GC106 Practical Issues In Antibiotic Use
Practical issues in antibiotic use encompass clinical considerations such as appropriate drug selection, dosing, route of administration, duration of therapy, spectrum of activity, drug interactions, adverse effects, and antibiotic stewardship to optimize efficacy and minimize resistance.
Practical Issues in Antibiotic Use
Lecture Map: The Big Idea
This lecture, delivered by Dr. HO Pak Leung (Microbiology, HKU), is fundamentally about the clinical decision-making process when choosing antibiotics — not just "which drug kills which bug," but when to prescribe, when NOT to prescribe, and how to think through empirical therapy in both outpatient and inpatient settings [1].
The lecture sits at the intersection of three pillars:
- The Patient (host factors, severity, immune status)
- The Bacteria (likely pathogens, resistance patterns)
- The Antibiotic (spectrum, route, pharmacology)
This lecture is tested directly in MCQs (matching antibiotic to clinical scenario), SAQs (empirical antibiotic choice, URTI management), and minicases (infection management). The 2025 Fourth Summative MCQ had an entire EMQ section (Questions 16–20) testing antibiotic selection for specific clinical scenarios — directly from this lecture's content [3].
Antibiotics ←→ Patient ←→ BacteriaThe antibiotic is only ONE part of the equation. The host-microbe interaction (virulence vs. host defense) determines whether infection occurs. The antibiotic is the tool we add to tip the balance. [1]
Why this matters: A young, immunocompetent patient with a viral URTI does NOT need an antibiotic — their host defense is sufficient. An elderly, immunocompromised patient with the same symptoms MAY need empirical antibiotics because their host defense is compromised and the consequences of missing a bacterial infection are severe.
Before choosing ANY antibiotic, you MUST make a working clinical diagnosis based on History, Physical Examination, and Investigations. [1]
The lecture emphasizes that you are pattern-matching the clinical syndrome to the likely organisms, then choosing an antibiotic that covers those organisms based on historical susceptibility data.
The Key Infectious Syndromes and Their "Educated Guess" Organisms
| Clinical Syndrome | "Likely" Organisms (Educated Guess) |
|---|---|
| Meningitis | Strep. pneumoniae, N. meningitidis, Listeria (elderly/immunocompromised) |
| Pneumonia | Strep. pneumoniae, H. influenzae, atypicals (Mycoplasma, Legionella) |
| Intra-abdominal | E. coli, Klebsiella, other Enterobacteriaceae, anaerobes (Bacteroides) |
| Bone/joint | S. aureus (MSSA/MRSA), Streptococci |
| Endocarditis | Viridans streptococci, S. aureus, Enterococcus |
| UTI | E. coli (most important), Klebsiella, other Enterobacteriaceae |
| SSTI (skin/soft tissue) | S. aureus (MSSA), Streptococcus pyogenes (GAS) |
| Bacteremia | Depends on source — any of the above |
This "educated guess" approach = empirical therapy. You do not know exactly what organism you are treating. You are making a probabilistic decision. [1]
Why Empirical Therapy Is Not Random Guessing
Empirical therapy is NOT random. It is based on: (1) the clinical syndrome → which narrows the likely organisms, (2) local resistance data → which tells you what antibiotics those organisms are likely to be susceptible to, and (3) patient factors → severity, allergies, renal/hepatic function, prior antibiotic exposure, recent culture results.
Step 2: Know Your Antibiotic Arsenal
The following antibiotic spectrum table is directly from the lecture and is HIGH YIELD for matching questions. [1]
| Class | Examples | Key Spectrum | Special Notes |
|---|---|---|---|
| Penicillins | Penicillin G, Cloxacillin, Piperacillin | Pen G = strep; Cloxacillin = staph; Piperacillin = Pseudomonas | Narrow spectrum individually |
| 1st Gen Cephalosporin (1GC) | Cephalexin (PO), Cefazolin (IV) | Gram +ve (Staph, Strep) | Surgical prophylaxis standard [4] |
| 2nd Gen Cephalosporin (2GC) | Cefuroxime | + Gram negative coverage | Cefoxitin has anti-anaerobic activity* |
| 3rd Gen Cephalosporin (3GC) | Ceftriaxone, Ceftazidime | ++ Gram negative; Ceftazidime covers non-fermenters (Pseudomonas) | Ceftriaxone = workhorse for serious infections |
| 4th Gen Cephalosporin (4GC) | Cefepime | + Gram positive coverage back | Broader than 3GC |
| Anti-MRSA Cephalosporin | Ceftaroline | MRSA coverage | Unique among cephalosporins |
| BLBLI (Beta-lactam/Beta-lactamase inhibitor) | Amoxicillin-clavulanate*, Piperacillin-tazobactam*, Ceftazidime-avibactam* | Reverse resistance by some β-lactamases | * = good anti-anaerobic activity |
| Carbapenems | Ertapenem, Imipenem, Meropenem | Broadest Gram +ve/−ve spectrum | Reserve for severe/resistant infections |
| Fluoroquinolones | Levofloxacin, Ciprofloxacin, Moxifloxacin* | Broad Gram +ve/−ve; Moxifloxacin better for Gram +ve | * = anti-anaerobic; Cipro best anti-Pseudomonal FQ |
| Aminoglycosides | Gentamicin, Amikacin | Gram negative; used in combination/synergy | Nephrotoxic, ototoxic; monitor levels |
| Oxazolidinone | Linezolid | Resistant Gram +ve (MRSA, VRE) | Oral bioavailability excellent |
| Tetracyclines | Doxycycline, Minocycline | Broad; atypicals, STIs | |
| Glycylcyclines | Tigecycline* | Broad-spectrum including anaerobes | IV only; NOT for bloodstream infections |
| Macrolides | Erythromycin, Clarithromycin, Azithromycin | Atypicals, Gram +ve | Mycoplasma Tx of choice [5] |
| Lincosamides | Clindamycin* | Gram +ve, anaerobes | Risk of C. diff colitis |
| Glycopeptides | Vancomycin, Teicoplanin | Gram +ve only (including MRSA) | IV vancomycin for MRSA; PO vancomycin for C. diff |
| Others | Chloramphenicol, Fosfomycin, Nitrofurantoin, Metronidazole* | Nitrofurantoin/Fosfomycin = UTI only; Metronidazole = anaerobes |
Key footnotes from the lecture:
- * = good anti-anaerobic activity (Penicillin G for strep doesn't have this; but amox-clav, pip-tazo, cefoxitin, carbapenems, metronidazole, clindamycin, moxifloxacin, tigecycline DO)
- Beta-lactamase inhibitors include avibactam, clavulanate, sulbactam, tazobactam
High Yield: Antibiotics with Anti-Anaerobic Activity
Remember the asterisk (*) drugs from the lecture slide — they have good anti-anaerobic activity. This is commonly tested. Key ones: amoxicillin-clavulanate, piperacillin-tazobactam, carbapenems, metronidazole, clindamycin, moxifloxacin, cefoxitin, tigecycline. [1]
The beta-lactams form the backbone of antibiotic therapy. Understanding their spectrum progression is essential:
Penicillin G (strep only)
→ Cloxacillin (adds staph)
→ Ampicillin/Amoxicillin (adds some Gram −ve)
→ Amox-clav (adds β-lactamase producers + anaerobes)
→ Cefuroxime (2GC: broader Gram −ve)
→ Ceftriaxone (3GC: much broader Gram −ve)
→ Ceftazidime (3GC + non-fermenters like Pseudomonas)
→ Cefepime (4GC: Gram +ve back + Gram −ve)
→ Pip-Tazo (broadest BLBLI, anti-Pseudomonal + anaerobes)
→ Carbapenems (broadest of all β-lactams)Infection in Outpatient Settings
The majority of outpatient infections are VIRAL and managed WITHOUT antibiotics. [1]
Ask yourself these questions before prescribing empirically:
- Is there a good reason (justification) to use antibiotic empirically?
- Is the patient very ill? Are the signs/symptoms getting worse?
- Is the patient very young, very old, or immunocompromised?
- Always explain the follow-up actions to the patient. [1]
| Drug | Typical Use |
|---|---|
| Penicillin V, Ampicillin/Amoxicillin, Cloxacillin | GAS pharyngitis (Pen V), SSTI (cloxacillin) |
| Amoxicillin-clavulanate | Bite wounds, sinusitis, AOM when antibiotics indicated |
| Cephalexin, Cefuroxime, Cefpodoxime | SSTI, UTI, step-down therapy |
| Macrolides | Atypical pneumonia, penicillin-allergic pharyngitis |
| Tetracyclines (Doxycycline) | Atypical pneumonia, STIs, skin infections (acne) |
| Fluoroquinolones (Cipro, Levofloxacin) | Complicated UTI, some respiratory (reserve use) |
| Nitrofurantoin | Uncomplicated cystitis ONLY |
The lecture provides an algorithm dividing inpatient infections into LOW RISK and HIGH RISK categories. [1]
Factors Determining Risk Stratification
| Factor | What to Assess |
|---|---|
| Age | Extremes of age = higher risk |
| Underlying disease | Diabetes, CKD, liver disease, malignancy, immunosuppression |
| Infection source | Lungs, abdomen, bloodstream, CNS — each has different severity implications |
| Severity scores | APACHE, qSOFA, etc. |
| VIP (Very Important Pathogens) | MRSA, ESBL, Pseudomonas — has the patient been colonized? |
| Antibiotic history | Recent broad-spectrum antibiotics → higher risk of resistant organisms |
| Recent culture results | Previous isolates guide current empirical therapy |
High Yield: The VIP Concept
VIP = Very Important Pathogens. When stratifying risk, always consider whether the patient has risk factors for resistant organisms (MRSA, ESBL-producing Enterobacteriaceae, Pseudomonas). Recent hospitalization, prior antibiotics, known colonization, and ICU stay are key risk factors. [1]
Key pathogens: Staphylococcus aureus (MSSA) and Streptococcus pyogenes (Group A Streptococcus) [1]
Key Principles
- Pus collection needs DRAINAGE — antibiotics alone are insufficient for abscesses
- Empirical options:
- Ampicillin + Cloxacillin
- Amoxicillin-clavulanate [1]
Why these choices?
- Cloxacillin specifically targets MSSA (penicillinase-resistant penicillin)
- Ampicillin covers streptococci well
- Amox-clav covers both plus anaerobes (useful for bite wounds, mixed infections)
Source Control Principle
A common exam mistake: treating an abscess with antibiotics alone without drainage. The lecture explicitly states "Pus collection – need drainage." Antibiotics cannot penetrate well into walled-off pus collections. Source control (I&D) is essential. [1]
Urinary Tract Infections (UTI)
Escherichia coli is the most important organism in UTI. [1]
| Feature | Uncomplicated UTI | Complicated UTI |
|---|---|---|
| Definition | Infection in structurally and neurologically normal urinary tract in otherwise healthy individuals | UTI associated with factors that compromise the urinary tract or host defense |
| Examples | Young woman with cystitis | Urinary obstruction, neurogenic bladder, renal transplant, pregnancy, stones, indwelling catheter, immunosuppression |
| Special categories | Infection in MEN, pregnant women, and children may be considered complicated |
High Yield: Who Gets Classified as 'Complicated'?
Men, pregnant women, and children with UTI are automatically considered to have complicated UTI even if the urinary tract is structurally normal. This is because UTI in these populations is unusual and warrants further investigation for underlying pathology. [1]
Uncomplicated cystitis is the most common UTI in outpatient settings. [1]
First-line treatment:
- Nitrofurantoin (the lecture specifically highlights this as the preferred agent)
Why nitrofurantoin?
- Concentrated in urine → high urinary levels, low systemic levels
- Low resistance rates among E. coli (even when resistance to other agents is high)
- Minimal impact on gut flora → less collateral damage
- Limitation: Only works for LOWER UTI (cystitis). It does NOT achieve adequate tissue/blood levels, so it is useless for pyelonephritis or systemic infections [1]
Request urine culture if:
- Recurrent UTI
- Complicated UTI
- Failure to respond
- Children
- Pregnant women
- Postmenopausal women [1]
Why? In straightforward uncomplicated cystitis in a young woman, you can treat empirically because the microbiology is predictable (E. coli, susceptible to nitrofurantoin). In the listed situations, the organism may be resistant, unusual, or there may be structural/host factors — so you NEED culture data to guide therapy.
The lecture shows that ~15% of outpatient urinary E. coli are resistant to common oral antibiotics, and resistance increases with age. [1]
This is why:
- Nitrofurantoin remains first-line — resistance rates remain low
- Amoxicillin-clavulanate and fluoroquinolones have higher resistance rates in HK
- Elderly patients (postmenopausal women, older men) have higher resistance rates due to prior antibiotic exposure and healthcare contact
Q17: Empirical treatment of uncomplicated cystitis in a 30-year-old woman → Answer: Nitrofurantoin (H) [3]
The lecture references the CDC STI treatment guidelines chart. [1]
This is covered in detail in other lectures, but the key exam-relevant point from THIS lecture is:
- Gonorrhea: Ceftriaxone IM (single dose)
- Chlamydia: Doxycycline (or azithromycin)
- Always treat both if one is suspected (co-infection is common)
Upper Respiratory Tract Infections (URTI) — The Antibiotic Stewardship Section
This is the most extensively covered section in the lecture and directly addresses Learning Objective 3: strategies to reduce unnecessary antibiotic treatment.
URTI (also called common cold) – viral infection of nasal mucosa, nasopharynx, oropharynx (tonsils), larynx, paranasal sinuses. [1]
Symptoms: Fever, systemic symptoms, running nose, nasal obstruction, cough, hoarseness, sore throat
Treatment is symptomatic. Antibiotic has NO role in URTI. [1]
Educate the patient about the natural course:
- Worse on initial 2–3 days, then improve
- Illness may be prolonged – 1+ week
- Yellow nasal discharge and phlegm is COMMON (from inflammation) [1]
Critical Exam Trap: Yellow Discharge ≠ Bacterial Infection
Students (and patients) commonly believe that yellow/green nasal discharge means bacterial infection requiring antibiotics. The lecture explicitly states that yellow nasal discharge and phlegm is COMMON and is due to INFLAMMATION, not bacterial superinfection. This is a classic exam trap. [1]
Viral infection, as an extension of URTI. [1] Main symptom: retrosternal discomfort and severe cough. Treatment: symptomatic. [1]
DDx for severe and persistent cough > 2 weeks:
- Asthma (airway hypersensitivity)
- Pertussis
- Tuberculosis [1]
Why this matters: Acute bronchitis is one of the most over-prescribed conditions. The lecture is explicit: it is viral, and antibiotics are NOT indicated. However, if cough persists > 2 weeks, you must think beyond simple viral illness.
Pneumonia is bacterial (or secondary to preceding viral infection) and DOES need antibiotics. [1]
Clinical features that distinguish pneumonia from viral bronchitis:
- Abnormal vital signs (tachycardia, tachypnea, persistent high fever)
- Asymmetrical lung sounds or signs of consolidation
- May need referral for a chest X-ray [1]
Why is this distinction critical? Because the whole point of this lecture section is: most respiratory infections are viral and don't need antibiotics, BUT pneumonia is the important bacterial exception. You must be able to clinically distinguish the two.
Causative agents:
- EBV (Epstein-Barr virus), Adenovirus → viral
- Streptococcus pyogenes (Group A Streptococcus) → bacterial [1]
May not be possible to distinguish viral from bacterial tonsillitis clinically. [1]
This is a key teaching point. The lecture presents the Centor/McIsaac criteria and rapid antigen testing as tools to help differentiate:
Modified Centor (McIsaac) Criteria for GAS Pharyngitis
| Criterion | Points |
|---|---|
| Tonsillar exudates | +1 |
| Tender anterior cervical lymphadenopathy | +1 |
| Fever (history or T > 38°C) | +1 |
| Absence of cough | +1 |
| Age 3–14 years | +1 |
| Age 15–44 years | 0 |
| Age ≥ 45 years | −1 |
Interpretation:
- Score 0–1: No testing, no antibiotics (viral overwhelmingly likely)
- Score 2–3: Perform rapid antigen test → treat if positive
- Score ≥ 4: High probability of GAS → can treat empirically or test and treat
Rapid antigen test: yields results in 5 minutes. [1]
Antibiotic of choice for GAS pharyngitis:
Why Penicillin V?
- GAS has NEVER developed resistance to penicillin
- Narrow spectrum → minimal collateral damage
- Cheap and effective
- 10-day course needed to eradicate pharyngeal carriage and prevent rheumatic fever
2025 Past Paper Q20: Treatment of group A streptococcal pharyngitis → Answer: Penicillin V (I) [3]
Why Treat GAS Pharyngitis At All?
The main reason to treat GAS pharyngitis with antibiotics is NOT primarily to shorten symptom duration (though it does reduce symptoms by ~1 day). The main reasons are: (1) Prevent acute rheumatic fever (ARF), (2) Prevent suppurative complications (peritonsillar abscess, retropharyngeal abscess), (3) Reduce transmission to close contacts. Post-streptococcal glomerulonephritis is NOT prevented by antibiotics.
Sinusitis is COMMON in viral URTI. [1] It will resolve with symptomatic treatment. [1] Symptoms may linger for weeks in patients with underlying allergic sinusitis. [1]
Only very few patients need antibiotics. [1] When antibiotics ARE indicated → Amoxicillin [1]
When to consider antibiotics for sinusitis:
- Symptoms persist > 10 days without improvement
- Severe symptoms (high fever ≥ 39°C, purulent nasal discharge, facial pain) lasting ≥ 3 consecutive days
- "Double worsening" — symptoms that were improving then suddenly worsen
Most sinusitis is viral. The sinuses are anatomically connected to the nasal passages, so any viral URTI causes mucosal inflammation in the sinuses. This is NOT bacterial sinusitis and does NOT need antibiotics.
Otitis Media
Episodes of otitis media should be classified as acute otitis media (AOM) or otitis media with effusion (OME). [1]
| Feature | AOM (Acute Otitis Media) | OME (Otitis Media with Effusion) |
|---|---|---|
| Pathology | Acute bacterial infection of middle ear | Fluid in middle ear WITHOUT acute infection |
| Antibiotics? | May be indicated (see criteria below) | Do NOT use antibiotics for initial treatment |
Antibiotics should be reserved for AOM with:
- Signs/symptoms of acute local illness (otorrhea, bulging or red TM with loss of light reflex, ear pain)
- Systemic illness (fever T > 38°C or malaise)
- If confirmed by myringotomy or tympanocentesis [1]
"AOM infection spontaneously resolves in 80%–90% of cases with most symptoms subsiding within 24 hours after presentation" [1]
Many experts emphasize a "watchful waiting approach." [1]
Why watchful waiting? Most AOM episodes are self-limiting. The NNT (number needed to treat) with antibiotics to benefit one child is approximately 20 — meaning you would need to treat 20 children with antibiotics for one to benefit. Meanwhile, all 20 are exposed to side effects.
Effusion is NOT an indication for re-treatment. [1]
| Time Since Diagnosis | % with Persistent MEE |
|---|---|
| 2 weeks | 70% |
| 1 month | 40% |
| 2 months | 20% |
| 3 months | 10% |
Antibiotics for OME may be indicated if it persists > 3 months. [1]
Common Exam Mistake: Treating Persistent Effusion with More Antibiotics
After treating AOM, the middle ear effusion takes WEEKS to MONTHS to resolve. At 2 weeks post-treatment, 70% of patients still have effusion — this is NORMAL and does NOT require re-treatment. Only consider antibiotics for OME persisting > 3 months. This table of MEE resolution is a high-yield exam fact. [1]
| Finding | Interpretation |
|---|---|
| Normal TM | Pearly grey, light reflex visible, translucent |
| AOM | Bulging, erythematous TM, loss of light reflex, pus/air-fluid level visible behind TM |
| OME | TM retracted or neutral, fluid level visible, but NOT bulging or erythematous |
This EMQ from the 2025 Fourth Summative directly tests this lecture [3]:
| Question | Scenario | Answer | Rationale |
|---|---|---|---|
| Q16 | Antibiotic prophylaxis for close contact of meningococcal meningitis | Ciprofloxacin (not listed → likely answer from options = none perfectly match, but if rifampicin listed → J. Rifampicin) | Post-exposure prophylaxis for meningococcal contacts [4] |
| Q17 | Empirical treatment of uncomplicated cystitis in 30-year-old woman | H. Nitrofurantoin | First-line for uncomplicated cystitis; concentrated in urine; low resistance [1] |
| Q18 | Surgical antibiotic prophylaxis for total hip replacement | B. Cefazolin | Standard surgical prophylaxis = 1GC; good anti-Staph/Strep activity; long half-life [4] |
| Q19 | Treatment of cat bite wound infection | A. Amoxicillin-clavulanic acid | Bite wounds = polymicrobial (Pasteurella, anaerobes, strep, staph); amox-clav covers all [4] |
| Q20 | Treatment of GAS pharyngitis | I. Penicillin V | GAS never resistant to penicillin; narrow spectrum; gold standard [1] |
Integration with Related Lectures
Key principles from the prophylaxis lecture that connect here [4]:
- Surgical prophylaxis: Cefazolin (1GC) is standard; give within 30 min of incision; do NOT continue post-operatively
- Vancomycin for penicillin allergy or high MRSA incidence
- Cefuroxime + metronidazole for colorectal surgery (need anaerobic coverage)
- Bite wound prophylaxis: Amoxicillin-clavulanate
Key connections [6]:
- ESBL-producing Enterobacteriaceae → carbapenems are the drug of choice for severe infections
- MRSA → vancomycin, linezolid, or ceftaroline
- Intrinsic resistance examples:
- Salmonella: intrinsically resistant to cefuroxime (active in-vitro but NOT in-vivo)
- Morganella, Providencia, Proteus: intrinsically resistant to nitrofurantoin
- Listeria: intrinsically resistant to 3rd gen cephalosporins (must add ampicillin)
Q85 (2023): Mycoplasma pneumoniae detected → Which antibiotic class? → Answer: C. Macrolides [5]
Why? Mycoplasma lacks a cell wall → beta-lactams (which target cell wall synthesis) are useless. Macrolides, tetracyclines, and fluoroquinolones work against Mycoplasma because they target protein synthesis or DNA replication.
From the pediatric leukemia and hematology lectures [7]:
- Medical emergency → empirical broad-spectrum antibiotics ASAP (within 1–2 hours)
- Must cover Pseudomonas aeruginosa
- Typical regimen: Piperacillin-tazobactam or cefepime or meropenem
| Clinical Pearl | Explanation |
|---|---|
| "You do not know exactly what organism(s) you are treating" | Empirical therapy is probabilistic, not definitive |
| Always make a working diagnosis FIRST | The clinical syndrome determines the likely organisms |
| Yellow nasal discharge ≠ bacterial infection | Common misconception; caused by inflammation |
| Pus needs drainage | Antibiotics alone insufficient for abscesses |
| Most URTI is viral → no antibiotics | The single most important stewardship message |
| Nitrofurantoin for cystitis ONLY | Does not achieve tissue/blood levels; useless for pyelonephritis |
| Effusion after AOM is normal and resolves over months | Not an indication for retreatment |
| AOM spontaneously resolves in 80–90% | Watchful waiting is appropriate for most cases |
Likely Exam Questions
-
A 25-year-old woman presents with dysuria and frequency for 2 days. She is afebrile and has no loin pain. What is the most appropriate empirical treatment?
- Answer: Nitrofurantoin
-
A 4-year-old child has been treated for AOM. At the 2-week follow-up, there is still fluid behind the TM but no fever or ear pain. What is the most appropriate management?
- Answer: Reassurance and follow-up (effusion is normal at 2 weeks; present in 70% of cases)
-
Which of the following is NOT an indication for urine culture in a patient with UTI? (A) Recurrent UTI, (B) Uncomplicated cystitis in a young woman, (C) Failure to respond to empirical therapy, (D) Pregnancy
- Answer: B — uncomplicated cystitis can be treated empirically without culture
-
A patient with acute pharyngitis has a Centor score of 1. What is the appropriate management?
- Answer: Symptomatic treatment; no antibiotics; no rapid antigen test needed
-
List 4 factors that determine whether an inpatient infection is high-risk or low-risk for antibiotic selection.
- Age, underlying disease, infection source, severity (APACHE), VIP pathogens, antibiotic history, recent culture results
-
A patient presents with 3 days of rhinorrhea, cough, and low-grade fever. The nasal discharge is yellow-green. The patient requests antibiotics. How would you manage this patient?
- Clinical diagnosis: URTI (common cold). Symptomatic treatment only. Explain that yellow discharge is from inflammation, not bacterial infection. Educate about natural course (worse first 2–3 days, then improve; may last > 1 week). Arrange safety-net follow-up. No antibiotics indicated.
High Yield Summary
Key takeaways from GC 106 — Practical Issues in Antibiotic Use:
- Always make a working clinical diagnosis BEFORE choosing an antibiotic — the clinical syndrome determines the likely organisms.
- Empirical therapy = educated guess based on syndrome, local resistance data, and patient factors.
- Most outpatient infections (URTI, bronchitis, sinusitis) are VIRAL → no antibiotics. Yellow nasal discharge is inflammation, NOT an indication for antibiotics.
- Pneumonia is the key bacterial exception in respiratory infections → needs antibiotics.
- GAS pharyngitis → Penicillin V × 10 days. Use Centor criteria and/or rapid antigen test to guide decision.
- Uncomplicated cystitis → Nitrofurantoin (first-line; concentrated in urine; low resistance). Nitrofurantoin is useless for pyelonephritis.
- E. coli is the most important UTI organism. Request urine culture for: recurrent, complicated, failure to respond, children, pregnant, postmenopausal.
- SSTI → S. aureus (MSSA) + S. pyogenes. Treat with ampicillin + cloxacillin or amox-clav. Drain any pus collection.
- AOM resolves spontaneously in 80–90%. Reserve antibiotics for severe AOM. OME does NOT need antibiotics initially. Persistent MEE is normal after AOM (70% at 2 weeks → 10% at 3 months).
- Inpatient risk stratification considers age, comorbidities, infection source, severity, VIP pathogens, antibiotic history, and recent cultures.
- Know which antibiotics have anti-anaerobic activity (asterisk drugs: amox-clav, pip-tazo, carbapenems, metronidazole, clindamycin, moxifloxacin, cefoxitin, tigecycline).
- Surgical prophylaxis = Cefazolin (1GC); given within 30 min of first incision; do NOT continue post-op.
Active Recall - Practical Issues in Antibiotic Use
[1] Lecture slides: GC 106. Practical issues in antibiotic use.pdf (all pages) [2] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf [3] Past papers: 2025 Fourth Summative MCQ.pdf (Questions 16–20, p.35) [4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (Antibiotic prophylaxis section) [5] Past papers: 2023 Fourth Summative MCQ.pdf (Question 85, p.33) [6] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (Antimicrobial resistance section) [7] Senior notes: Adrian Lui Pediatrics Notes.pdf (Neutropenic fever, p.423)
GC105 Medically Important Microbes What Every Doctor Should Know
A foundational medical microbiology framework covering the essential bacteria, viruses, fungi, and parasites that clinicians must recognize for accurate diagnosis, appropriate antimicrobial therapy, and effective infection control.
GC106 Practical Issues In Antibotic Use
Practical issues in antibiotic use encompass clinical considerations such as appropriate drug selection, dosing, route of administration, duration of therapy, spectrum coverage, de-escalation strategies, and awareness of resistance patterns to optimize therapeutic efficacy and minimize adverse effects.