GC101 Diagnosis Of Infections
Diagnosis of infections is the systematic process of identifying causative pathogens through clinical assessment, microbiological techniques (culture, microscopy, serology, molecular methods), and laboratory markers to guide appropriate antimicrobial therapy.
Diagnosis of Infections: Collection & Handling of Clinical Specimens for Microbiological Tests
Lecture Map
This lecture is fundamentally about how to correctly diagnose infections in the laboratory — not just what tests to order, but why specimen quality determines whether you get a meaningful answer or misleading garbage. The lecture walks through a real clinical case (Legionella pneumonia in a CAPD patient), then systematically covers every major specimen type, what tests apply to each, and common pitfalls.
"The usefulness of microbiological tests is directly related to the quality of specimens." — This is the single most important principle of the entire lecture. [1]
- Understand the clinical approach to a patient with suspected infection (6-step framework)
- Know the two broad categories of laboratory diagnosis: detecting the microbe vs. detecting the host response
- Learn general principles of specimen collection, transport, and labelling
- Know specimen-specific requirements for blood, CSF, urine, sputum, stool, genital, wound, and other body fluids
- Recognize specimens of dubious significance and avoid ordering tests that produce misleading results
- Understand when and why to consult a microbiologist
The 6-step clinical approach from the lecture handout is the backbone of how HKU teaches infection management: [1]
| Step | Action | Why It Matters |
|---|---|---|
| 1 | History, PE, preliminary Ix → formulate clinical diagnosis | You must suspect infection before you can diagnose it |
| 2 | Localize the focus of infection (clinical + radiological) | Determines which specimens to collect |
| 3 | Collect & transport relevant specimens for microbiology | The quality here determines everything downstream |
| 4 | Start empirical antimicrobial therapy if indicated | Don't wait for culture results in serious infection |
| 5 | Modify therapy when rapid tests (Gram stain) or final C/ST come back | De-escalation or targeted therapy |
| 6 | Monitor therapy — further specimens, drug levels | Ensure response; catch resistance or complications |
Critical Sequence
Specimens should ideally be collected before antibiotics, but collection must not create a dangerous delay. Current adult sepsis guidance is probability- and severity-based: give antimicrobials immediately (ideally within 1 hour) for septic shock or probable/definite sepsis; in possible sepsis without shock, perform rapid investigation and give antimicrobials within 3 hours if concern persists. [10]
The Case: Legionella Pneumonia in a CAPD Patient
This case is used throughout the lecture to illustrate diagnostic principles. Understanding it helps you see how the framework works in practice.
- Patient: Elderly male, extensive comorbidities (HT ×30yr, DM ×15yr on insulin, mild CAD, hyperlipidaemia, gout, CKD on CAPD)
- Presentation: Acute fever + SOB ×1 day. Given ciprofloxacin 250mg q12h by GP → worsened → admitted QMH
- On admission: Tachycardic (120/min, irregular), febrile (39°C), desaturating (SpO₂ 70% on RA, 95% on CPAP), slow mentation, bilateral ankle oedema, decreased air entry left posterior chest, coarse inspiratory crepitus
- Labs: Hb 7.5 (dropped from 11), WBC 8.6, N 7.4, Cr 1299, K 5.1 — uraemic patient in acute LHF
What the serial chest radiographs add: the sitting films show cardiomegaly with a progressive left basal/retrocardiac air-space opacity and probable pleural fluid. In this overloaded dialysis patient, pneumonia and pulmonary oedema can coexist; the image localizes the problem but does not identify the organism.
Key history elicited on retrospective questioning: Travel to a hotel and zoo in Guangzhou 6 days before admission [1] — This completely changed the differential diagnosis.
| Investigation | Result | Teaching Point |
|---|---|---|
| Blood culture | Negative | Blood cultures are often negative in atypical pneumonia — doesn't rule out serious infection |
| Cold agglutinin | Negative | Rules against Mycoplasma (though sensitivity is low) |
| NPA viral IF | Negative | Direct fluorescent antibody for respiratory viruses |
| Resplex II RT-PCR (16 viruses) | Negative | Comprehensive viral panel — important to systematically exclude |
| Urine antigen (Binax) for Legionella SG1 & S. pneumoniae | Negative | Binax detects only serogroup 1 — other serogroups missed; also timing matters |
| Stool C/ST & C. difficile toxin | Negative | Patient had diarrhoea — must investigate |
| PD fluid | Clear, normal cell count, culture negative | Made PD-related peritonitis much less likely in this presentation |
| RT-PCR for Legionella pneumophila | POSITIVE (NPA Day 1 and sputum Day 4) | This was the definitive diagnosis |
The urine antigen for Legionella was negative because it detects only serogroup 1. The RT-PCR detected the organism. This illustrates why relying on a single test modality is dangerous. [1]
The amplification plot is not just decoration: the patient NPA and later sputum traces cross the assay threshold in the expected exponential phase while standards and controls validate the run. A cycle-threshold value must still be interpreted with the assay's controls, specimen type and clinical syndrome; it is not a stand-alone measure of disease severity.
- Stop meropenem and zanamivir after targeted results returned
- Continue levofloxacin alone in this renal-adjusted historical case
- Notify CHP (Centre for Health Protection) — Legionella is a notifiable disease
Historical 2011 empirical regimen (before Legionella was confirmed): [1]
- Levofloxacin 0.5 g q48h (renal-adjusted for CAPD)
- Meropenem 0.5 g q24h (renal-adjusted)
- Zanamivir while influenza testing was pending
These doses and this combination reflect one critically ill CAPD patient and must not be copied as a current empirical-pneumonia regimen; selection and renal dosing depend on syndrome severity, local resistance, organ function, and current protocols.
- In the historical investigation, 13/38 water samples from a newly commissioned Government Office grew Legionella (reported range 0.7–13 CFU/mL), including the patient's private shower.
- This distribution made the water system an epidemiologically plausible source, but a positive environmental sample alone does not prove transmission from a particular outlet. Culture enables comparison of clinical and environmental isolates; molecular relatedness plus exposure data provides stronger source attribution. [17]
Exam Pearl
Legionella is usually acquired by inhaling or aspirating contaminated water aerosols (for example from building water systems, showers, cooling towers, or hot tubs). Travel and water-system exposure histories are therefore important. Current diagnostic practice pairs urinary antigen (mainly L. pneumophila serogroup 1) with lower-respiratory culture or a validated molecular assay, because the latter detects a wider range of species/serogroups. [17]
The lecture presents a decision-making flowchart that complements the 6-step framework: [1]
Key concept: diagnosis is iteratively refined. A microbiology result changes the probability of a diagnosis; it rarely “rules in” or “rules out” infection without considering specimen quality, test limitations, host factors, and prior antimicrobials.
Laboratory Diagnosis of Infection: The Two Pillars
The lecture divides laboratory diagnosis into two fundamental approaches: [1]
| Method | What It Detects | Examples |
|---|---|---|
| Visualization | Morphology of organisms | Gram stain (bacteria), acid-fast stain (mycobacteria), wet mount (selected parasites/amoebae); electron microscopy is largely a historical/specialist method rather than routine viral diagnosis |
| Culture | Growth of living organisms | Blood culture broth, agar plates, viral culture in cell lines |
| Detection of microbial components | ||
| — Proteins | Antigen by EIA/LA | SARS-CoV-2 RAT, Cryptococcal antigen, Dengue NS1 |
| — Polysaccharides | Antigen by EIA/LA | Pneumococcal urinary antigen, Legionella urinary antigen |
| — Lipids | Chromatography | Gas-liquid chromatography for anaerobes |
| Nucleic acid detection | Specific DNA/RNA | PCR, probe hybridization, RT-PCR, next-generation sequencing (NGS) |
| Method | What It Detects | Examples |
|---|---|---|
| Antibody response | IgM (acute), IgG (past/rising titre) | Brucella, Rickettsia, VDRL, Toxoplasma IgM; 4× rise in IgG titre between acute and convalescent sera |
| Cellular immune response | T-cell activation | Mantoux/TST (tuberculin skin test), IGRA (IFN-γ release assay) — detecting activated CD4/CD8 lymphocytes expressing IFN-γ, TNF-α after stimulation by specific antigens |
Why Two Approaches?
Direct detection finds an organism or one of its components, but detection does not always equal causation: NAAT may detect colonization or residual nucleic acid, and non-sterile-site culture may recover flora. Host-response tests may reflect past exposure, vaccination, cross-reaction, or a delayed response. TST/IGRA indicate immune sensitization to TB antigens and cannot distinguish latent infection from active TB disease. Interpret both approaches against pre-test probability.
"Garbage In, Garbage Out" — Improperly collected specimens → Misleading test results [1]
This is a deliberately provocative slide to emphasize that the entire diagnostic chain fails if the specimen is poor. The three determinants of specimen quality are:
- Representative of the pathophysiological process — Is the specimen actually from the site of infection?
- Manner of collection — Was aseptic technique used? Was the right container chosen?
- Manner of transportation — Was it delivered promptly? Were transport conditions appropriate?
The photograph of many containers awaiting processing makes the operational point visible: even a well-collected sample degrades if it sits unlabelled, leaks, dries out, warms, or permits commensal overgrowth. Record collection time, use the specified transport system, and alert the laboratory before unusual or high-risk specimens arrive.
These principles appear repeatedly in the lecture and handout: [1]
| Principle | Explanation |
|---|---|
| Relevant specimens with clinical indication | Don't send random swabs without clinical suspicion |
| Correct containers | Prevent leakage and contamination |
| Expedient transport | Preserve microbial viability; prevent overgrowth of contaminants |
| Proper request form entry | Clinical information guides the lab — wrong info = wrong tests |
| Standard Precautions | Protect the patient, collector, courier, and laboratory staff; “universal precautions” is the lecture's historical terminology |
| Known hazards — inform lab | If you suspect high-risk pathogens (TB, SARS, avian flu), TELL the lab |
| Unusual infections — consult microbiologist | Don't just send specimens blindly for rare organisms |
Common Exam Trap
"Fill the entries in computer form properly!!!" — The lecture specifically emphasizes that clinicians often fail to provide adequate clinical information on request forms. Without clinical context, the lab cannot select appropriate tests. A sputum sent "for C/ST" without mentioning immunosuppression will not be tested for fungi or Nocardia. [1]
The visual contrast between good and bad requests: “swab + fever” is not an adequate request. State the exact anatomical site and specimen type, syndrome and onset, host risks/exposures, current or recent antimicrobials, and the suspected high-consequence organism. Place the leak-proof primary container in a sealed biohazard bag and keep paperwork in the separate outer pouch—the slide's blood-smeared form is an avoidable exposure and may make the request unreadable. Electronic reporting does not close the loop by itself: urgent and actionable results require an identified recipient, acknowledgement, and follow-up.
Specimen-by-Specimen Guide
Blood culture collection has specific rules regarding indication, timing, number, site, and technique: [1]
| Parameter | Details | Why |
|---|---|---|
| Indications | Suspected bloodstream/endovascular infection when the result can change management—not fever alone | Avoid indiscriminate cultures and false positives |
| Timing | Ideally before antibiotics; routine sets may be drawn sequentially without waiting. For suspected endocarditis, current ESC guidance uses ≥3 sets at 30-minute intervals before antibiotics. [13] | Maximizes recovery and helps document continuous bacteraemia |
| Number and volume | Usually ≥2 adult sets; target 20–30 mL per set (typically split across aerobic/anaerobic bottles), following local bottle instructions | Blood volume is a major determinant of yield; multiple sets also help distinguish contamination. [19] |
| Site | Separate peripheral venepunctures; paired line + peripheral cultures only when evaluating a catheter source | Routine line draws contaminate more easily; paired samples permit differential time-to-positivity |
| Broth | Some modern bottles contain antibiotic-binding resin/charcoal | May improve recovery after prior antimicrobial exposure; use the local blood-culture system correctly |
| Aseptic technique | Skin antisepsis (chlorhexidine/alcohol), no-touch technique | Reduces contamination with coagulase-negative staphylococci (CoNS) |
Pseudobacteraemia Outbreak
The reproduced 1996 report describes Burkholderia pickettii (now Ralstonia pickettii) pseudobacteraemia traced to a contaminated citrate product used during coagulation studies; PFGE/AP-PCR supported a common source. The visual lesson is broader: a cluster of an unusual environmental organism, especially without a compatible syndrome, should trigger review of collection devices, shared fluids, timing, and processing—not automatic treatment of every positive bottle. [1]
Blood culture-negative endocarditis (BCNIE): previous antibiotics are the commonest explanation. If suspicion remains high, involve microbiology/endocarditis specialists and tailor serology, blood/tissue PCR, valve culture, histology, and 16S/18S sequencing to exposure and epidemiology. Current ESC guidance particularly includes Coxiella burnetii, Bartonella spp., Brucella spp., Tropheryma whipplei, fungi and mycobacteria, with rarer agents considered locally. The lecture's undifferentiated list (including Leptospira and Chlamydia) should be read as historical, not a universal panel. [13]
Differential Time to Positivity (DTP)
In patients with central venous catheters, blood cultures drawn simultaneously from the catheter and a peripheral vein can help diagnose catheter-related bloodstream infection (CRBSI). If the catheter culture turns positive ≥2 hours before the peripheral culture, it suggests the catheter is the source. [3]
Interpreting the polymicrobial report images: multiple enteric species in a compatible patient can represent true polymicrobial bacteraemia from a gastrointestinal, pressure-injury, or necrotic soft-tissue source; “many organisms” is not synonymous with contamination. Conversely, repeated recovery of an unusual water organism across patients may indicate pseudobacteraemia. Judge organism identity, number of positive sets, time-to-positivity, source, and clinical trajectory together.
CSF collection and testing is a critical exam topic: [1]
Before and during LP:
- Do not obtain cranial imaging routinely before LP. WHO 2025 recommends pre-LP imaging when available if there is GCS <10, focal neurological signs, cranial nerve deficits, papilloedema, new-onset seizure in an adult, or severe immunocompromise. [12]
- If LP or imaging must be deferred, obtain blood cultures and start indicated empirical treatment promptly; suspected bacterial meningitis must not wait for imaging or CSF results. [12]
- Use sterile, single-use, laboratory-approved tubes. The lecture's phrase “non-reused bottle” reflects an older local teaching image; current tube allocation, minimum volumes, and transport conditions should follow the receiving laboratory.
- LP level: the diagram shows a needle entering below the conus among the cauda equina, commonly at L3–L4 or L4–L5—not “after CT” by default.
- Send CSF urgently and provide matched blood glucose where relevant; record opening pressure when safe and feasible. Cell count/differential, protein and glucose complement the microbiology tubes.
Tests to order on CSF:
| Test | Target | Why |
|---|---|---|
| Gram stain | Bacteria (pyogenic meningitis) | Rapid — guides empirical therapy within hours |
| ZN smear | AFB (TB meningitis) | Low sensitivity (~10-20%) but highly specific |
| Wet mount | Free-living amoeba (primary amoebic meningoencephalitis) | Rare but rapidly fatal — must consider if freshwater exposure |
| Bacterial culture | Standard pathogens | Gold standard but takes 24-48h |
| PCR + AFB culture for TB | M. tuberculosis | PCR faster than culture (which takes weeks) |
| Cryptococcal antigen (latex agglutination) + fungal culture | Cryptococcus neoformans | Especially in HIV/immunocompromised — cryptococcal antigen is highly sensitive |
| PCR for HSV1, HSV2, VZV | Herpes encephalitis | PCR is the single most important method for diagnosing CNS viral infections [4] |
| RT-PCR for enterovirus | Acute meningitis/encephalitis | Common viral cause of aseptic meningitis |
| CSF IgM for Japanese encephalitis | Epidemiologically compatible encephalitis | Useful in the appropriate exposure context; the lecture also lists M. pneumoniae IgM, but testing should be syndrome- and laboratory-guided |
Historical slide indications for considering a removed-tip culture include: [1]
- Local signs of phlebitis at the insertion site
- Persistent fever without other localizing signs
Threshold: ≥15 CFU of bacteria on roll-plate culture = significant (Maki technique)
The paired photographs show the method: cut the distal catheter segment aseptically, roll it across the agar surface with sterile forceps, then count colonies. Do not routinely culture a removed catheter tip. The ≥15-CFU threshold supports catheter colonization; catheter-related bloodstream infection still requires compatible illness and blood-culture correlation (or another validated paired-culture method). [11]
There is a critical hierarchy of specimen quality for skin/soft tissue infections: [1]
| Specimen Type | Representation | Contamination Risk | Comment |
|---|---|---|---|
| Tissue biopsy | Excellent — directly from infected site | Low | Best specimen for deep infections |
| Sterile aspirate / Abscess fluid | Excellent | Low | Use needle aspiration, not swabs |
| Drain fluid | Variable | Moderate-High | Long-term drains become colonized |
| Wound swab | Poor | High — picks up superficial colonizing flora | Degree of representation low, degree of contamination high |
For selected normally sterile body fluids (joint, pleural, peritoneal, pericardial), bedside inoculation into appropriate blood-culture bottles may improve recovery. Also send the laboratory's required sterile container for Gram stain, cell count, and any tests not validated from a blood-culture bottle. [1,11]
This is a practical pearl: enrichment broth and, in some systems, antibiotic-neutralizing media can improve recovery of low-count or fastidious organisms.
Container lesson from the photographs: never place a microbiology tissue specimen in formalin. Send a separate fresh tissue/fluid specimen in the sterile container and transport conditions specified by the laboratory; place the pathology portion in fixative separately. A swab is a fallback for superficial aerobic culture, not a substitute for obtainable tissue, pus or body fluid. [11]
Actinomycosis
The histology image labelled “actinomycosis” shows basophilic bacterial colonies with radiating filamentous forms within inflamed tissue. Actinomyces are anaerobic/microaerophilic Gram-positive filamentous bacteria. Submit deep tissue or aspirate for histopathology and prolonged anaerobic culture; “sulfur granules” are supportive but are neither always present nor unique to one species. Superficial swabs are poor specimens. [1,11]
Stool specimens have unique challenges: [1]
| Challenge | Explanation | Solution |
|---|---|---|
| Overgrowth of commensal flora | Normal gut has 10¹¹ bacteria/g — pathogens easily overgrown | Selective media (e.g., MacConkey, XLD agar) |
| Death/loss of architecture of parasites | Trophozoites of amoeba die quickly at room temperature | Examine fresh stool within 30 min; fix specimens for ova & cysts |
| Antibiotic-related diarrhoea | Clostridioides difficile infection is toxin-mediated; NAAT may also detect colonization | Test only an appropriate unformed stool from a symptomatic patient; use the locally validated NAAT and/or multistep GDH/toxin algorithm. [18] |
There is no single “standard stool panel” for everyone. Choose culture, Shiga-toxin testing, multiplex NAAT, ova/parasite examination, or C. difficile testing from the diarrhoeal syndrome, duration, travel/exposure, immune status, outbreak/public-health needs, and local laboratory pathway. A diarrhoeal stool that takes the shape of the container is preferred; formed stool and routine rectal swabs are generally unsuitable. [11,18]
Historical viral workflow shown in the slides:
- Rotavirus: EIA for antigen detection
- Norovirus, sapovirus, astrovirus, enteric adenovirus 40/41: faeces in viral transport medium for RT-PCR
The slide's viral-transport-medium tubes document a historical local workflow. Modern enteric PCR platforms have assay-specific containers and transport requirements; use the current laboratory manual rather than automatically placing every stool or rectal swab in VTM. [11]
Special requests must be exposure- and syndrome-led: mycobacteria, Cryptosporidium, Cystoisospora (historically “Isospora”), Cyclospora, microsporidia, and Yersinia enterocolitica may require different methods. Suspected high-consequence agents should be discussed with microbiology/public health before collection rather than added as a routine tick-box. [1,11]
Three collection routes shown in the lecture: [1]
| Method | Contamination Risk | Indication |
|---|---|---|
| Mid-stream clean-catch urine (MSU) | Requires clear instruction; collect the midstream portion without touching the inside of the cup | Standard when the patient can void |
| Catheter urine | Biofilm makes bacteriuria increasingly common over time | Collect aseptically from the disinfected sampling port—never from the drainage bag. [14] |
| Suprapubic aspirate (SPA) | Bypasses urethral flora but is invasive | Selected paediatric or diagnostically difficult situations, not a universal first-line “gold standard” |
Urine can be used for:
- Bacterial culture (UTI)
- Yeast (Candida)
- PCR for N. gonorrhoeae & C. trachomatis (first-void urine)
- Antigenuria: Pneumococcal & Legionella urinary antigen [1]
Transport promptly or refrigerate/use an approved preservative if delayed. Pyuria or bacteriuria alone does not diagnose symptomatic UTI. After about one month of indwelling catheterization, bacteriuria is nearly universal; catheter-associated asymptomatic bacteriuria usually does not require antibiotics. If a long-standing catheter must be cultured for suspected CAUTI, follow the local catheter-replacement and specimen protocol. [14]
The lecture specifically covers the approach to suspected gonococcal infection: [1]
| Specimen | Test | Purpose |
|---|---|---|
| Urethral discharge | Gram stain + culture when indicated | Intracellular Gram-negative diplococci are highly specific in symptomatic men; a negative smear does not exclude infection |
| NAAT specimen | First-catch urine is optimal for men; vaginal swab is optimal for women on many platforms | Test for N. gonorrhoeae and C. trachomatis using an assay validated for that specimen [15] |
| Extragenital sites | Pharyngeal and rectal swabs | Test each anatomical site of sexual exposure; “nasopharyngeal throat swab” on the slide is anatomically incorrect [15] |
| Culture | Relevant genital/extragenital site | Essential when treatment failure or resistance is suspected because NAAT does not provide phenotypic susceptibility [15] |
The lecture's “triple swab” is a useful reminder not to miss extragenital infection, but site selection should follow sexual exposure and symptoms rather than a fixed bundle for every patient.
Two main specimen types with different pathogen targets: [1]
| Specimen | Pathogens Detected | Key Points |
|---|---|---|
| Nasopharyngeal swab/aspirate (NPA) | Bordetella pertussis and respiratory viruses, depending on assay | Correct site and depth matter; specimen choice varies with pathogen, age, assay, and current laboratory guidance |
| Throat swab | Streptococcus pyogenes, Neisseria gonorrhoeae, Corynebacterium diphtheriae, and selected respiratory-virus assays | Sensitivity is pathogen-, site- and assay-dependent; it is not interchangeable with an NP specimen |
Historical slide caveat: “all RT-PCR samples must be in unreused gamma-irradiated bottles” was a local contamination-control rule, not a universal modern requirement. Use the manufacturer- and laboratory-approved sterile collection device and transport medium for the requested NAAT. [11]
Transport medium: VTM is used for some respiratory assays and cultures, but molecular and rapid-antigen platforms may specify other swabs/media. Follow the active assay instructions.
Throat swab technique: swab both tonsillar pillars/fossae and the posterior pharynx while avoiding the tongue, teeth, and buccal mucosa. The adjacent NPA diagram shows posterior nasopharyngeal aspiration into transport medium—this is a different anatomical specimen.
Rapid antigen testing (RAT): Uses immunochromatographic EIA (as with SARS-CoV-2 rapid tests). Lower sensitivity than RT-PCR but provides point-of-care results.
RAT interpretation: The lateral-flow cassette has a C (control) line and T (test) line. A valid negative result shows a band at C only; a valid positive shows bands at both C and T. Any result without the control line is invalid. [1]
The diagram moves from easily obtained but contamination-prone samples toward invasive, more anatomically representative samples. Yield is syndrome- and test-dependent, so this is not a simple “higher is always better” ladder. [1]
| Specimen | Indication | Quality |
|---|---|---|
| Expectorated sputum | Unusual chest infections (instructed/assisted by physiotherapist) | Oropharyngeal / saliva contamination is the main problem |
| Endotracheal aspirate | Intubated patients | Samples the lower airway but may reflect tube/airway colonization |
| Bronchoscopic aspirate | Selected patients | Better access does not eliminate oropharyngeal contamination |
| Bronchoalveolar lavage (BAL) | Severe/persistent/undetermined pneumonia or immunocompromise | Enables targeted bacterial, fungal, mycobacterial, viral, cytological and other testing |
| Protected catheter brush | As above | Minimizes upper airway contamination |
| Transbronchial lung biopsy | Immunocompromised with undetermined pneumonia | Tissue diagnosis |
| Open lung biopsy | Last resort | Highest yield but most invasive |
Sputum Quality
Saliva-rich sputum is poor for routine bacterial culture. Squamous epithelial cells suggest oropharyngeal contamination; neutrophils support inflammation. The often-taught “>25 WBC and <10 squamous epithelial cells/low-power field” rule is a laboratory-specific screening convention, not a universal cut-off, and does not apply identically to mycobacterial, fungal, viral or Legionella testing. The case patient did not produce sputum until day 4, so an earlier alternative specimen was needed. [1,11]
Sputum report examples from the lecture: [1]
| Scenario | Gram Stain | Culture | Interpretation |
|---|---|---|---|
| F/28, oral swab | Mixed bacterial morphotypes, WBC ++, epithelial cells negative | Oral commensals only | A specimen labelled only “swab” cannot reliably represent a deep dental infection; site and sampling depth matter |
| M/65, CAP on 1 week antibiotics from GP | WBC +++, epithelial cells negative, muco-purulent | Candida albicans (heavy) | Prior antibiotics suppressed bacterial pathogens; heavy Candida likely represents overgrowth, not true pathogen — illustrates how antibiotics before specimen collection compromise results |
The BAL report image grows several upper-airway organisms in scant quantities. Bronchoscopy passes through colonized airways, so a mixed low-burden culture without a dominant organism can represent contamination or colonization. Likewise, the tracheostomy photograph reminds you that chronic artificial airways become colonized: treat a compatible pneumonia syndrome, not the airway culture in isolation.
Causes of Febrile Respiratory Illness
The lecture provides a comprehensive list of bacterial, viral, fungal, and parasitic causes. This is HIGH YIELD: [1]
| Organism | Key Association / Notes |
|---|---|
| Streptococcus pneumoniae | Common CAP pathogen; urinary antigen is available in selected hospitalized adults |
| Staphylococcus aureus / Streptococcus pyogenes | Post-influenza pneumonia, skin/wound source |
| Haemophilus influenzae / Neisseria meningitidis | H. influenzae is associated with COPD; meningococcal pneumonia is rare |
| Klebsiella pneumoniae | Consider with relevant host/healthcare factors; “red-currant-jelly sputum” is a classic but insensitive, non-specific historical descriptor |
| Oral aerobes/anaerobes | Aspiration pneumonia |
| Acinetobacter baumannii | Hospital-acquired, multidrug-resistant |
| Pseudomonas aeruginosa | Hospital-acquired, structural lung disease, ICU |
| Burkholderia pseudomallei | Melioidosis — SE Asia, travel history, DM |
| Legionella pneumophila | Urinary antigen EIA (but only SG1); travel/water exposure |
| Mycoplasma pneumoniae | RT-PCR or Ab — atypical pneumonia |
| Chlamydophila pneumoniae/psittaci | Atypical pneumonia; psittaci = bird contact |
| Coxiella burnetii | Q fever — animal/farm exposure |
| Mycobacterium tuberculosis | Sputum for AFB smear/culture |
| Virus | Notes |
|---|---|
| Influenza A (H3N2, H1N1, avian H5N1/H9N2/H7N9), B, C | Seasonal and zoonotic epidemiology determine testing/public-health implications |
| Adenovirus | Can cause severe pneumonia in immunocompromised |
| RSV, Metapneumovirus | Important in children and elderly |
| Parainfluenza 1-4 | Croup in children |
| Rhinovirus Clade A, B, C | Most common cause of common cold; can cause pneumonia |
| Coronavirus: MERS, SARS-CoV-1/2, OC43, HKU1, 229E | Pandemic potential; HKU1 discovered in HK |
| Enterovirus, Bocavirus | Less common respiratory pathogens |
| Organism | Notes |
|---|---|
| Cryptococcus | HIV/AIDS, cryptococcal meningitis |
| Aspergillus | Invasive aspergillosis in neutropenic patients |
| Dimorphic fungi: Talaromyces marneffei (historically Penicillium marneffei), Histoplasma, Coccidioides | Geography and exposure matter |
| Mucorales (historically “zygomycetes”) | Diabetic ketoacidosis, neutropenia, transplantation, iron overload |
| Pneumocystis | PCP in HIV (CD4 < 200) or other immunosuppression |
| Organism | Notes |
|---|---|
| Paragonimus westermanii | Lung fluke — raw crabs/crayfish |
| Ascaris lumbricoides | Löffler syndrome (pulmonary eosinophilia) |
| Strongyloides stercoralis | Hyperinfection in immunosuppressed |
Eosinophilia can be a clue during tissue-invasive helminth migration, but its absence does not exclude parasitic lung disease; it may be absent in severe Strongyloides hyperinfection.
Beyond blood cultures, serum/blood provides three categories of diagnostic information: [1]
| Category | Tests | Examples |
|---|---|---|
| Therapeutic drug monitoring | Drug exposure interpreted with dose timing, renal function and pharmacokinetic target | Aminoglycosides and vancomycin; the slide's “serum bactericidal titre” is now a niche/historical test rather than routine monitoring |
| Serum antibody detection | Acute + convalescent sera; IgM for viruses; 4× rise in IgG titre | Brucella, Rickettsia/Orientia, ASO titre, VDRL, Toxoplasma, Aspergillus IgG |
| Serum/blood antigen detection | Direct antigen assays | Cryptococcal antigen, Aspergillus galactomannan, 1,3-beta-D-glucan (a non-specific fungal cell-wall marker), Dengue NS1; CMV pp65 antigenemia is a historical/local workflow now often replaced by quantitative nucleic-acid testing |
The 4× Rise Rule
For some pathogens, a fourfold rise in agent-specific antibody titre between appropriately timed acute and convalescent sera supports recent infection. It is not a universal “gold standard,” and collection intervals/assays vary. A single IgG result may represent past exposure; IgM can be early-negative, persist, cross-react, or be falsely positive. [1,11]
This is a critical concept the lecture emphasizes — ordering the wrong test wastes resources and produces misleading results: [1]
| Dubious Specimen | Why It's Problematic |
|---|---|
| Bedsore / superficial wound swabs | Colonized by multiple organisms — culture reflects flora, not the pathogen causing deep infection |
| Routine culture of penile/perineal/nasal swabs | Same problem — surface colonization ≠ infection |
| Long-term drain fluids (e.g., PTBD) | Drains become colonized over time — culture grows colonizers, not pathogens |
| Urine from chronic catheterized patients without a compatible syndrome | Biofilm-associated bacteriuria becomes nearly universal with prolonged catheterization; culture and treatment of asymptomatic bacteriuria are usually inappropriate. [14] |
| Colonic biopsy for routine bacterial culture (without clinical info) | Without context, the lab cannot interpret results — need histological confirmation of infection |
The lecture shows images asking “Does this need Rx?”—a long-term PTBD with a seven-organism culture, a large open abdominal wound with drains, a macerated perineal wound yielding MRSA/Candida, and a necrotic wound yielding mixed aerobes/anaerobes. The organism list alone cannot answer the question. [1]
The Teaching Slide With Three Organisms
A superficial or drain specimen growing WBCs plus Enterococcus, Acinetobacter and Stenotrophomonas may reflect colonization, but the photographed necrosis also warns against dismissing a clinically infected wound. Assess new pain, spreading erythema/induration, purulence, malodour, crepitus, systemic toxicity, tissue ischaemia/necrosis and source-control needs. If infection is suspected, obtain deep tissue/aspirate (including anaerobic culture where appropriate) and involve surgery urgently for a necrotizing phenotype. Treat the syndrome and source, not the organism list alone. [11]
This is a deliberate clinical scenario testing whether you understand the significance of a positive AFB smear: [1]
- M/56, old TB, occasional dry cough
- CBP: Hb 11, WBC 6.8, ESR 30
- LFT/RFT: normal
- CXR image: chronic upper-zone fibrotic change consistent with previous TB, but radiography alone cannot determine activity
- Sputum AFB smear/culture ×3 → one specimen AFB smear +++; culture identified M. tuberculosis
The positive culture establishes active pulmonary TB in this clinical setting. A 3+/+++ smear indicates high bacillary burden and greater infectiousness, but an AFB smear alone is not species-specific and a negative smear would not exclude TB. Current evaluation also includes rapid molecular detection of M. tuberculosis and resistance on at least one respiratory specimen, culture, and drug-susceptibility testing. [16]
Immediate actions include:
- Notification
- Contact tracing
- Airborne precautions and appropriate sputum-collection controls
- Multidrug therapy selected with TB specialists and adjusted to rapid/genotypic and phenotypic susceptibility results—not an unqualified automatic “RIPE” label
The lecture specifically highlights that blood cultures, pleural fluid cultures, and BAL cultures are often neglected but critical: [1]
- Blood cultures: useful in severe pneumonia or when bacteraemia/endovascular infection is suspected; collect adequate volume before antibiotics when feasible.
- Pleural fluid: sample a clinically significant parapneumonic effusion when indicated; send direct sterile-fluid studies and, where validated, additionally inoculate blood-culture bottles.
- BAL: useful when immunocompromise, severity, unusual pathogens or non-diagnostic non-invasive samples justify bronchoscopy; select tests before the procedure with microbiology.
Integration with Related Lectures
- MSU collection technique matters — contamination with periurethral flora causes false-positive cultures
- Pyuria and bacteriuria support—but do not by themselves establish—symptomatic UTI; require compatible symptoms/signs and exclude another source. [14]
- Empirical therapy should be narrowed once C/ST results return
- TDM for aminoglycosides and vancomycin — lecture mentions these specifically
- CSF specimen handling directly feeds into this lecture's content
- PCR for HSV is the single most important test for viral encephalitis
- Blood cultures are part of the septic workup
- The 6-step approach applies to all febrile patients
- The Legionella case demonstrates travel-associated infection
- Travel history changes the differential and specimen requirements
- Most common MDR Gram-positive (contact): MRSA
- Most common MDR Gram-negative (contact): ESBL-producing Enterobacteriaceae (or Acinetobacter baumannii)
- Most common MDR yeast (contact): Candida auris
- Most common bacterial cause of infectious diarrhoea (contact, hospital): Clostridioides difficile
- Airborne: TB, measles, varicella (chickenpox)
- Blood-borne: HBV, HCV, HIV
High Yield Summary
-
Clinical approach
- : 6 steps — suspect → localize → collect specimens → empirical Rx → modify with results → monitor
-
GIGO
- : Specimen quality determines result quality. Three pillars: representative of disease, proper collection, proper transport.
-
Blood cultures
- : Usually ≥2 adult sets from separate peripheral venepunctures; target 20–30 mL per set, collect before antibiotics when feasible, and prioritize asepsis. BCNIE work-up is exposure- and epidemiology-led.
-
CSF
- : Prompt LP unless contraindicated; do not CT everyone. Image first only for defined risk features and never delay indicated empirical treatment. Use sterile approved tubes and urgent transport.
-
Sputum quality
- : Saliva/squamous epithelial cells suggest poor routine-bacterial-culture quality. Cut-offs are laboratory-specific; choose sputum, ETA, BAL or tissue from the syndrome and requested test.
-
Urine
- : Instruct clean-catch MSU; sample an indwelling catheter from the disinfected port, never the bag. SPA is reserved for selected cases. Do not treat asymptomatic catheter bacteriuria in most patients.
-
Stool
- : Submit unformed diarrhoeal stool and choose tests by syndrome/exposure. C. difficile NAAT can detect colonization; use an appropriate patient plus the local validated algorithm.
-
Genital
- : NAAT at every exposed anatomical site (urogenital, pharyngeal, rectal); culture when susceptibility is needed. First-catch urine is preferred for men and vaginal swab for women on many platforms.
-
Dubious specimens
- : Bedsore swabs, long-term drain cultures, catheterized urine — treat the patient, not the culture.
-
Serology
- : A fourfold rise on paired sera supports recent infection for selected validated assays; single IgG/IgM results require context. Antigen/biomarker detection can be organism-specific or non-specific.
- Legionella case
- : Environmental water source, travel history critical, urine antigen detects only SG1, PCR was diagnostic.
Active Recall - Diagnosis of Infections
[1] Lecture slides: GC 101. Diagnosis of infections.pdf; GC 101. Diagnosis of infections [Handouts].pdf [2] Past papers: 2024 Fourth Summative SAQ.pdf (Q12) [3] Senior notes: Maksim Medicine Notes.pdf (Infectious diseases, p186) [4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p508) [5] Lecture slides: GC 210. Urinary tract infection.pdf [6] Lecture slides: GC 106. Practical issues in antibiotic use.pdf [7] Lecture slides: GC 051. Fever and confusion_meningitis and encephalitis; suppurative brain infection.pdf [8] Lecture slides: GC 048. Fever.pdf [9] Lecture slides: GC 103. Fever after travelling.pdf [10] Society of Critical Care Medicine. Surviving Sepsis Campaign Adult Guidelines (2026). [11] IDSA/ASM. Guide to Utilization of the Microbiology Laboratory for Diagnosis of Infectious Diseases: 2024 Update. Clinical Infectious Diseases. 2024; ciae104. [12] World Health Organization. WHO guidelines on meningitis diagnosis, treatment and care. 2025. [13] European Society of Cardiology. 2023 ESC Guidelines for the management of endocarditis. European Heart Journal. 2023;44:3948–4042. [14] US CDC. Indwelling Urinary Catheter Culture Stewardship. Updated 2024. [15] US CDC. Sexually Transmitted Infections Treatment Guidelines. 2021. [16] US CDC. Clinical and Laboratory Diagnosis for Tuberculosis. Updated 2025. [17] US CDC. Laboratory Testing for Legionella. Updated 2026. [18] US CDC. Clinical Guidance for C. difficile Infection Prevention in Acute Care Facilities. Updated 2026. [19] US CDC. Collect Adult Blood Culture Sets. Updated 2026.
GC100 Defense Against Microbes
The host defense against microbes encompasses the integrated system of innate and adaptive immune mechanisms—including physical barriers, phagocytes, complement, and lymphocyte-mediated responses—that collectively recognize, contain, and eliminate pathogenic microorganisms.
GC102 Fever After Chemotherapy Infections In Immunocompromised Hosts
Febrile neutropenia and opportunistic infections occurring in immunocompromised patients following chemotherapy, requiring urgent evaluation and empiric broad-spectrum antimicrobial therapy due to impaired host defenses.