Inflamed appendix lying adjacent to ureter/bladder → reactive pyuria | RIF pain, peritonism, clinical picture inconsistent with UTI |
High Yield: Sterile Pyuria DDx
The classic exam question: "A patient has persistent pyuria but negative urine cultures. What is your differential?" Answer with: TB, stones, cancer, partially treated UTI, interstitial cystitis, ketamine cystitis, interstitial nephritis. TB is the must-not-miss diagnosis — always send early morning urine for AFB culture/TB-PCR in unexplained sterile pyuria.
When a patient presents with the classic pyelonephritis triad (fever + loin pain + renal angle tenderness), the differential includes [2][4]:
| Condition | How to Distinguish from Pyelonephritis |
|---|---|
| Pyelonephritis | Frequently presents with fever, flank pain and pyuria [2]; positive urine culture; responds to antibiotics |
| Renal abscess / perinephric abscess | More insidious onset; may not respond to antibiotics alone; CT shows rim-enhancing collection |
| Renal stones | Fever is uncommon in patients with renal stones unless it is complicated [2]; colicky pain; stone visible on CT KUB |
| Infected obstructed kidney (pyonephrosis) | Fever + hydronephrosis on USG + positive urine culture → urological emergency requiring drainage |
| Renal infarction | Sudden-onset loin pain + haematuria; risk factors: AF, endocarditis; CT with contrast shows wedge-shaped non-enhancing area |
| Renal cell carcinoma | Bleeding within the kidney can produce blood clots which lodge temporarily in the ureter leading to renal colic [2]; constitutional symptoms; mass on imaging |
Some abdominal and gynaecological conditions can mimic UTI or present alongside it [2]:
| System | Condition | Key Feature |
|---|---|---|
| Abdominal | Intestinal obstruction | Distension, vomiting, absolute constipation |
| Acute diverticulitis | LIF pain, fever, altered bowel habit; may cause fistula to bladder → pneumaturia + recurrent UTI | |
| Acute appendicitis | RIF pain, anorexia, migration of pain from periumbilical; inflamed appendix near ureter may cause reactive pyuria | |
| Abdominal aortic aneurysm | Pulsatile abdominal mass, back pain, hypotension if ruptured | |
| Gynaecological | Dysmenorrhoea | Rarely presents with flank pain that begins just before or concurrent with the onset of menstruation [2] |
| Ectopic pregnancy | Underlying cause of pain can be clarified by obtaining a renal or pelvic USG [2]; always do β-hCG in reproductive-age women | |
| Pelvic inflammatory disease (PID) | Bilateral lower abdominal pain, cervical motion tenderness, vaginal discharge; may coexist with UTI in sexually active women | |
| Ovarian torsion / ruptured ovarian cyst | Acute pelvic pain, may have haematuria from adjacent inflammation |
When a patient has recurrent UTI (≥ 2 in 6 months or ≥ 3 in 12 months), you must differentiate between [1][4][6]:
-
Bacterial reinfection (> 95% of cases): caused by different organisms or same organism but separated by documented periods of no growth in urine; source of organisms likely reservoir in faecal flora; indicates underlying susceptibility (e.g. genetic) to UTI [1]
-
Bacterial persistence/relapse: recurrent UTI caused by the same organism, frequently from a focus within the urinary tract (e.g. stones, urethral diverticulum) [1]
The approach to recurrent UTI [6]:
- Check documented culture: urine C/ST if not available
- Culture negative: consider stones / TB / tumour
- Rule out treatment failure: resistant strain / incorrect antibiotics / poor compliance to treatment
- True recurrent UTI: check culture pattern to differentiate reinfection from persistence
When a male presents with dysuria and discharge, the differential is specifically [9]:
| Category | Conditions |
|---|---|
| Physiological | Sexual arousal, prostatorrhoea/spermatorrhoea, phosphaturia |
| STDs | Gonorrhoea; Non-gonococcal urethritis e.g. Chlamydia trachomatis, Mycoplasma genitalium, Ureaplasma urealyticum, Trichomonas vaginalis, Candida, HSV; BOTH |
| Non-STDs | Bacterial urethritis/prostatitis, phimosis with poor hygiene, catheterisation & instrumentation of urethra, allergy (SJS), irritants (e.g. alcohol, caffeine, physical trauma), foreign bodies |
Key distinguishing features [9]:
| Feature | Gonococcal | Non-gonococcal |
|---|---|---|
| Discharge | Profuse, purulent | Mucoid, scanty |
| Dysuria, urethral itching | Greater extent, within 4 days | Less extent, lasts 1–5 weeks (peak at 2–3 weeks) |
| Frequency, haematuria, urgency | Seldom | Seldom |
When gas is seen in the bladder on imaging, the DDx includes [1]:
- Instrumentation (recent cystoscopy, catheterisation)
- Fistula to hollow viscus (colovesical fistula from diverticulitis, Crohn's, colorectal cancer)
- Tissue infarct with necrosis
- Infection (emphysematous cystitis)
The following mermaid diagram provides a systematic approach to the differential diagnosis when a patient presents with symptoms suggestive of UTI:
| Diagnosis | Dysuria | Frequency/Urgency | Fever | Loin Pain | Discharge | Culture | Unique Clue |
|---|---|---|---|---|---|---|---|
| Cystitis | ✓ | ✓ | ✗ | ✗ | ✗ | +ve | Suprapubic pain, turbid urine |
| Pyelonephritis | ± | ± | ✓ | ✓ | ✗ | +ve | CVA tenderness, rigors |
| STD urethritis | ✓ | Seldom | ✗ | ✗ | ✓ | UTI culture –ve | Morning discharge, sexual Hx |
| Prostatitis (acute) | ✓ | ✓ | ✓ | ✗ | ✗ | +ve | Tender boggy prostate on DRE |
| Interstitial cystitis | ✓ | ✓ | ✗ | ✗ | ✗ | –ve | Chronic, refractory; dx of exclusion |
| Bladder stone | ✓ | ✓ | ✗ | ✗ | ✗ | ± | Interrupted stream, positional pain |
| CA bladder | ± | ✓ | ✗ | ✗ | ✗ | –ve | Painless haematuria, age > 35, smoker |
| Renal TB | ± | ± | Low-grade | ± | ✗ | Sterile pyuria | TB exposure, chronic course |
| Ketamine cystitis | ✓ | ✓ | ✗ | ✗ | ✗ | –ve | Drug history, contracted bladder |
| Epididymitis | ✓ | ✓ | ✓ | ✗ | ✗ | +ve | Unilateral testicular pain/swelling |
High Yield Summary: Differential Diagnosis of UTI
-
Dysuria DDx by sex: Female — UTI, STD/vaginitis, PID, interstitial cystitis, bladder stones. Male — UTI, STD urethritis, acute/chronic prostatitis, bladder stones, epididymitis.
-
Cystitis vs Pyelonephritis: The dividing line is systemic symptoms (fever, rigors) and flank pain/CVA tenderness. Lower UTI should NOT cause fever.
-
Sterile pyuria (pyuria + negative culture): Think TB, stones, cancer, partially treated UTI, interstitial cystitis, ketamine cystitis, interstitial nephritis.
-
Recurrent UTI: > 95% are reinfection (genetic susceptibility). Always rule out bacterial persistence (stones, diverticulum) and treatment failure (resistance, poor compliance). Culture-negative recurrence → consider stones/TB/tumour.
-
Persistent haematuria after treated UTI: Must exclude malignancy (cystoscopy + upper tract imaging), especially if age > 35, male, smoker.
-
Pneumaturia: Instrumentation, colovesical fistula, emphysematous cystitis, tissue necrosis.
-
Always ask about: Ketamine use, sexual history, TB exposure, previous urological surgery/stones, and medication history.
Active Recall - Differential Diagnosis of UTI
References
[1] Lecture slides: GC 210. Urinary tract infection.pdf [2] Senior notes: felixlai.md (UTI section, including renal colic DDx) [3] Senior notes: Ryan Ho Fundamentals.pdf (p346, Dysuria) [4] Senior notes: Ryan Ho Urogenital.pdf (p121–128, Approach to Dysuria and UTI) [5] Senior notes: felixlai.md (LUTS differential diagnosis section) [6] Senior notes: maxim.md (Recurrent UTI approach) [7] Senior notes: Ryan Ho Urogenital.pdf (p130–132, Approach to Haematuria) and Ryan Ho Fundamentals.pdf (p340) [8] Senior notes: Ryan Ho Respiratory.pdf (p78, Genitourinary TB) [9] Senior notes: Ryan Ho Urogenital.pdf (p248, Urethritis)
Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities
Before diving into tests and criteria, let's establish the conceptual framework. UTI is fundamentally a clinical diagnosis supported by laboratory findings. You need three things:
- Clinical symptoms/signs of urinary tract inflammation
- Pyuria (evidence of inflammatory response)
- Significant bacteriuria (evidence of causative pathogen)
Pyuria (more than 10 leucocytes per field) and bacteriuria in a fresh urine sample will reinforce the clinical diagnosis of UTI. [1]
Epithelial cells strongly suggestive of contamination. [1]
WBC casts pathognomonic of pyelonephritis. [1]
A positive culture alone without symptoms is asymptomatic bacteriuria, not UTI. Pyuria alone without bacteriuria is sterile pyuria — a different diagnostic entity entirely. +ve culture ≠ UTI → UTI = significant bacteriuria + pyuria [3]
The Diagnostic Trinity of UTI
Symptoms + Pyuria + Significant bacteriuria = UTI. Miss any one component and you either have a different diagnosis (sterile pyuria, asymptomatic bacteriuria) or insufficient evidence for treatment.
The EAU classification system provides formal diagnostic criteria, and this was presented in the lecture slides [1]:
| Category | Clinical Criteria | Laboratory Criteria |
|---|---|---|
| 1. Acute uncomplicated cystitis in women | Dysuria, urgency, frequency, suprapubic pain; no urinary symptoms in 4 weeks before this episode | > 10 WBC/mm³; > 10³ CFU/mL |
| 2. Acute uncomplicated pyelonephritis | Fever, chills, flank pain; other diagnoses excluded; no history or clinical evidence of urological abnormalities (ultrasonography, radiography) | > 10 WBC/mm³; > 10⁴ CFU/mL |
| 3. Complicated UTI | Any combination of symptoms from categories 1 and 2 above; one or more factors associated with a complicated UTI | > 10 WBC/mm³; > 10⁵ CFU/mL in women; > 10⁴ CFU/mL in men, or in straight catheter urine in women |
| 4. Asymptomatic bacteriuria | No urinary symptoms | > 10 WBC/mm³; > 10⁵ CFU/mL in two consecutive MSU cultures > 24 hours apart |
| 5. Recurrent UTI | At least 3 episodes of uncomplicated infection documented by culture in last 12 months; women only; no structural/functional abnormalities | < 10³ CFU/mL (between episodes, confirming clearance) |
Why do the CFU/mL thresholds differ?
The classic Kass criterion of ≥ 10⁵ CFU/mL was established in the 1950s for asymptomatic bacteriuria in women using MSU. But we now know that:
- In symptomatic cystitis, a lower count of ≥ 10³ CFU/mL is significant because the patient has symptoms AND pyuria — the pre-test probability is already high, so a lower bacterial count still represents true infection (bacteria haven't had time to multiply to 10⁵ because the patient is voiding frequently due to urgency)
- In pyelonephritis, the threshold is ≥ 10⁴ CFU/mL — an intermediate value because upper tract infection involves larger bacterial burden than early cystitis
- In men, even ≥ 10³ CFU/mL is significant because male UTI is inherently uncommon — any bacteriuria in a symptomatic male deserves attention [4]
- In catheter specimens, contamination risk is lower, so ≥ 10⁴ CFU/mL suffices
- In suprapubic aspirate (SPA), ANY growth is significant because the bladder is normally sterile, and this specimen bypasses all sources of contamination
| Specimen Type | Significant Bacteriuria Threshold | Rationale |
|---|---|---|
| MSU (female, symptomatic) | ≥ 10³ CFU/mL | High pre-test probability; frequent voiding ↓bacterial concentration |
| MSU (male, symptomatic) | ≥ 10³ CFU/mL | Male UTI inherently uncommon → lower threshold |
| MSU (asymptomatic screening) | ≥ 10⁵ CFU/mL × 2 specimens | Need high specificity to avoid treating colonisation |
| Catheter specimen | ≥ 10⁴ CFU/mL | ↓Contamination compared to MSU |
| Suprapubic aspirate | Any growth | Bladder normally sterile; bypasses all contamination |
| Bag urine (paediatric) | NOT valid for culture | Too contaminated by perineal flora |
3. Diagnostic Approach and Algorithm
Diagnostic evaluation: defined clinically, may be difficult in elderly [4]
The approach involves:
General approach [2]:
- Screening test: sent for microscopy for pyuria or bacteriuria, dipstick for leukocyte esterase, nitrite and ANY positive results should lead to confirmatory tests
- Choice of urine collection: bag urine
- Confirmatory test: sent for microscopy (pyuria/bacteriuria), leukocyte esterase, nitrite and culture with sensitivity test
- Choice of urine collection:
- < 12 months: suprapubic aspiration / clean-catch urine / catheterised urine
- > 12 months: midstream urine / clean-catch urine / catheterised urine
- Choice of urine collection:
Workup of suspected UTI should include [1]:
- Associated gross haematuria?
- Previous antibiotic treatment
- Presence of "complicated" features
- E.g. neuropathic bladder, renal stone, previous surgery to urinary tract
- Recurrent attack?
- Social & drug history: ?ketamine use
4. Investigation Modalities — Detailed Breakdown
The specimen collection method critically determines the reliability of results. ↑Invasiveness = ↓contamination risk [4].
| Method | Description | Validity | Key Points |
|---|---|---|---|
| Bag urine | Adhesive bag attached to perineum | Send for urinalysis ONLY (visual + chemical + microscopic). Do NOT send bag urine specimen for culture since it is prone to contamination by flora around the perineum [2] | Screening tool only; high false-positive rate for culture |
| Mid-stream urine (MSU) | Commonest method done. Prone to contamination particularly in females. NOT applicable in young children [2] | Can send for both urinalysis and culture with sensitivity testing [2] | Patient must clean perineum first; void initial stream to flush urethral commensals; collect mid-portion |
| Clean-catch urine | Urine caught mid-stream without cleaning | Can send for both urinalysis and culture with sensitivity testing [2] | Used for toddlers who can void on cue; parent "catches" mid-stream |
| Urethral catheterisation | NOT indicated unless patients are unable to void. Risk of iatrogenic UTI [2] | Can send for both urinalysis and culture with sensitivity testing [2] | Lower contamination than MSU; threshold ≥ 10⁴ CFU/mL; risk of introducing infection (1–3%) |
| Suprapubic aspiration (SPA) | Most accurate method. Requires a distended bladder. Indicated in paediatric patients and spinal cord injury with paraplegia [2] | Can send for both urinalysis and culture with sensitivity testing [2] | Gold standard — any growth is significant; done under USS guidance; bypasses all contamination |
Exam Tip: Bag Urine
A common exam trap: a bag urine culture grows 10⁵ CFU/mL of E. coli. Is this UTI? No — bag urine is valid for screening urinalysis only and is NOT reliable for culture. You must confirm with a properly collected specimen (SPA, catheter, or clean-catch) before diagnosing UTI in a child.
The urine dipstick is a rapid bedside screening tool. Two key markers are used for UTI:
| Marker | What It Detects | Mechanism | Performance | Pitfalls |
|---|---|---|---|---|
| Nitrite (more specific) [2] | Indicates bacteriuria | Enterobacteriaceae express nitrate reductase which converts urinary nitrate into nitrite [2] | High specificity (~95%) but moderate sensitivity (~50%) | False-negative results in Gram-positive or Pseudomonas UTI [2] (these organisms lack nitrate reductase). Also FN if urine has been in bladder < 4 hours (insufficient time for conversion), or in patients on low-nitrate diet |
| Leukocyte esterase (LE) (more sensitive) [2] | Indicates pyuria | Leukocyte esterase is released by lysed neutrophils and macrophages and is a marker for the presence of WBC [2] | High sensitivity (~80–90%) but lower specificity (~70%) | False-positive in vaginal contamination, genital inflammation. False-negative with very dilute urine, high glucose, high ascorbic acid |
| Additional Dipstick Parameters | Interpretation |
|---|---|
| Urine pH | Normal urinary pH 5.5–6.5. An overly alkaline urine ( > 7.5) suggests infection by an urease-producing organism, especially in the presence of stones. E.g. Proteus, Klebsiella, Pseudomonas etc. Urea converted by bacteria to ammonia → raises pH → facilitates precipitation of struvite calculi (magnesium ammonium phosphate or "triple phosphate stone") [1] |
| Urine specific gravity (SG) | Reflects concentration of urine and therefore hydration status. SG is defined as the weight of solution compared with weight of an equal volume of distilled water [2]. A very dilute specimen (low SG) may give false-negative results |
| Blood (haem) | Detects haemoglobin — positive in haematuria, haemoglobinuria, myoglobinuria. Must correlate with microscopy to confirm actual RBCs present |
| Protein | If significantly positive → consider glomerular disease rather than simple UTI |
Dipstick interpretation logic for UTI:
| LE | Nitrite | Interpretation |
|---|---|---|
| + | + | Strongly suggestive of UTI — both pyuria and bacteriuria present |
| + | − | Pyuria present but nitrite-negative — could be early UTI, Gram-positive UTI, or non-infective cause of pyuria |
| − | + | Bacteriuria without pyuria — possible colonisation/contamination, or very early infection |
| − | − | UTI very unlikely (negative predictive value > 95%) |
Practical approach: If either LE or nitrite is positive, send urine for culture and treat based on clinical picture. If both are negative, UTI is very unlikely and you should reconsider the diagnosis [2].
Microscopy provides direct visualisation of cellular elements and is more informative than dipstick alone.
| Finding | Definition/Threshold | Clinical Significance |
|---|---|---|
| Pyuria | > 2 WBC/HPF (OR) > 10 WBC/mL [2] | Inflammatory response in urinary tract. Pyuria and bacteriuria together suggests genuine UTI [2] |
| Sterile pyuria | Pyuria without bacteriuria | May indicate TB, partially treated UTI, urinary tract stones, bladder cancer and inflammatory conditions such as interstitial cystitis and ketamine cystitis [2] |
| Bacteriuria (microscopic) | Only seen when > 10⁵ CFU/mL [3] | Does not equate genuine bacteriuria. May be normal flora or contamination [3] |
| WBC casts | Casts containing WBCs formed in renal tubules | Pathognomonic of pyelonephritis [1] — because WBCs are trapped in a Tamm-Horsfall protein matrix within the renal tubules, proving the inflammatory process is in the kidney |
| RBC / dysmorphic RBC | Dysmorphic RBCs have been distorted passing through glomerular basement membrane | Dysmorphic RBCs and RBC casts → glomerular origin. Isomorphic RBCs → urological (post-renal) origin |
| Epithelial cells | Squamous epithelial cells from skin/vaginal mucosa | Strongly suggestive of contamination [1] — specimen should be recollected |
| Crystals | Various types based on composition | Struvite crystal due to urease-positive bacterial infection [3]. Uric acid crystal in acid urine. Calcium oxalate in alkaline urine |
This is the gold standard for confirming UTI and guiding antibiotic therapy.
When is urine C/ST indicated?
| Scenario | C/ST Indicated? | Rationale |
|---|---|---|
| Uncomplicated cystitis in young women | Not always (can treat empirically) | High pre-test probability; predictable microbiology; short course treatment |
| Complicated UTI | Always | Broader pathogen spectrum; risk of resistance; need directed therapy |
| Pyelonephritis | Always | Serious infection; blood cultures also indicated |
| Male UTI | Always — routine in males [4] | 50% have underlying urological abnormalities → need to identify pathogen |
| Recurrent UTI | Always | To differentiate reinfection vs persistence; guide prophylaxis |
| Paediatric UTI | Always | Need to confirm diagnosis; guide subsequent imaging decisions |
| Pregnancy | Always (including screening) | Asymptomatic bacteriuria in pregnancy requires treatment due to risk of pyelonephritis, prematurity, and low birth weight [2] |
| Catheter-associated | Only if symptomatic | Asymptomatic catheter bacteriuria is nearly universal and should NOT be treated |
Interpreting culture results:
| Result | Interpretation |
|---|---|
| Single organism, count above threshold, with pyuria | True UTI — treat according to sensitivity |
| Multiple organisms | Likely contamination — repeat specimen (unless catheter/complicated UTI where polymicrobial is possible) |
| Growth below threshold | Usually colonisation or contamination; correlate clinically |
| No growth but pyuria present | Sterile pyuria — consider TB (send EMU × 3 for AFB culture), stones, cancer, partially treated UTI, interstitial cystitis |
| Test | When to Order | Expected Findings and Rationale |
|---|---|---|
| CBC with differentials [2] | Pyelonephritis, complicated UTI, sepsis | Leukocytosis with neutrophilia; left shift in severe infection |
| RFT (renal function tests) [2] | Pyelonephritis, complicated UTI, obstruction suspected | ↑Creatinine may indicate obstructive uropathy, AKI from sepsis, or pre-existing CKD |
| Serum inflammatory markers (CRP, ESR) [2] | ↑ESR and CRP for acute pyelonephritis [2] | Differentiates upper from lower UTI (CRP is typically normal or minimally elevated in uncomplicated cystitis) |
| Blood culture | Pyelonephritis, urosepsis, febrile UTI | Positive in 15–30% of pyelonephritis cases; essential for guiding IV antibiotic therapy in sepsis |
| Procalcitonin (PCT) | Paediatric UTI, sepsis evaluation | PCT > 0.5 ng/mL suggests upper tract involvement; useful in distinguishing cystitis from pyelonephritis in children |
| PSA | Acute prostatitis (may be incidentally elevated) | ↑PSA in acute prostatitis [4]; do NOT use for cancer screening during active infection as it will be falsely elevated |
CRP in UTI: Upper vs Lower
CRP is a useful discriminator. In uncomplicated cystitis, CRP should be normal or minimally raised ( < 20 mg/L). A significantly elevated CRP ( > 50–100 mg/L) with urinary symptoms should make you think pyelonephritis or complicated UTI, even if the patient doesn't have classic flank pain. This is especially useful in elderly patients with atypical presentations.
Imaging is not required in most UTI → to look for underlying factors amenable to non-medical means and diagnosis of focus of bacterial persistence [4]
| Modality | Indications in UTI | Key Findings | Advantages / Limitations |
|---|---|---|---|
| USG Kidneys | Mandatory for severe infection to rule out obstruction [4]; first-line imaging in all paediatric UTI; recurrent UTI | Hydronephrosis (obstruction), renal abscess, perinephric collection, renal stones, cortical thinning (chronic reflux), bladder wall thickening, post-void residual volume | Bedside, no radiation, no contrast. Limitation: poor at visualising ureters (only proximal and distal ends) and small stones [7] |
| NCCT (non-contrast CT) | Standard Ix if flank pain or ureteric colic [7]; suspected urolithiasis as complicating factor | Allows assessment of level, size, density and degree of obstruction of calculi [7]; perinephric stranding in pyelonephritis | Gold standard for stones; no contrast needed. Limitation: radiation exposure |
| CT A+P with contrast | If no improvement > 72 hours in pyelonephritis [4]; suspected abscess, EPN, EC | Rim-enhancing abscess, gas within renal parenchyma (EPN) or bladder wall (EC), extent of perinephric disease, obstructing cause | Detection of anatomic abnormalities; detection of processes that delay response to therapy including urinary stones, urinary tract obstruction or papillary necrosis; diagnose complications of infection such as renal or perinephric abscess; more sensitive than ultrasound to detect renal abnormalities [2] |
| CT urogram (CTU) | Persistent haematuria after UTI resolution; recurrent UTI with suspected structural abnormality | Allows delineation of all significant upper tract pathology [7]; three phases (non-contrast, nephrographic, delayed/excretory) | Largely replaced IVU; excellent anatomical detail. Limitation: radiation + contrast |
| XR KUB | Initial screening, largely superseded by CT | Radio-opaque stones (calcium-containing), loss of psoas shadow (perinephric abscess) | Quick and cheap. Limitation: misses radiolucent stones (uric acid), poor sensitivity |
| Cystoscopy | Persistent haematuria; recurrent UTI with suspected lower tract pathology; Ix of choice for lower urinary tract [7] | Bladder tumour, bladder stones, urethral stricture, bladder diverticulum, ureteric orifice abnormalities | Direct visualisation + biopsy capability. Limitation: invasive |
| MCUG (micturating cystourethrogram) | Paediatric UTI with suspected VUR or PUV | Grades VUR (I–V); posterior urethral valves (dilated posterior urethra + bladder trabeculation) | Gold standard for VUR diagnosis. Indication in paediatrics: abnormal USG / recurrent UTI / atypical UTI / abnormal DMSA [2] |
| DMSA scan | Paediatric UTI — to detect renal scarring | Photopenic areas = scarring; differential renal function | Done 4–6 months after acute infection to detect the formation of scarring [2]; Indications: VUR ≥ Grade 3 / recurrent UTI [2] |
After acute UTI in children, imaging is performed to detect VUR, obstruction, and renal scarring — because these are correctable/monitorable causes that can prevent long-term renal damage.
| Guideline | RBUS (Renal-Bladder USG) | MCUG | DMSA |
|---|---|---|---|
| AAP guidelines | ALL infants and children 2–24 months old | Infants 2–24 months old if: abnormal USG showing hydronephrosis, scarring, or other findings suggesting high-grade VUR or obstructive uropathy; presence of other atypical or complex clinical circumstances | N/A |
| NICE guidelines | ALL infants < 6 months old; atypical or recurrent UTI in children 6 months – 3 years old | Atypical or recurrent UTI in infants < 6 months old; atypical or recurrent UTI in children 6 months – 3 years old if: abnormal USG (ureteral dilation), poor urine flow, non-E. coli infection, family history of VUR | Atypical or recurrent UTI in children < 3 years old; recurrent UTI in children > 3 years old |
NICE Definitions [2]:
- Recurrent UTI: ≥ 2 episodes of upper UTI; 1 episode of upper UTI + ≥ 1 episode of lower UTI; ≥ 3 episodes of lower UTI
- Atypical UTI: seriously ill; poor urine flow; abdominal or bladder mass; elevated serum creatinine; septicaemia; failure to respond to treatment with suitable antibiotics within 48 hours; infection with non-E. coli organisms
| Test | Indication | Rationale |
|---|---|---|
| Early morning urine (EMU) × 3 for AFB culture | Sterile pyuria, chronic UTI symptoms, TB exposure history | M. tuberculosis requires Lowenstein-Jensen media or BACTEC system; standard urine culture will be negative. EMU has highest mycobacterial concentration [6] |
| TB-PCR on urine | As above, faster result | Sensitivity 87–100%, specificity 93–98% [6] |
| Urine cytology | Persistent haematuria after UTI treatment, recurrent UTI in patients with malignancy risk factors | Can detect high-grade transitional cell carcinoma before gross lesion becomes noticeable (carcinoma-in-situ of bladder). Low detection rate for low-grade cancer [7]. Interpretation: normal / atypical / suspicious / malignant [7] |
| Urodynamic study | Recurrent UTI with suspected voiding dysfunction, neurogenic bladder | Measures detrusor pressure, flow rate, bladder compliance, residual volume; identifies detrusor overactivity or underactivity |
| Bladder diary | Recurrent UTI, OAB symptoms | Records voiding frequency, volumes, fluid intake, incontinence episodes — objective assessment of bladder function [5] |
5. Approach to Specific Clinical Scenarios
- Diagnosis is primarily clinical — classic triad of dysuria + frequency + urgency without systemic symptoms
- Dipstick is usually sufficient for confirmation (LE+ and/or Nitrite+)
- Urine C/ST not mandatory for first episode — can treat empirically [4]
- No imaging required
- Clinical diagnosis supported by fever ≥ 38°C + flank pain/CVA tenderness
- Urine C/ST always required [4]
- Blood culture should be obtained [4]
- Bloods: CBC, RFT, CRP/ESR
- USG kidney: mandatory for severe infection to rule out obstruction [4]
- Contrast CT A+P if no improvement > 72 hours → to rule out renal abscess and obstruction [4]
- Dx: clinical + urinalysis (WBC > 10/mm³) + culture (routine in males, > 10³ CFU/mL) [4]
- Need for further evaluation: 50% have underlying urological abnormalities [4]
- Indications for further evaluation (should focus on lower urinary tract): severe UTI (febrile UTI, pyelonephritis), recurrent UTI → to rule out chronic prostatitis, history of voiding difficulties and AROU, persistent microscopic haematuria [4]
- Always obtain urine C/ST to document organisms and sensitivities
- Check documented culture: urine C/ST if not available [5]
- Imaging (USG ± CT) if evidence of bacterial persistence, recurrent Proteus spp, history of stones, persistent haematuria [4]
- Cystoscopy if persistent haematuria or suspected lower tract pathology
Asymptomatic bacteriuria refers to ≥ 10³ CFU/mL in males and ≥ 10⁵ CFU/mL in females and requires screening and treatment ONLY in pregnancy or patients undergoing genitourinary procedure with risk of mucosal bleeding and sepsis [2].
Routine screening and treatment of asymptomatic bacteriuria in pregnant patients is due to higher risk of progression into UTI and its association with prematurity and low birth weight [2].
| Population | Screen for ASB? | Treat if Found? | Rationale |
|---|---|---|---|
| Pregnant women | Yes | Yes | 20–40% of untreated ASB in pregnancy progresses to pyelonephritis; a/w preterm labour and low birth weight |
| Pre-urological procedure | Yes | Yes | Mucosal breach during instrumentation → bacteraemia risk |
| Elderly / catheterised | No | No | ASB is extremely common (up to 50% in institutionalised elderly); treatment does NOT improve outcomes and promotes resistance |
| Diabetic patients | No | No | Despite higher ASB rates, treatment of ASB in DM does not prevent symptomatic UTI |
| Spinal cord injury | No | No | Chronic colonisation is near-universal; treat only if symptomatic |
Do NOT Treat Asymptomatic Bacteriuria
One of the most common clinical errors in hospital medicine is treating a positive urine culture in an asymptomatic catheterised or elderly patient. This drives antimicrobial resistance without benefit. The ONLY populations requiring ASB screening and treatment are pregnant women and patients about to undergo invasive urological procedures.
| Investigation | Uncomplicated Cystitis | Pyelonephritis | Complicated UTI | Male UTI | Recurrent UTI | Paediatric UTI |
|---|---|---|---|---|---|---|
| Dipstick | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ (screening) |
| Urine microscopy | ± | ✓ | ✓ | ✓ | ✓ | ✓ |
| Urine C/ST | Optional (1st episode) | Always | Always | Always | Always | Always |
| CBC, RFT, CRP | Not routine | ✓ | ✓ | If febrile | ± | ✓ |
| Blood culture | No | Yes | If febrile/septic | If febrile | No | If febrile |
| USG | No | Mandatory | Yes | If complicated features | ± | Yes (all < 2y) |
| CT | No | If no improvement > 72h | If complications suspected | ± | If persistence suspected | If atypical |
| MCUG | No | No | No | No | No | If VUR suspected |
| DMSA | No | No | No | No | No | If scarring suspected |
| Cystoscopy | No | No | ± | If persistent haematuria | If persistent haematuria | No |
High Yield Summary: Diagnostics of UTI
Diagnosis = Symptoms + Pyuria + Significant Bacteriuria. +ve culture alone is NOT UTI.
Dipstick: LE (sensitive, detects pyuria) + Nitrite (specific, detects Gram-negative bacteriuria). Both negative → UTI very unlikely (NPV > 95%).
Microscopy: Pyuria > 10 WBC/mL; epithelial cells = contamination; WBC casts = pathognomonic of pyelonephritis; sterile pyuria → think TB, stones, cancer.
Culture thresholds: Symptomatic cystitis ≥ 10³; pyelonephritis ≥ 10⁴; asymptomatic bacteriuria ≥ 10⁵ × 2; SPA = any growth significant; bag urine = invalid for culture.
Urine pH > 7.5 → urease-producing organism (Proteus, Klebsiella, Pseudomonas) → struvite stones.
Imaging: Not needed in uncomplicated cystitis. USG mandatory in pyelonephritis to rule out obstruction. CT if no improvement at 72h. Paediatric: RBUS for all < 2y with UTI; MCUG if abnormal USG/atypical/recurrent UTI; DMSA for renal scarring.
Asymptomatic bacteriuria: Screen and treat ONLY in pregnancy and pre-urological procedures. Do NOT treat in elderly, catheterised, or diabetic patients.
Male UTI: Always culture; 50% have underlying abnormality; always consider further evaluation.
Active Recall - Diagnostic Criteria and Investigations for UTI
References
[1] Lecture slides: GC 210. Urinary tract infection.pdf [2] Senior notes: felixlai.md (UTI section — diagnosis, urine collection, biochemical tests, paediatric imaging) [3] Senior notes: Ryan Ho Fundamentals.pdf (p478, Urinalysis interpretation) [4] Senior notes: Ryan Ho Urogenital.pdf (p122–128, UTI diagnostic evaluation and clinical syndromes) [5] Senior notes: maxim.md (Recurrent UTI approach, urinary incontinence investigations) [6] Senior notes: Ryan Ho Respiratory.pdf (p78, Genitourinary TB — EMU AFB culture and TB-PCR) [7] Senior notes: Ryan Ho Urogenital.pdf (p130–134, Approach to Haematuria — imaging modalities) and Ryan Ho Fundamentals.pdf (p343–344)
Management of Urinary Tract Infection
Before jumping into specific regimens, let's establish the overarching management principles. These are the "why" behind every treatment decision:
Principle 1: Classify before you treat. The management differs drastically depending on whether this is uncomplicated vs complicated, upper vs lower, first episode vs recurrent. Getting the classification right determines antibiotic choice, duration, route, and need for imaging/intervention.
Principle 2: Treat the infection AND address the underlying cause. Antibiotics alone won't cure a UTI if there's an obstructed kidney, a stone acting as a nidus, or a neurogenic bladder with 500 mL residual. Measures should be made to eradicate the complicating feature [4] — this is the single most important concept in complicated UTI management.
Principle 3: Antibiotic choice is guided by: Choice of antibiotics: based on [4]:
- Likely microbiology and local resistance pattern
- Any recent antimicrobial use
- Costs, allergy, S/E profile…
In Hong Kong, this is particularly important because outpatient E. coli urine isolates show high resistance to ampicillin (64–67%), levofloxacin (36–46%), and co-trimoxazole (31–32%), while resistance remains low to nitrofurantoin (1–2%) and fosfomycin (~2%). Empirical fluoroquinolone and co-trimoxazole use is therefore increasingly unreliable locally.
Principle 4: Collect cultures before antibiotics in complicated cases. For uncomplicated cystitis, empirical treatment without culture is acceptable. For everything else — pyelonephritis, complicated UTI, male UTI, recurrent UTI, paediatric UTI — cultures should be obtained BEFORE starting antibiotics, because you may need to adjust therapy based on sensitivity results.
MUST identify underlying G6PD deficiency [2] — this is critical before prescribing nitrofurantoin or co-trimoxazole (septrin), both of which can cause haemolytic crisis in G6PD-deficient patients (common in Hong Kong: ~4.5% of males).
Clinically treat as upper UTI if fever ≥ 38.0°C or loin pain or tenderness [2] — when in doubt, treat as the more serious entity. You don't want to under-treat pyelonephritis with a short course of oral nitrofurantoin.
3. Management by Clinical Syndrome
This is the bread-and-butter scenario: a young, otherwise healthy woman with dysuria, frequency, and urgency, no fever, no flank pain.
Treatment can be given empirically without urine C/ST for uncomplicated cases [4]
General measures:
- ↑Fluid intake [4] — why? Increased urine output "washes out" bacteria and dilutes bacterial concentration. Aim 2–3 L/day
- Pyridium (phenazopyridine) to ↓discomfort [4] — this is a urinary analgesic that acts directly on the bladder mucosa to reduce pain, burning, and urgency. It turns urine bright orange (warn the patient!)
Antibiotic regimen — current Hong Kong guidance / IMPACT 6th edition [4][8][9]:
| Line | Agent | Duration | Notes |
|---|---|---|---|
| 1st line | PO nitrofurantoin | 5 days | Low local E. coli resistance (1–2%); narrow lower-tract agent with low collateral damage. Concentrated in urine only — not suitable for pyelonephritis |
| 1st line | PO Augmentin (amoxicillin-clavulanate) | 5–7 days | Used in Hong Kong; beta-lactams achieve high urinary concentrations even where intermediate susceptibility is reported |
| Second line / alternative | PO cefpodoxime or cefuroxime | 7 days (cefpodoxime); 5–7 days (cefuroxime) | Broader spectrum; use when first-line agents are unsuitable and adjust for renal function |
| Second line / alternative | PO fosfomycin trometamol | 3 g single dose | Low local E. coli resistance (~2%); IMPACT notes it can be used for acute uncomplicated UTI in premenopausal, non-pregnant women without urological abnormalities/co-morbidities, but may have inferior efficacy vs standard therapy |
Avoid empirical fluoroquinolones for uncomplicated cystitis unless other commonly prescribed options are inappropriate — local E. coli levofloxacin resistance is 36–46%, and fluoroquinolones have potentially disabling or long-lasting adverse effects [8]. Co-trimoxazole is not recommended first-line because local E. coli resistance is 31–32% [8].
Why Not Fluoroquinolones First-Line?
Fluoroquinolones (levofloxacin, ciprofloxacin) are powerful broad-spectrum antibiotics, but Hong Kong outpatient E. coli urinary isolates show 36–46% levofloxacin resistance. Using them first-line for uncomplicated cystitis accelerates resistance and exposes patients to avoidable severe adverse effects. Reserve them for situations where narrower agents are unsuitable and culture results support use.
Understanding each antibiotic from first principles:
| Antibiotic | Mechanism | Spectrum | Why It Works for UTI | Key Contraindications / S/E |
|---|---|---|---|---|
| Nitrofurantoin ("nitro" = nitrogen; "furantoin" = furan ring) | Reduced by bacterial flavoproteins → reactive intermediates → damage bacterial DNA, RNA, proteins, and cell wall | Gram-positives including Enterococcus; most E. coli; NOT effective against Proteus, Pseudomonas, Klebsiella (~50% resistant) | Concentrated in urine (achieves therapeutic levels ONLY in the lower tract) → excellent for cystitis but useless for pyelonephritis or systemic infection | C/I: CrCl < 30 mL/min (inadequate urinary concentration), G6PD deficiency (haemolytic anaemia), pregnancy at term (38–42 weeks). S/E: GI upset, pulmonary fibrosis (chronic use), peripheral neuropathy |
| Augmentin (amoxicillin + clavulanic acid) | Amoxicillin: β-lactam → inhibits PBP → ↓cell wall synthesis. Clavulanate: β-lactamase inhibitor → protects amoxicillin from degradation | Broad: Gram-positives, Gram-negatives including many Enterobacteriaceae, anaerobes | Good oral bioavailability; covers most community uropathogens; clavulanate extends spectrum to β-lactamase producers | S/E: diarrhoea (clavulanate irritates gut), allergic reactions; may not cover ESBL-producers |
| Levofloxacin ("levo" = levorotatory isomer; "floxacin" = fluoroquinolone) | Inhibits DNA gyrase (topoisomerase II) and topoisomerase IV → prevents bacterial DNA supercoiling and replication | Very broad: Gram-negatives, some Gram-positives, atypicals | Excellent oral bioavailability (~99%); achieves high urinary AND tissue concentrations; penetrates prostate | C/I: pregnancy, children (cartilage toxicity), known QT prolongation. S/E: tendon rupture (especially with corticosteroids), C. difficile, QT prolongation, CNS effects. Avoid if possible in > 60 y/o on steroids |
| Co-trimoxazole (Septrin) (trimethoprim + sulfamethoxazole) | Sequential blockade of folate synthesis: sulfamethoxazole inhibits dihydropteroate synthase; trimethoprim inhibits dihydrofolate reductase | Gram-negatives, some Gram-positives | Useful only when susceptibility is known; not first-line empirically in HK due to 31–32% local E. coli resistance | C/I: G6PD deficiency, sulfa allergy, pregnancy (1st trimester — teratogenic). S/E: hyperkalaemia, bone marrow suppression, SJS/TEN (rare) |
Complicated cystitis: refer to cystitis in functionally/structurally abnormal urinary tracts [4]
Complicating features: pregnancy, DM, immunocompromised states, recent Abx use, recent urinary tract intervention, nosocomial infection [4]
Treatment differs in three important ways:
- Generally require longer duration treatment, e.g. 10–14d culture-guided therapy; use fluoroquinolones only if susceptibility and patient factors support it [4]
- Measures should be made to eradicate the complicating feature [4]
- Urine C/ST is mandatory — broader pathogen spectrum, higher resistance rates
| Aspect | Approach |
|---|---|
| Duration | 10–14 days (vs 3–7 days for uncomplicated) |
| Route | Oral if mild; IV if severe or cannot tolerate oral |
| Antibiotic choice | Guided by C/ST; empirical with broader spectrum based on severity, recent antibiotics, and local resistance (fluoroquinolone only if appropriate/susceptible) |
| Complicating feature | Must identify and address: remove catheter, relieve obstruction, optimize glucose control in DM, drain abscess |
Management [4]:
- Hospitalisation if severe or suspicious of obstruction
- Prompt empirical Abx:
- Choice: IV Augmentin → Tazocin if suspect P. aeruginosa → Imipenem or Meropenem if severe or rapidly deteriorating
- Regimen: IV until afebrile 24–48h → complete 14-day course with oral drugs
- Contrast CT A+P if no improvement > 72h → to rule out renal abscess and obstruction
Step-by-step approach:
| Step | Action | Rationale |
|---|---|---|
| 1 | Assess severity: vitals, sepsis screen, volume status | Determine need for admission vs outpatient management |
| 2 | Obtain cultures: urine C/ST + blood cultures | Before antibiotics; blood cultures positive in 15–30% |
| 3 | Bloods: CBC, RFT, CRP | Assess inflammatory burden, renal function |
| 4 | USG kidney: mandatory in severe infection | Rule out obstruction (pyonephrosis = urological emergency requiring drainage) |
| 5 | Empirical IV antibiotics | Start immediately after cultures obtained; don't wait for results |
| 6 | Monitor response | Expect clinical improvement (defervescence) within 48–72 hours |
| 7 | If no improvement at 72h: CT A+P with contrast | Look for abscess, EPN, obstruction, or other complications requiring intervention |
| 8 | Step-down to oral once afebrile 24–48h | Complete total 14-day course |
Antibiotic escalation ladder for pyelonephritis:
Mild (outpatient, selected stable patients):
PO Augmentin or another culture-appropriate oral agent
Avoid empirical fluoroquinolone unless other options are unsuitable
and susceptibility/local resistance supports use
↓ (if worsening or not tolerating PO)
Moderate (inpatient):
IV Augmentin (amoxicillin-clavulanate 1.2g TDS)
↓ (if P. aeruginosa suspected — catheter, instrumentation, nosocomial)
IV Tazocin (piperacillin-tazobactam 4.5g TDS)
↓ (if severe / rapidly deteriorating / ESBL suspected)
IV Meropenem (1g TDS) or Imipenem (500mg QDS)Why these specific escalation choices?
- Augmentin: Good first-line IV; covers most community E. coli and Enterobacteriaceae; β-lactamase inhibitor extends spectrum
- Tazocin ("tazo" = tazobactam, a β-lactamase inhibitor; "cin" from piperacillin): Broader Gram-negative coverage including Pseudomonas aeruginosa; the go-to when Pseudomonas is suspected
- Meropenem/Imipenem: Carbapenems — the "big guns." Stable against ESBL enzymes. Reserved for severe/resistant cases because overuse drives carbapenem-resistant Enterobacteriaceae (CRE), which is a public health catastrophe
EPN management [1]:
- High index of suspicion in patient failing medical treatment for acute pyelonephritis
- Active resuscitation — ABC
- Medical management (MM):
- O₂, IVF, acid-base balance, Abx, good glycaemic control
- Keep SBP > 100 with IVF ± inotropes
- Empirical Abx: aminoglycoside, β-lactamase inhibitor, cephalosporin, quinolones — until C/ST available
- Renal support if ARF
- ICU care if multiorgan support needed
Surgical / interventional management:
- Percutaneous drainage (PCD): CT-guided or USG-guided insertion of drain into gas-containing collection. First-line intervention in most EPN
- Nephrectomy: Reserved for patients failing PCD + medical management, or those with non-functioning kidney (destroyed by gas-forming infection)
- The traditional approach of immediate nephrectomy has shifted to a stepwise approach: medical → PCD → nephrectomy only if failing [1]
Male UTI: very uncommon, incidence only 5–8/10k/year [4]
Mx: generally need longer (7d) treatment for male cystitis [4]
Why longer? Males have a longer urethra and the prostate can harbour bacteria — a 3-day course may sterilise the urine but leave bacteria in the prostate → relapse. Seven days provides a better chance of eradicating the prostatic reservoir.
Mx: Empirical Abx — prefer quinolone (excellent prostatic penetration) for 2–6 weeks [4]
Why quinolones specifically for prostatitis?
- The prostate has a blood-prostate barrier (similar in concept to the blood-brain barrier) composed of prostatic epithelial cells with tight junctions
- Most antibiotics penetrate poorly into prostatic tissue
- Fluoroquinolones are small, lipophilic molecules with excellent penetration across this barrier (achieving prostatic concentrations 2–3× serum levels)
- Co-trimoxazole also penetrates well and is an alternative
- β-lactams and aminoglycosides penetrate poorly → NOT suitable for prostatitis
This was covered in the diagnostic section but the management implications are critical:
| Population | Management | Rationale |
|---|---|---|
| Pregnant patients | Screen and treat per sensitivity | ↑Risk of ascending infection due to physiological changes → ↑risk of preterm delivery and low birth weight infants [4] |
| Pending urological procedure where mucosal bleeding is anticipated | Usually require antibiotic prophylaxis | A/w ↑risk of infective post-procedure complications [4] |
| Non-pregnant women, men, DM, catheterised patients, SCI, nephrostomy/JJ stent | Do NOT treat | Benign natural history; treatment does not ↓risk of symptomatic UTI, complications or death. S/E of Abx e.g. GI upset, C. difficile colitis [4] |
Recurrent cystitis: 20% of females with UTI will recur within 6 months → common, genetically determined [4]
Management follows a stepwise approach:
Step 1: Behavioural changes [4]
| Measure | Mechanism | Evidence Level |
|---|---|---|
| Avoid use of spermicides and diaphragms → use alternative methods | Spermicides disrupt vaginal Lactobacilli flora → ↑periurethral colonisation | Good evidence |
| Observe personal hygiene → wipe from front to back after voiding | ↓Faecal contamination of periurethral area | Never shown in case-controlled studies to be associated with ↓risk of recurrent UTI [4] — but still sensible advice |
| Post-coital voiding | Flushes bacteria introduced during intercourse | Same caveat — not proven in case-controlled studies [4] |
| Hydration to maintain adequate urine output → to aim 2–3 L/day fluid intake | ↑Urine flow → bacterial washout | Same caveat [4] |
Step 2: Topical oestrogen for postmenopausal women
- Topical oestrogen for postmenopausal women: ↓75% incidence of cystitis in RCTs [4]
- Mechanism: Restores vaginal glycogen → ↑Lactobacilli → ↓vaginal pH → ↓periurethral colonisation by uropathogens
- Applied intravaginally (cream or pessary), NOT systemic HRT
- This is one of the most effective interventions for recurrent UTI in postmenopausal women
Step 3: Antimicrobial prophylaxis [4]
Indication: recurrent symptoms that are specific for UTI persists despite non-Rx measures [4]
Don't give Abx for non-specific symptoms, e.g. mental status changes without genitourinary symptoms in elderly [4]
| Type | Indication | Regimen | Notes |
|---|---|---|---|
| Continuous prophylaxis | If no temporal relation to sexual activity | Usually co-trimoxazole (Septrin) or nitrofurantoin [2][4] | Low-dose, taken once nightly; can give trial for a few months to assess response first [4] |
| Post-coital prophylaxis | If temporally related to sexual activity | Single post-coital dose | May be a more efficient and acceptable method of prevention than continuous prophylaxis [2]; same agents as above |
| Intermittent self-treatment | Women who prefer to minimise their intake of antimicrobials | Patient keeps antibiotics at home and self-treats at symptom onset | Patients should be candidates for self-diagnosis and self-treatment [2] |
Step 4: Investigate and correct bacterial persistence (if suspected)
When the same organism keeps recurring, consider:
- Source of bacterial persistence: infected urinary stones, infected atrophic kidneys, ureteral stump after nephrectomy, medullary sponge kidney, papillary necrosis, infected urachal cyst, urethral diverticulum and urinary fistula [2]
- Imaging (USG ± CT) to identify the focus
- Surgical correction of the focus
Antibiotic prophylaxis should NOT be routinely prescribed to young children with first episode of UTI [2]
Acute treatment:
| Severity | Route | Agents | Duration |
|---|---|---|---|
| Lower UTI (afebrile) | Oral | Augmentin, cephalosporin (cefuroxime, cefixime), co-trimoxazole | 3–5 days |
| Upper UTI (febrile, < 3 months) | IV initially | IV ampicillin + gentamicin; or IV ceftriaxone | 7–10 days total (IV then step-down to PO) |
| Upper UTI (febrile, > 3 months, not toxic) | Oral (if tolerating) or brief IV → PO | As above; oral cephalosporin acceptable if well-appearing | 7–10 days |
Aminoglycosides (e.g. gentamicin, amikacin): should not be used as monotherapy (with 3rd generation cephalosporin). NOT effective in treating Enterococcus infection [2] Enterococcus is intrinsically resistant to cephalosporins and aminoglycosides and therefore should use Augmentin or nitrofurantoin [2]
Prophylaxis in children [2]:
| Indication | Regimen |
|---|---|
| VUR Grade ≥ 3 | Prophylactic antibiotics with co-trimoxazole |
| When MCUG is performed | Prophylactic antibiotics with Zinnat (cefuroxime) 10 mg/kg |
| First episode, no VUR | Antibiotic prophylaxis should NOT be routinely prescribed |
Surgical intervention for VUR [1][2]:
Surgical intervention for: breakthrough infections, poor compliance with meds, renal scarring [1]
Indications for surgical referral [2]:
- Recurrence of UTI (2nd episode) + VUR > Grade 3 + significant scarring on DMSA
- Recurrence of UTI (3rd episode) despite antibiotic prophylaxis + VUR > Grade 3
- NOT suggested for patients with Grade 1 and 2 VUR since there is a high likelihood of spontaneous resolution and low risk of renal scarring
| Surgical Option | Description |
|---|---|
| Ureteral reimplantation (open surgical / robotic-assisted laparoscopic) | Ureters are reimplanted by tunnelling a ureteral segment through the detrusor muscle thereby creating a submucosal tunnel that is long enough to act as a flap valve. Approaches: intravesical (bladder is opened) and extravesical approach (bladder not opened) which is associated with shorter hospital stay [2] |
| Endoscopic correction | Less invasive ambulatory procedure which injects a periurethral bulking agent via a cystoscope which changes the angle and fixation of intravesical ureter. Techniques: HIT (Hydrodistension Implantation Technique) — places bulking agent within the ureteral tunnel; STING (Subureteral Transurethral Injection) — places bulking agent outside ureteral orifice [2] |
Catheter-associated UTI is the most common nosocomial infection and deserves special mention:
| Principle | Rationale |
|---|---|
| Remove or replace the catheter | Biofilm on the old catheter harbours bacteria that antibiotics cannot penetrate; the new catheter should be in place before antibiotics are started so culture is from a fresh specimen |
| Treat only if symptomatic | Asymptomatic catheter bacteriuria is universal after 30 days; treatment does not improve outcomes and promotes resistance |
| Duration: 7 days (uncomplicated), 10–14 days (complicated) | Shorter courses reduce resistance selection; longer if slow to respond |
| Antibiotic choice: guided by C/ST | Polymicrobial and resistant organisms are common in CAUTI |
| Prevention: daily assessment of catheter necessity; aseptic insertion; closed drainage system; avoid unnecessary catheter placement | Most effective strategy to prevent CAUTI is to not catheterise or to remove the catheter as early as possible |
An obstructed + infected collecting system (pyonephrosis) is a urological emergency. Antibiotics alone cannot sterilise pus under pressure. Immediate drainage is required:
| Method | Description | Advantages | Limitations |
|---|---|---|---|
| Percutaneous nephrostomy (PCN) | USG/fluoroscopy-guided percutaneous drainage of the renal pelvis | Quicker — preferred in septic shock [7]; can be done at bedside by interventional radiology | C/I: bleeding tendency, distorted surface anatomy, obesity [7]; risk of bleeding, infection, pneumothorax |
| JJ stent (retrograde) | Cystoscopic placement of an internal ureteric stent from bladder up to renal pelvis | More comfortable [7]; no external tube | Not possible in BPH, incompliant bladder, stone impaction [7]; requires theatre/cystoscopy suite |
Indications for urgent drainage [2][7]:
- Uncontrolled sepsis
- Progressively worsening renal function
- Pyonephrosis (NOT simple pyelonephritis — the distinction matters)
- Intractable pain
| Clinical Syndrome | First-Line | Alternative | Duration | Route | Special Notes |
|---|---|---|---|---|---|
| Uncomplicated cystitis (F) | Nitrofurantoin or Augmentin | Cefpodoxime / cefuroxime / fosfomycin | 5d (nitrofurantoin); 5–7d (Augmentin); fosfomycin single dose | PO | No imaging; no culture needed for classic 1st episode |
| Complicated cystitis | Culture-guided therapy; Augmentin/cephalosporin/FQ if susceptible | — | 10–14 days | PO or IV | Eradicate complicating feature |
| Male cystitis | As per uncomplicated but longer | — | 7 days | PO | Always culture; consider further evaluation |
| Acute pyelonephritis (mild) | PO Augmentin or culture-guided oral agent | FQ only if appropriate/susceptible | Usually 10–14 days | PO | Outpatient only if tolerating PO and stable; culture and rule out obstruction |
| Acute pyelonephritis (severe) | IV Augmentin → Tazocin → Meropenem | — | 14 days (IV → PO step-down) | IV then PO | Admit; USG mandatory; CT if no improvement 72h |
| Acute prostatitis | Fluoroquinolone (levofloxacin/ciprofloxacin) | Co-trimoxazole | 2–6 weeks | IV then PO | Quinolone preferred for prostatic penetration |
| EPN | AG + β-lactamase inhibitor + cephalosporin + quinolone (broad cover until C/ST) | Carbapenems | Prolonged | IV | ABC resuscitation; PCD ± nephrectomy |
| Paediatric lower UTI | Augmentin, cephalosporin, co-trimoxazole | — | 3–5 days | PO | Check G6PD; no prophylaxis for 1st episode |
| Paediatric upper UTI | Ampicillin + gentamicin; or ceftriaxone | — | 7–10 days | IV then PO | Aminoglycosides NOT monotherapy; NOT for Enterococcus |
Enterococcus: The Antibiotic Trap
Enterococcus is intrinsically resistant to cephalosporins and aminoglycosides. If your urine culture grows Enterococcus, do NOT treat with cefuroxime, ceftriaxone, or gentamicin — they will all fail. Use Augmentin or nitrofurantoin for lower tract; ampicillin ± gentamicin (synergy, not monotherapy) or vancomycin for serious Enterococcal infections.
High Yield Summary: UTI Management
General Principles: Classify first; treat infection AND complicating factors; choose antibiotics based on local resistance patterns; collect cultures before antibiotics in complicated cases; check G6PD before nitrofurantoin/co-trimoxazole.
Uncomplicated Cystitis: Empirical PO nitrofurantoin 5d or augmentin 5–7d (current HK guidance / IMPACT). Fosfomycin 3 g single dose is an alternative for suitable premenopausal, non-pregnant women, though IMPACT notes possibly inferior efficacy. Avoid fluoroquinolones empirically unless other options are inappropriate.
Complicated Cystitis: Longer course (10–14d); eradicate the complicating feature; culture-guided.
Pyelonephritis: IV Augmentin → Tazocin (if Pseudomonas) → Meropenem (if ESBL/severe). IV until afebrile 24–48h → step-down PO. Total 14 days. CT if no improvement at 72h.
Prostatitis: Quinolone 2–6 weeks (excellent prostatic penetration).
Male UTI: 7 days minimum; always culture; 50% have underlying abnormality.
Asymptomatic Bacteriuria: Treat ONLY in pregnancy and pre-urological procedures. Do NOT treat in elderly/catheterised/DM.
Recurrent UTI: Behavioural measures → topical oestrogen (postmenopausal) → antimicrobial prophylaxis (continuous vs post-coital). Investigate for bacterial persistence if same organism recurs.
Paediatric: No routine prophylaxis for 1st UTI; co-trimoxazole prophylaxis for VUR ≥ Grade 3; surgical referral for recurrent UTI with VUR > 3 + scarring.
Obstructed infected kidney = Emergency: PCN or JJ stent for drainage; antibiotics alone are insufficient.
Active Recall - UTI Management
References
[1] Lecture slides: GC 210. Urinary tract infection.pdf (EPN management, paediatric UTI management) [2] Senior notes: felixlai.md (UTI treatment, paediatric management, antibiotic prophylaxis, surgical options for VUR, G6PD) [3] Lecture slides: Pediatric urology.pdf (VUR surgical intervention indications) [4] Senior notes: Ryan Ho Urogenital.pdf (p125–128, Acute cystitis treatment, pyelonephritis management, male UTI, prostatitis, recurrent UTI, asymptomatic bacteriuria) [5] Senior notes: maxim.md (Recurrent UTI approach) [7] Senior notes: Ryan Ho Urogenital.pdf (p140, Urinary drainage — PCN vs JJ stent indications and contraindications) [8] Centre for Health Protection / HKMJ: New guidance notes to drive rational prescription of antimicrobials for community settings in Hong Kong (acute uncomplicated cystitis in women; resistance rates and antibiotic choice) [9] IMPACT 6th edition (2025): Part IV empirical therapy for UTI and Part III fosfomycin trometamol
Complications of Urinary Tract Infection
UTI complications are best understood by thinking about what happens when infection is not contained or treated adequately — bacteria can spread locally, spread systemically, cause structural damage, or establish a chronic focus. The complications differ depending on whether the UTI is lower tract, upper tract, or complicated, and whether the patient is an adult, a child, or has special risk factors.
Let's work through these systematically.
1. Local Complications (Spread of Infection Within the Urinary Tract)
The most fundamental "complication" of lower UTI is its ascent into the upper tract. Uncomplicated cystitis in a healthy woman rarely progresses, but in the presence of complicating factors (VUR, obstruction, catheter, DM, immunosuppression), bacteria can ascend the ureters and invade the renal parenchyma.
Why does this matter? Because the kidney, unlike the bladder, has a rich blood supply that communicates directly with the systemic circulation. Once bacteria reach the renal parenchyma, they have a portal into the bloodstream → potential for bacteraemia and sepsis. The bladder mucosa, by contrast, is relatively avascular and acts as a barrier to systemic spread — which is why uncomplicated cystitis almost never causes fever or bacteraemia.
± Complications: especially if obstruction, recent instrumentation, prior urinary tract abnormalities, DM, immunocompromised [4]
Spread of infection: renal corticomedullary abscess, perinephric abscess [4]
| Complication | Pathophysiology | Clinical Features | Diagnosis | Management |
|---|---|---|---|---|
| Renal abscess (intrarenal) | Coalescence of microabscesses within the renal parenchyma, arising from severe pyelonephritis (ascending, usually Gram-negative) or haematogenous seeding (usually S. aureus) | Insidious onset of: vague loin pain, CVA tenderness, erythema; systemic upset: low-grade fever, pallor, fatigue, sweats, weight loss; ± S/S of background UTI (not always present) [4] | CT A+P with contrast: rim-enhancing hypodense lesion within renal parenchyma | Small ( < 3 cm): IV antibiotics alone (4–6 weeks). Large ( > 3–5 cm) or failing medical Rx: percutaneous drainage + antibiotics |
| Perinephric abscess | Rupture of renal abscess through the renal capsule into the perinephric space (between renal capsule and Gerota's fascia) | As above but more indolent; may have psoas spasm (hip flexion) if abscess irritates psoas muscle | CT A+P: collection in perinephric space, loss of perinephric fat planes | Percutaneous or surgical drainage mandatory + prolonged IV antibiotics |
Risk factors: DM, pregnancy, anatomical abnormalities (renal stones, PCKD, renal cyst, obstructive tumour), functional abnormalities (neurogenic bladder, VUR) [4]
Microbiology: E. coli (51.4%), S. aureus (10.0%), K. pneumoniae (8.6%) [4]
Why does DM keep appearing as a risk factor? Diabetes creates a "perfect storm" for complicated UTI:
- Glycosuria = substrate for bacterial growth
- Diabetic cystopathy = neurogenic bladder → incomplete emptying → stasis
- Impaired neutrophil chemotaxis and phagocytosis → ↓bacterial clearance
- Microangiopathy → impaired tissue perfusion → ↓antibiotic delivery to infected tissue
"Pyo" (Greek: πύον) = pus + "nephrosis" = kidney condition. This is pus under pressure in an obstructed collecting system — a urological emergency.
| Feature | Detail |
|---|---|
| Pathophysiology | Infected urine trapped behind an obstruction (stone, tumour, stricture) → pus accumulates in the dilated collecting system → increasing pressure → renal parenchymal damage → bacteraemia |
| Why is it an emergency? | Antibiotics CANNOT sterilise pus under pressure (poor penetration into a non-perfused, obstructed system). Untreated, it progresses rapidly to urosepsis, septic shock, and death |
| Clinical features | Fever, rigors, loin pain, CVA tenderness + evidence of obstruction (hydronephrosis on USG with echogenic debris in collecting system) |
| Management | Emergency drainage: PCN (preferred in septic shock — quicker) or JJ stent (retrograde). Antibiotics are adjunctive, not sufficient alone |
This is a life-threatening necrotising infection characterised by gas formation within the renal parenchyma and surrounding tissues. Covered in detail in the management section. Key complication points:
- Mortality historically 50–80% if treated medically alone; with modern approach (PCD + antibiotics ± nephrectomy), mortality ~20%
- EPN associated factors: DM — single most common factor; Female > Male; urinary tract obstruction; stones; immunocompromised [1]
Gas formation in the bladder wall. Less immediately dangerous than EPN but can progress:
Various S/S: asymptomatic, pneumaturia, irritative voiding, acute abdomen to severe sepsis [1]
A rare, chronic destructive granulomatous process of the kidney, almost always unilateral:
| Feature | Detail |
|---|---|
| Pathophysiology | Chronic obstruction (often staghorn calculus) + chronic infection → macrophages laden with lipid ("foamy macrophages") infiltrate and destroy renal parenchyma |
| Associations | Staghorn stones, Proteus or E. coli infection, DM |
| Clinical features | Chronic malaise, weight loss, recurrent UTI, non-functioning kidney on imaging |
| Key imaging | CT: enlarged non-functioning kidney with calcification + multiple hypodense areas (replacing parenchyma) — the "bear paw" sign |
| Management | Nephrectomy (medical treatment alone is insufficient) |
2. Systemic Complications
Urosepsis: systemic sepsis, shock, multi-organ failure [4]
Urosepsis is the most feared systemic complication of UTI. It occurs when bacteria and their products (especially Gram-negative LPS endotoxin) enter the bloodstream from an infected urinary tract → triggering the systemic inflammatory response syndrome (SIRS) → sepsis → septic shock → multi-organ dysfunction.
| Stage | Definition | Pathophysiology |
|---|---|---|
| Bacteraemia | Bacteria detectable in the bloodstream | Passage of bacteria from renal parenchyma (or instrumented urinary tract) into venous blood |
| Sepsis | Life-threatening organ dysfunction caused by a dysregulated host response to infection (Sepsis-3) | LPS activates TLR4 → massive cytokine release (IL-1, IL-6, TNF-α) → endothelial activation → widespread vasodilation, capillary leak, microthrombi |
| Septic shock | Sepsis + vasopressor requirement to maintain MAP ≥ 65 mmHg + lactate > 2 mmol/L despite adequate fluid resuscitation | Refractory vasodilation from overwhelming NO production; myocardial depression; DIC → end-organ hypoperfusion |
| Multi-organ failure | ≥ 2 organ systems failing | Kidneys (AKI), lungs (ARDS), liver (cholestatic jaundice), coagulopathy (DIC), brain (encephalopathy) |
Risk factors for urosepsis:
- Obstruction (the single most important factor — pus under pressure)
- Recent urological instrumentation
- DM and immunosuppression
- Complicated UTI: ↑risk of progression into sepsis/death [4]
- Elderly and frail patients
Management: Sepsis bundles (SSC guidelines) — early recognition, blood cultures, empirical broad-spectrum antibiotics within 1 hour, aggressive fluid resuscitation, vasopressors if needed, source control (drainage of obstructed/infected system).
Acute renal failure [4] can complicate UTI through multiple mechanisms:
| Mechanism | Example | Category |
|---|---|---|
| Post-renal (obstructive) | UTI listed as a post-renal cause of AKI: infected obstructing stone, pyonephrosis → must be bilateral to cause AKI from obstruction alone [6] | Post-renal |
| Pre-renal | Sepsis-induced hypotension → ↓renal perfusion → ↓GFR | Pre-renal |
| Intrinsic renal | Severe pyelonephritis → ATN from sepsis; aminoglycoside nephrotoxicity from treatment | Renal |
Prolonged pre-renal and post-renal disease will progress to become ATN and tubulointerstitial fibrosis respectively, i.e. intrinsic renal disease [6]
This is why early recognition and treatment matter — reversible pre-renal and post-renal AKI become irreversible intrinsic renal damage if left too long.
3. Complications of Specific UTI Subtypes
± Complications: bacteraemia and sepsis, endocarditis, epididymitis, chronic prostatitis, prostatic abscess [4]
| Complication | Pathophysiology | Clinical Clue |
|---|---|---|
| Bacteraemia/sepsis | Rich prostatic blood supply → easy systemic dissemination | Spiking fevers, rigors, positive blood cultures |
| Endocarditis | Haematogenous seeding of heart valves from prostatic bacteraemia | New murmur, embolic phenomena (rare but devastating) |
| Epididymitis | Infection travels retrogradely via vas deferens from prostate to epididymis | Unilateral scrotal pain + swelling following prostatitis symptoms |
| Chronic prostatitis | Inadequately treated acute prostatitis → chronic bacterial focus within prostate | Recurrent UTI with same organism; perineal/pelvic pain; haematospermia |
| Prostatic abscess | Liquefactive necrosis within prostate parenchyma | Failure to defervesce despite antibiotics; fluctuant prostate on DRE. Dx: CT or TRUS. Mx: drainage |
UTI (and especially untreated asymptomatic bacteriuria) in pregnancy carries specific risks:
| Complication | Mechanism |
|---|---|
| Pyelonephritis | 20–40% of untreated ASB in pregnancy progresses to pyelonephritis (progesterone-mediated ureteral dilation + mechanical compression by gravid uterus → stasis) |
| Preterm delivery | Endotoxin and prostaglandins from UTI stimulate uterine contractions |
| Low birth weight | Chronic infection/inflammation impairs placental function |
| Pre-eclampsia | Epidemiological association with recurrent UTI/pyelonephritis |
This is precisely why asymptomatic bacteriuria is screened for and treated in pregnancy — it's the only reliable way to prevent these complications.
4. Chronic / Long-Term Complications
This is the most important long-term complication in paediatric UTI:
| Feature | Detail |
|---|---|
| Pathophysiology | VUR → infected urine refluxes into the kidney → intrarenal reflux (compound papillae in the renal poles allow reflux into collecting ducts) → parenchymal inflammation → fibrosis and scarring |
| Why children? | (1) VUR is more common (30–50% of children with UTI); (2) Growing kidneys are more susceptible to scarring; (3) Compound papillae (which allow intrarenal reflux) are more common at the renal poles in children |
| Consequences | CKD (segmental scarring → loss of functioning nephrons → progressive renal failure); hypertension (renin-angiotensin activation from ischaemic scarred segments); proteinuria |
| Detection | DMSA scan: done 4–6 months after acute infection to detect the formation of scarring [2] |
| Prevention | Prompt treatment of UTI; antibiotic prophylaxis in VUR ≥ Grade 3; surgical correction if recurrent UTI + scarring + high-grade VUR |
Urinary tract TB: often diagnosed late with complications such as urethral or ureteric strictures, renal failure, infertility [1]
Recurrent UTI with urease-producing organisms (Proteus, Klebsiella, Pseudomonas, Staphylococcus) creates a vicious cycle:
Urease-producing bacteria colonise urinary tract
↓
Urease breaks down urea → ammonia + CO₂
↓
Ammonia alkalinises urine (pH > 7.5)
↓
Supersaturation with magnesium ammonium phosphate
↓
STRUVITE STONE formation (can grow rapidly into staghorn calculi)
↓
Stone acts as nidus for bacterial persistence → recurrent UTI
↓
Cycle perpetuatesStruvite crystal due to urease-positive bacterial infection [3]. This is why repeated Proteus spp isolation should prompt imaging to look for stones [4].
Repeated episodes of pyelonephritis (particularly with VUR or obstruction) → progressive tubulointerstitial fibrosis → cortical thinning and scarring → CKD → eventually end-stage renal disease in severe cases.
Chronic UTI and chronic indwelling catheter use (years) → chronic irritation → squamous metaplasia of the urothelium → squamous cell carcinoma (not the usual transitional cell carcinoma). This is:
- Seen in patients with long-term indwelling catheters (> 10 years)
- Associated with chronic UTI as a risk factor [8]
- Also associated with schistosomiasis (S. haematobium — not common in HK but important for travel history)
Since UTI is intimately linked with catheterisation, the complications of catheterisation itself overlap with UTI complications [2]:
| Complication | Pathophysiology |
|---|---|
| Catheter-associated UTI (CAUTI) | Biofilm formation on catheter surface; ascending infection along extraluminal or intraluminal surfaces; 3–7% per day of bacteriuria |
| Epididymo-orchitis | Ascending infection via vas deferens from catheter-colonised bladder |
| Bladder stone formation | Catheter acts as foreign body nidus; encrustation with calcium/struvite deposits |
| Urethral stricture | Chronic catheter-related urethral trauma → fibrosis → stricture |
| Periurethral abscess | Infection tracks along catheter in periurethral tissue |
| Incontinence | Chronic catheter → sphincter damage → incontinence after removal |
| Bladder perforation | Traumatic insertion or overinflated balloon |
Long-term catheterisation problems (from retention of urine notes) [7]:
- Risk of UTI/urosepsis, trauma, stones, urethral strictures, prostatitis and SCC of bladder
5.1 Post-Obstructive Diuresis (After Relief of Obstruction)
When you drain an obstructed infected kidney (e.g. insert PCN or catheter for retention), you can trigger post-obstructive diuresis (POD):
Post-obstructive diuresis: > 200 mL/h urine × ≥ 2h or > 3L urine in 24h [7]
MoA: tubular damage → ↓concentrating ability → rapid fluid and solute loss [7]
Significance: represents a physiological response to remove excess fluid in body (especially in CROU), but may result in fluid and electrolyte imbalance (pathological POD) [7]
Ix: may have hypoNa, hypoK, hypovolaemia [7]
Mx: close monitoring of I/O, fluid/electrolyte status with appropriate replacement and resuscitation (prefer oral hydration, aim to replace 1/2 of UO in the past hour) [7]
Other complications of acute drainage:
- Haemorrhage ex-vacuo (transient haematuria): bladder mucosal disruption with sudden emptying of greatly distended bladder. Usually self-limiting, rarely significant [7]
- Transient hypotension due to vagovagal response or relief of pelvic venous congestion [7]
Tuberculosis TB: the most common cause of infection-related death globally. 5–45% extra-pulmonary manifestations. 30 to 40% of cases involve the urogenital tract [1]
GUTB: following pulmonary tuberculosis, around 2 to 20% of individuals may develop genitourinary tuberculosis after a latency of 5 to 40 years [1]
GUTB: often diagnosed late with complications such as urethral or ureteric strictures, renal failure, infertility [1]
| Complication | Mechanism | Clinical Consequence |
|---|---|---|
| Ureteric strictures | Granulomatous inflammation → fibrosis → circumferential narrowing of ureter | Obstruction → hydronephrosis → renal damage. "Corkscrew" ureter (segmental strictures and dilatation) or "pipe-stem" ureter (shortened, rigid) [4] |
| Contracted "thimble" bladder | Chronic TB cystitis → fibrosis → loss of bladder capacity | Severe storage LUTS (frequency up to every 15–30 minutes); functionally devastating |
| Autonephrectomy | Progressive caseous necrosis → dystrophic calcification → non-functioning, calcified kidney | Loss of renal function on affected side; small calcified kidney on imaging [4] |
| Infertility | Male: epididymal/vas deferens involvement → obstructive azoospermia. Female: fallopian tube involvement (90–100%) → tubal occlusion | Important consequence in reproductive-age patients [4] |
| Renal failure | Bilateral strictures, bilateral parenchymal destruction, or amyloidosis | May require dialysis/transplant |
UTI is itself a common complication in many clinical settings:
| Clinical Setting | Why UTI Occurs |
|---|---|
| Stroke | Immobility, catheterisation, neurogenic bladder → UTI listed as a systemic complication of stroke [9] |
| Spinal cord injury | Neurogenic bladder → incomplete emptying → catheter dependence → recurrent UTI |
| Post-surgical (any major surgery) | Catheterisation, immobility, post-anaesthetic voiding difficulty → UTI: pathology includes catheterisation, ineffective voiding (post-anaesthesia, wound pain, posture, nerve injury) [5] |
| Diabetes mellitus | Glycosuria, autonomic neuropathy → diabetic cystopathy, immunosuppression |
| BPH / prostate cancer | BOO → incomplete emptying → urinary stasis |
| Long-term catheterisation | Problems: risk of UTI/urosepsis, trauma, stones, urethral strictures, prostatitis and SCC of bladder [7] |
| Category | Complication | Key Mechanism | Urgency |
|---|---|---|---|
| Local — ascending | Pyelonephritis | Bacteria ascend from bladder to kidney | Urgent — needs antibiotics and possible imaging |
| Local — abscess | Renal / perinephric abscess | Coalescence of microabscesses; rupture through capsule | Urgent — needs drainage if > 3–5 cm |
| Local — obstructed infection | Pyonephrosis | Pus under pressure in obstructed system | Emergency — needs immediate drainage |
| Local — gas-forming | EPN / EC | Glucose fermentation by G-negative bacteria in DM | Emergency for EPN; EC less acute |
| Local — chronic | XGP | Chronic obstruction + infection → granulomatous destruction | Nephrectomy needed |
| Systemic | Urosepsis → septic shock → MOF | Bacteraemia → cytokine storm → vasodilation, DIC | Emergency — sepsis bundles |
| Systemic | AKI | Obstruction (post-renal), sepsis (pre-renal/renal) | Urgent — identify and treat cause |
| Long-term — renal | Renal scarring (reflux nephropathy) | VUR + infection → intrarenal reflux → fibrosis | Prevention in children (prophylaxis, VUR surgery) |
| Long-term — stones | Struvite / infection stones | Urease → alkaline urine → MgNH₄PO₄ precipitation | Treat infection + remove stone (cycle perpetuation) |
| Long-term — malignancy | SCC bladder | Chronic irritation → squamous metaplasia → malignant transformation | Long-term catheter > 10 years |
| Long-term — TB | Strictures, contracted bladder, autonephrectomy, infertility | Granulomatous inflammation → fibrosis | Diagnosed late; requires prolonged anti-TB Rx |
| Prostatitis-specific | Abscess, chronic prostatitis, endocarditis | Inadequately treated acute prostatitis; haematogenous seeding | Drainage for abscess; prolonged Abx for chronic |
| Pregnancy-specific | Preterm delivery, low birth weight | Endotoxin/prostaglandins stimulate uterine contractions | Screen and treat ASB to prevent |
High Yield Summary: Complications of UTI
Immediate life-threatening complications:
- Urosepsis — the most feared; especially with obstruction. Manage with sepsis bundles + source control.
- Pyonephrosis — pus under pressure = emergency drainage (PCN or JJ stent). Antibiotics alone are INSUFFICIENT.
- EPN — gas-forming infection in DM. High mortality. Medical Mx + PCD ± nephrectomy.
Local complications: 4. Renal/perinephric abscess — insidious onset; CT for diagnosis; drain if > 3–5 cm. 5. Prostatic abscess — failure to defervesce despite antibiotics for prostatitis.
Long-term complications: 6. Renal scarring (reflux nephropathy) — the key paediatric complication. VUR + UTI → scarring → CKD + HTN. Prevent with prophylaxis and surgery. 7. Struvite stones — urease-producing organisms → alkaline urine → stone formation → perpetuates infection cycle. 8. SCC bladder — chronic UTI + long-term catheter ( > 10 years). 9. GU-TB complications — strictures, contracted bladder, autonephrectomy, infertility. Diagnosed late.
Pregnancy complications: Pyelonephritis (20–40% of untreated ASB), preterm delivery, low birth weight → screen and treat ASB.
Post-drainage complications: Post-obstructive diuresis ( > 200 mL/h), haemorrhage ex-vacuo, transient hypotension. Monitor I/O closely.
Active Recall - Complications of UTI
References
[1] Lecture slides: GC 210. Urinary tract infection.pdf (EPN, EC, GU-TB complications) [2] Senior notes: felixlai.md (UTI complications, DMSA timing, paediatric imaging, catheterisation complications) [3] Senior notes: Ryan Ho Fundamentals.pdf (p478, Urinalysis — struvite crystals) [4] Senior notes: Ryan Ho Urogenital.pdf (p124–129, Pyelonephritis complications, renal/perinephric abscess, prostatitis complications, GU-TB complications) [5] Senior notes: maxim.md (Post-surgical UTI — catheterisation and ineffective voiding) [6] Senior notes: Ryan Ho Critical Care.pdf (p25, AKI aetiology — post-renal disease including UTI) [7] Senior notes: Ryan Ho Fundamentals.pdf (p352–353, Post-obstructive diuresis, long-term catheterisation complications) [8] Senior notes: Ryan Ho Fundamentals.pdf (p342, Risk factors for malignancy — chronic UTI, chronic indwelling FB) [9] Senior notes: Ryan Ho Neurology.pdf (p80, Stroke complications — UTI as systemic complication)
High Yield Summary
Definition: UTI = inflammatory response of urothelium to bacterial invasion, associated with bacteriuria AND pyuria. Bacteriuria alone ≠ UTI. Pyuria alone ≠ UTI.
Epidemiology: Commonest bacterial infection; 50% of women will have UTI in lifetime; F >>> M (except neonates where M > F); incidence ↑ with age and sexual activity.
Risk Factors:
- Female sex (short urethra), sexual activity, lack of circumcision, VUR, urinary obstruction, BBD, constipation, catheterisation, DM, oestrogen deficiency
- Recurrent UTI: premenopausal = sexual activity + anatomy + genetics; postmenopausal = oestrogen deficiency + bladder mechanics
Microbiology: E. coli dominates (75% uncomplicated, 65% complicated, 50% nosocomial). S. saprophyticus in young sexually active women. Proteus = struvite stones. Broader spectrum + resistance in complicated/nosocomial UTI.
Pathogenesis: Ascending route commonest → faecal flora colonise periurethral area → ascend via urethra → adhere via pili (Type 1 for cystitis, Type P for pyelonephritis) → TLR activation → inflammatory cascade → symptoms.
Classification: Upper vs Lower; Uncomplicated vs Complicated; Isolated vs Recurrent (reinfection vs persistence); Clinical syndromes (ABU, cystitis, pyelonephritis, prostatitis, etc.).
Clinical Features:
- Lower UTI: dysuria, frequency, urgency, suprapubic pain, haematuria, NO systemic symptoms
- Upper UTI: fever ≥ 38°C, loin pain/CVA tenderness, rigors, ± lower tract symptoms
- Children: atypical — fever, irritability, poor feeding (always test urine in febrile child < 2 years)
- Elderly: atypical — confusion, falls, functional decline (but don't over-treat asymptomatic bacteriuria)
Host Defences: Vaginal flora (Lactobacilli, low pH), antegrade urine flow, Tamm-Horsfall protein, normal bladder emptying, urothelial innate immunity (TLR recognition of LPS), genetic susceptibility of epithelial cells.
Virulence Factors: Pili (Type 1 = cystitis, Type P = pyelonephritis), haemolysin, urease, capsular K antigen, LPS, siderophores, antimicrobial resistance.
High Yield Summary: Differential Diagnosis of UTI
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Dysuria DDx by sex: Female — UTI, STD/vaginitis, PID, interstitial cystitis, bladder stones. Male — UTI, STD urethritis, acute/chronic prostatitis, bladder stones, epididymitis.
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Cystitis vs Pyelonephritis: The dividing line is systemic symptoms (fever, rigors) and flank pain/CVA tenderness. Lower UTI should NOT cause fever.
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Sterile pyuria (pyuria + negative culture): Think TB, stones, cancer, partially treated UTI, interstitial cystitis, ketamine cystitis, interstitial nephritis.
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Recurrent UTI: > 95% are reinfection (genetic susceptibility). Always rule out bacterial persistence (stones, diverticulum) and treatment failure (resistance, poor compliance). Culture-negative recurrence → consider stones/TB/tumour.
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Persistent haematuria after treated UTI: Must exclude malignancy (cystoscopy + upper tract imaging), especially if age > 35, male, smoker.
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Pneumaturia: Instrumentation, colovesical fistula, emphysematous cystitis, tissue necrosis.
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Always ask about: Ketamine use, sexual history, TB exposure, previous urological surgery/stones, and medication history.
High Yield Summary: Diagnostics of UTI
Diagnosis = Symptoms + Pyuria + Significant Bacteriuria. +ve culture alone is NOT UTI.
Dipstick: LE (sensitive, detects pyuria) + Nitrite (specific, detects Gram-negative bacteriuria). Both negative → UTI very unlikely (NPV > 95%).
Microscopy: Pyuria > 10 WBC/mL; epithelial cells = contamination; WBC casts = pathognomonic of pyelonephritis; sterile pyuria → think TB, stones, cancer.
Culture thresholds: Symptomatic cystitis ≥ 10³; pyelonephritis ≥ 10⁴; asymptomatic bacteriuria ≥ 10⁵ × 2; SPA = any growth significant; bag urine = invalid for culture.
Urine pH > 7.5 → urease-producing organism (Proteus, Klebsiella, Pseudomonas) → struvite stones.
Imaging: Not needed in uncomplicated cystitis. USG mandatory in pyelonephritis to rule out obstruction. CT if no improvement at 72h. Paediatric: RBUS for all < 2y with UTI; MCUG if abnormal USG/atypical/recurrent UTI; DMSA for renal scarring.
Asymptomatic bacteriuria: Screen and treat ONLY in pregnancy and pre-urological procedures. Do NOT treat in elderly, catheterised, or diabetic patients.
Male UTI: Always culture; 50% have underlying abnormality; always consider further evaluation.
High Yield Summary: UTI Management
General Principles: Classify first; treat infection AND complicating factors; choose antibiotics based on local resistance patterns; collect cultures before antibiotics in complicated cases; check G6PD before nitrofurantoin/co-trimoxazole.
Uncomplicated Cystitis: Empirical PO nitrofurantoin 5d or augmentin 5–7d (current HK guidance / IMPACT). Fosfomycin 3 g single dose is an alternative in suitable premenopausal, non-pregnant women, though IMPACT notes possibly inferior efficacy. Avoid empirical fluoroquinolones unless other options are inappropriate.
Complicated Cystitis: Longer course (10–14d); eradicate the complicating feature; culture-guided.
Pyelonephritis: IV Augmentin → Tazocin (if Pseudomonas) → Meropenem (if ESBL/severe). IV until afebrile 24–48h → step-down PO. Total 14 days. CT if no improvement at 72h.
Prostatitis: Quinolone 2–6 weeks (excellent prostatic penetration).
Male UTI: 7 days minimum; always culture; 50% have underlying abnormality.
Asymptomatic Bacteriuria: Treat ONLY in pregnancy and pre-urological procedures. Do NOT treat in elderly/catheterised/DM.
Recurrent UTI: Behavioural measures → topical oestrogen (postmenopausal) → antimicrobial prophylaxis (continuous vs post-coital). Investigate for bacterial persistence if same organism recurs.
Paediatric: No routine prophylaxis for 1st UTI; co-trimoxazole prophylaxis for VUR ≥ Grade 3; surgical referral for recurrent UTI with VUR > 3 + scarring.
Obstructed infected kidney = Emergency: PCN or JJ stent for drainage; antibiotics alone are insufficient.
High Yield Summary: Complications of UTI
Immediate life-threatening complications:
- Urosepsis — the most feared; especially with obstruction. Manage with sepsis bundles + source control.
- Pyonephrosis — pus under pressure = emergency drainage (PCN or JJ stent). Antibiotics alone are INSUFFICIENT.
- EPN — gas-forming infection in DM. High mortality. Medical Mx + PCD ± nephrectomy.
Local complications: 4. Renal/perinephric abscess — insidious onset; CT for diagnosis; drain if > 3–5 cm. 5. Prostatic abscess — failure to defervesce despite antibiotics for prostatitis.
Long-term complications: 6. Renal scarring (reflux nephropathy) — the key paediatric complication. VUR + UTI → scarring → CKD + HTN. Prevent with prophylaxis and surgery. 7. Struvite stones — urease-producing organisms → alkaline urine → stone formation → perpetuates infection cycle. 8. SCC bladder — chronic UTI + long-term catheter ( > 10 years). 9. GU-TB complications — strictures, contracted bladder, autonephrectomy, infertility. Diagnosed late.
Pregnancy complications: Pyelonephritis (20–40% of untreated ASB), preterm delivery, low birth weight → screen and treat ASB.
Post-drainage complications: Post-obstructive diuresis ( > 200 mL/h), haemorrhage ex-vacuo, transient hypotension. Monitor I/O closely.