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.
Practical Issues in Antibiotic Use
Lecture Map: Big Idea, Learning Objectives & Exam Relevance
This lecture, delivered by Dr. Ho Pak Leung (Department of Microbiology, HKU), is the clinical application counterpart to the earlier GC lectures on antibiotic prophylaxis (GC 098), antimicrobial resistance (GC 099), host defense (GC 100), and diagnosis of infections (GC 101). While those lectures built the science, this lecture is about the practical "how" of choosing and using antibiotics at the bedside — the decisions you'll face on Day 1 as an intern.
1. Recognize the considerations in choosing an antibiotic on an empirical basis. 2. Formulate antibiotic options for common infectious syndromes. 3. Describe strategies to reduce unnecessary antibiotic treatment for acute upper respiratory tract infections.
Every clinical rotation — Medicine, Surgery, Paediatrics, O&G, Family Medicine — will require you to prescribe antibiotics. The examiners test:
- Can you pick the right empirical antibiotic for a given clinical syndrome?
- Can you identify when antibiotics are NOT indicated (antibiotic stewardship)?
- Do you know the practical constraints — allergy, G6PD, pregnancy, renal function, resistance patterns?
Past papers frequently embed antibiotic choices into clinical scenarios (e.g., UTI, pneumonia, skin infection, sore throat, neutropenic fever) [2][3]. This lecture is the direct source for many of those stems.
Core Concepts and Mechanisms: Why Empirical Antibiotic Prescribing Matters
Every time you prescribe an antibiotic, you face two competing risks:
- Under-treatment: If you don't cover the likely pathogen, the patient may deteriorate or die (especially in sepsis, neutropenic fever, meningitis).
- Over-treatment: If you prescribe unnecessarily or use an overly broad agent, you drive antimicrobial resistance (AMR), expose the patient to adverse effects, increase cost, and disrupt normal flora (→ C. difficile, fungal superinfection).
Empirical therapy = choosing an antibiotic BEFORE culture results are available, based on the clinical syndrome, likely pathogens, local resistance patterns, and patient factors.
The art is in balancing coverage with stewardship. This lecture teaches you the framework for that balance.
- Culture and sensitivity results take 48–72 hours (at minimum).
- Many infections (e.g., community-acquired pneumonia, cellulitis) are treated entirely empirically — you may never identify the organism.
- The decision to start empirical antibiotics must be made at the bedside, immediately. You cannot wait.
- Once culture results return, you should de-escalate (narrow the spectrum) or escalate (broaden if resistant organism found). This is called "streamlining".
Considerations in Choosing Empirical Antibiotics
This is the core framework of the lecture. Think of it as a mental checklist every time you write an antibiotic prescription.
The clinical syndrome tells you the site of infection, which in turn predicts the likely pathogens.
| Clinical Syndrome | Likely Pathogens | First-Line Empirical Options |
|---|---|---|
| Community-acquired pneumonia (CAP) | S. pneumoniae, H. influenzae, atypicals (Mycoplasma, Chlamydophila, Legionella) | Amoxicillin ± macrolide, or respiratory fluoroquinolone |
| Hospital-acquired pneumonia (HAP) | Gram-negatives (Pseudomonas, Acinetobacter), MRSA | Piperacillin-tazobactam, or carbapenem ± vancomycin |
| Acute uncomplicated UTI | E. coli, Klebsiella, Proteus | Nitrofurantoin, fosfomycin, or TMP-SMX |
| Cellulitis | S. aureus, Group A Strep | Flucloxacillin (or cloxacillin IV) |
| Intra-abdominal infection | Enterobacteriaceae, anaerobes, Enterococcus | Amoxicillin-clavulanate, or cefuroxime + metronidazole |
| Acute pharyngitis (if bacterial) | Group A Streptococcus (GAS) | Penicillin V or amoxicillin |
| Neutropenic fever | Gram-negatives, Pseudomonas, Gram-positives | Piperacillin-tazobactam or meropenem |
| Meningitis | S. pneumoniae, N. meningitidis, Listeria (elderly/neonates) | Ceftriaxone ± ampicillin (for Listeria) |
High Yield — Syndromic Approach
The lecture emphasizes a syndromic approach to empirical antibiotic selection: identify the site → predict the pathogen → choose the antibiotic. This is the foundation of practical prescribing. [1]
These modify your antibiotic choice significantly:
| Patient Factor | Why It Matters | Example |
|---|---|---|
| Drug allergy | Penicillin allergy → avoid all penicillins; if anaphylaxis, avoid cephalosporins too (cross-reactivity ~1–2%) | Use macrolide or fluoroquinolone for CAP if penicillin-allergic |
| Renal function | Many antibiotics are renally cleared; dose adjustment needed in CKD/AKI | Aminoglycosides, vancomycin, nitrofurantoin (contraindicated if eGFR < 30–45) |
| Hepatic function | Some drugs are hepatotoxic or hepatically metabolized | Metronidazole, isoniazid, fluconazole — monitor LFTs |
| G6PD deficiency | Oxidant drugs → haemolytic anaemia | Nitrofurantoin is contraindicated in G6PD deficiency [1] |
| Pregnancy | Teratogenicity concerns | Avoid tetracyclines (teeth discoloration, bone growth), fluoroquinolones (cartilage), TMP (folate antagonism in 1st trimester), metronidazole (1st trimester), aminoglycosides (ototoxicity) |
| Age | Affects pathogen epidemiology and drug metabolism | Neonates: Listeria, GBS → ampicillin + gentamicin; Elderly: atypical presentations, renal dose adjustment |
| Immunosuppression | Broader range of pathogens including opportunistic infections | Neutropenic fever → broad-spectrum cover including Pseudomonas |
| Prior antibiotic exposure | Selects for resistant organisms | Recent amoxicillin → consider amoxicillin-clavulanate or different class |
MCQ 5 from the lecture notes: Nitrofurantoin is an oxidant drug contraindicated in G6PD deficiency because it can cause haemolytic anaemia. [1]
Exam Trap — Nitrofurantoin
Students often forget that nitrofurantoin has TWO important contraindications: (1) G6PD deficiency (haemolysis) and (2) significant renal impairment (eGFR < 30–45 mL/min) because it requires adequate renal excretion to achieve therapeutic concentrations in the urine and also accumulates causing peripheral neuropathy and pulmonary fibrosis. It also should not be used for pyelonephritis — it only concentrates in the lower urinary tract.
- The lecture directs students to the IMPACT (Interhospital Multi-disciplinary Programme on Antimicrobial ChemoTherapy) data for Hong Kong resistance patterns [1].
- You must know which organisms are commonly resistant in your local setting. For example:
- MRSA prevalence in HK hospitals → affects choice for skin/soft tissue infection, line infections
- ESBL-producing E. coli/Klebsiella → amoxicillin-clavulanate may fail; may need carbapenem
- Fluoroquinolone-resistant E. coli → no longer first-line for UTI in many settings
| Concept | Explanation | Clinical Relevance |
|---|---|---|
| Concentration-dependent killing | Higher peak concentrations → better killing (e.g., aminoglycosides, fluoroquinolones) | Give aminoglycosides as once-daily high-dose ("extended interval") dosing |
| Time-dependent killing | Duration above MIC determines efficacy (e.g., beta-lactams, vancomycin) | Give beta-lactams more frequently (or as continuous infusion in ICU) |
| Tissue penetration | Some antibiotics don't reach certain sites well | CSF: ceftriaxone (good), aminoglycosides (poor); Bone: fluoroquinolones (good), vancomycin (moderate); Urine: nitrofurantoin (excellent in bladder only) |
| Oral bioavailability | Some antibiotics absorb well orally; others don't | Fluoroquinolones, metronidazole, linezolid have excellent oral bioavailability (~100%) → IV-to-oral switch candidates |
- IV for severe infections, sepsis, inability to take orally, poor oral bioavailability
- Oral for mild-moderate infections, step-down from IV when improving
- IV-to-oral switch: Important stewardship intervention. Switch when:
- Afebrile for 24–48 hours
- Clinically improving
- Able to tolerate oral intake
- No ongoing indication for IV (e.g., endocarditis, meningitis, osteomyelitis)
Antibiotic Options for Common Infectious Syndromes
A. Acute Upper Respiratory Tract Infections (URTI) — When NOT to Prescribe
This is a major focus of the lecture and directly addresses Learning Objective 3.
The lecture emphasizes strategies to REDUCE unnecessary antibiotic treatment for acute URTIs. [1]
- Most URTIs are viral — common cold (rhinovirus, coronavirus, adenovirus, RSV, parainfluenza, influenza), acute bronchitis, pharyngitis (majority viral).
- Inappropriate antibiotic prescribing for URTI is the single biggest driver of community AMR.
- Patients expect antibiotics → doctors prescribe to satisfy demand → resistance emerges → everyone loses.
| Feature | Details |
|---|---|
| Aetiology | Predominantly viral (rhinovirus most common) |
| Antibiotics indicated? | NO for common cold. NO for acute rhinosinusitis unless symptoms persist > 10 days, or worsen after initial improvement ("double-sickening"), or severe symptoms (high fever ≥ 39°C + purulent nasal discharge ≥ 3 consecutive days) |
| Management | Symptomatic: analgesics, decongestants, saline nasal irrigation |
| Feature | Details |
|---|---|
| Aetiology | Almost always viral |
| Antibiotics indicated? | NO. Antibiotics do NOT shorten duration or reduce severity. Even if sputum is purulent — purulent sputum ≠ bacterial infection in bronchitis |
| Management | Symptomatic: cough suppressants, bronchodilators if wheezing |
Classic Exam Trap — Purulent Sputum
A very common mistake: equating purulent (yellow/green) sputum with bacterial infection requiring antibiotics. In acute bronchitis, purulent sputum is caused by neutrophil-derived enzymes (myeloperoxidase) and does NOT indicate bacterial aetiology. Do not prescribe antibiotics just because sputum is yellow or green.
Acute Pharyngitis / Tonsillitis
This is heavily tested. The lecture includes MCQs on this topic [1].
Key question: Is this Group A Streptococcal (GAS) pharyngitis, or viral?
The Centor score estimates the probability that pharyngitis is streptococcal. 1 point each for: (1) Fever > 38°C, (2) Tonsillar exudate, (3) Absence of cough, (4) Anterior cervical lymphadenopathy, (5) Age 3–14 years (+1 point); Age 15–44 (0 points); Age ≥ 45 (−1 point). [1]
| Centor Score | Probability of GAS | Action |
|---|---|---|
| 0–1 | 1–10% | No antibiotic, no throat culture |
| 2–3 | 11–35% | Consider rapid antigen test or throat culture; treat if positive |
| ≥ 4 | ~50% | Empirical antibiotic treatment (or test-and-treat) |
MCQ 2 from the lecture: A 25-year-old man with fever (38°C), sore throat (no exudate), cough and running nose → Score = 1 (only fever scores). Presence of cough SUBTRACTS from the probability of GAS (i.e., cough suggests viral). Answer: 1 (1–10% probability). [1]
- Prevent acute rheumatic fever (ARF) — the main reason. ARF can cause permanent valvular heart disease.
- Prevent peritonsillar abscess (quinsy)
- Reduce duration of symptoms (by ~1 day)
- Reduce transmission
- Note: Antibiotics do NOT prevent post-streptococcal glomerulonephritis
- First line: Penicillin V (oral) for 10 days, or Amoxicillin for 10 days
- Penicillin-allergic: Azithromycin or clarithromycin (macrolides), or cephalexin (if not anaphylaxis-type allergy)
- GAS remains universally susceptible to penicillin — no resistance has ever been documented
MCQ 1: Epstein-Barr Virus (EBV) is a well-known cause of acute tonsillitis/pharyngitis (infectious mononucleosis). Cervical lymphadenopathy is common. Influenza does NOT cause purulent tonsillitis. Rhinovirus causes common cold. [1]
Clinical Pearl — EBV and Amoxicillin
If you give amoxicillin to a patient with EBV infectious mononucleosis, they may develop a characteristic widespread maculopapular rash. This is NOT a true penicillin allergy — it is an immune-mediated reaction specific to EBV infection. However, it is a classic trap in exams and a reason to consider EBV before prescribing amoxicillin for pharyngitis.
MCQ 3: Characteristic signs of Scarlet fever include: (1) Strawberry tongue — swollen, bumpy, bright red tongue resembling a strawberry; (2) Fine pink-red rash with a sandpaper feel; (3) Pastia's lines — bright red coloration of flexor creases (axilla, groin). Fever > 39°C and cervical lymphadenopathy are common but nonspecific. [1]
Mycoplasma pneumoniae
MCQ 4: Mycoplasma pneumoniae has the highest incidence in school-age children (2–11 years). In a local HK study: 7% in age 0–1y, 33% in age 2–11y, 30% in age 12–17y, 4% in age 18–54y, 1% in ≥65y. However, Streptococcus pneumoniae remains the most important cause of bacterial pneumonia in persons of ALL ages. [1]
| Pathogen | Key Features | Antibiotic |
|---|---|---|
| S. pneumoniae | Most common bacterial cause across ALL ages; lobar consolidation | Amoxicillin (mild), IV amoxicillin-clavulanate or ceftriaxone (moderate-severe) |
| Mycoplasma pneumoniae | School-age children peak; atypical pneumonia; dry cough; bilateral patchy infiltrates; extrapulmonary manifestations (rash, haemolytic anaemia, encephalitis) | Macrolide (azithromycin, clarithromycin) or doxycycline |
| H. influenzae | COPD patients, smokers | Amoxicillin-clavulanate |
| Legionella pneumophila | Travel, air-conditioning, severe pneumonia, hyponatraemia, diarrhoea | Fluoroquinolone or macrolide |
| Staphylococcus aureus | Post-influenza pneumonia, necrotizing | Flucloxacillin (MSSA) or vancomycin (MRSA) |
C. Urinary Tract Infection (UTI)
The lecture references the chapter on Nitrofurantoin, Fosfomycin, and Methenamine from Mandell's textbook [1].
| Agent | Key Points |
|---|---|
| Nitrofurantoin | First-line for uncomplicated cystitis. Concentrates in urine (not systemic). Contraindicated in G6PD deficiency (haemolysis), eGFR < 30–45 (ineffective + toxicity), pregnancy at term (neonatal haemolysis). Covers most E. coli, Enterococcus. Does NOT cover Proteus or Pseudomonas. |
| Fosfomycin | Single-dose oral treatment (3g sachet). Convenient, good compliance. Broad Gram-negative coverage. |
| Trimethoprim-sulfamethoxazole (TMP-SMX) | Only if local resistance < 20%. Avoid in 1st trimester (folate antagonism). |
| Amoxicillin-clavulanate | Second-line. Broader spectrum. |
- Oral fluoroquinolone (ciprofloxacin or levofloxacin) if mild-moderate
- IV ceftriaxone or IV amoxicillin-clavulanate if severe/vomiting
- Nitrofurantoin is NOT adequate — it only concentrates in the bladder, not in renal parenchyma
From related GC slide content (GC 071 "It is red and painful") [4]:
| Infection | Pathogen | Antibiotic |
|---|---|---|
| Impetigo | S. aureus, GAS | Topical mupirocin (mild), oral flucloxacillin (moderate) |
| Cellulitis | GAS, S. aureus | Oral flucloxacillin; IV cloxacillin if severe |
| Erysipelas | GAS | Penicillin |
| Necrotizing fasciitis | Polymicrobial or GAS | Surgical emergency + broad-spectrum IV (piperacillin-tazobactam + clindamycin) |
| MRSA skin infection | CA-MRSA | TMP-SMX, doxycycline, or clindamycin (community); vancomycin IV (hospital) |
From GC 098 (Antibiotic Prophylaxis) and CFB WCS27 (Surgical Infection) [5][6]:
- Surgical prophylaxis: Single dose of IV antibiotic given within 60 minutes before skin incision (30 min for vancomycin/fluoroquinolones — they need longer infusion)
- Choice depends on the site: Cefazolin for most clean procedures; cefuroxime + metronidazole for colorectal/abdominal
- Duration: Usually single dose; re-dose if surgery > 2 half-lives of antibiotic or significant blood loss; stop within 24 hours post-op
- Prophylaxis ≠ treatment: If infection is already present, you are treating, not prophylaxing
Strategies to Reduce Unnecessary Antibiotic Prescribing (Antibiotic Stewardship)
This is the third learning objective and a favourite exam topic [1].
-
Delayed/watchful prescribing: Give the patient a prescription but advise them to fill it only if symptoms worsen or don't improve in 48–72 hours. Works well for otitis media, mild sinusitis.
-
Use clinical prediction rules: The Centor/McIsaac score for pharyngitis. CURB-65 for pneumonia severity (guides whether outpatient oral or hospital IV antibiotics are needed).
-
Patient education: Explain that most URTIs are viral, self-limiting, and antibiotics won't help. Reassure about expected duration (common cold: 7–10 days; cough can last 2–3 weeks). Use phrases like "This is a viral infection. Antibiotics won't make you better faster and could cause side effects."
-
Point-of-care testing: Rapid strep test, CRP testing, procalcitonin — help distinguish viral from bacterial and guide antibiotic decisions.
The CDC materials referenced in the lecture emphasize evidence-based guidelines for when antibiotics are appropriate for: acute rhinosinusitis, acute uncomplicated bronchitis, common cold, and pharyngitis — in both adults and children. [1]
| Condition | Antibiotics Needed? |
|---|---|
| Common cold | No |
| Acute sinusitis | Only if persistent (> 10 days), severe, or worsening |
| Acute otitis media (AOM) | Depends on age and severity: < 6 months → always treat; 6 months–2 years → treat if bilateral or severe; > 2 years → observation option for mild unilateral |
| Pharyngitis | Only if GAS confirmed or strongly suspected (high Centor score) |
Drug-Specific Practical Points
| Property | Detail |
|---|---|
| Mechanism | Reduced by bacterial flavoproteins to reactive intermediates that damage DNA, ribosomes, and other macromolecules. Multiple mechanisms → low resistance development |
| Spectrum | Most E. coli, Enterococcus (including VRE in some cases), S. saprophyticus, Klebsiella (variable). NOT active against Proteus, Pseudomonas, Serratia |
| Pharmacokinetics | Rapidly absorbed orally; almost completely excreted in urine → therapeutic levels only in urine; NO systemic tissue levels → cannot treat systemic infections or pyelonephritis |
| Contraindications | G6PD deficiency (oxidant → haemolysis); CKD (eGFR < 30–45) (ineffective + neurotoxicity); Late pregnancy/term (neonatal haemolysis); Neonates < 1 month |
| Adverse effects | GI upset (take with food), peripheral neuropathy (long-term), pulmonary fibrosis (chronic use), hepatotoxicity (rare) |
| Formulations | Macrocrystalline form (Macrodantin) → fewer GI side effects than microcrystalline |
| Antibiotic | Key "Must-Know" Points |
|---|---|
| Penicillin | GAS remains 100% susceptible; drug of choice for GAS pharyngitis, rheumatic fever prophylaxis |
| Amoxicillin | Better oral absorption than ampicillin; use for CAP (mild), GAS pharyngitis, H. pylori triple therapy |
| Amoxicillin-clavulanate | Adds beta-lactamase inhibitor → covers beta-lactamase-producing H. influenzae, E. coli, anaerobes |
| Flucloxacillin/Cloxacillin | Anti-staphylococcal penicillin; drug of choice for MSSA infections |
| Cefazolin | 1st-gen cephalosporin; surgical prophylaxis; good Gram-positive, some Gram-negative |
| Ceftriaxone | 3rd-gen cephalosporin; excellent CSF penetration; meningitis, severe CAP, pyelonephritis |
| Macrolides (azithromycin, clarithromycin) | Cover atypicals (Mycoplasma, Legionella, Chlamydophila); alternative for penicillin allergy |
| Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) | Broad-spectrum; excellent bioavailability; BUT resistance rising, tendon damage, QT prolongation, C. diff risk; reserve for specific indications |
| Metronidazole | Anaerobic coverage; C. difficile (oral); intra-abdominal infections; almost 100% oral bioavailability |
| Vancomycin | MRSA, C. difficile (oral vancomycin); requires TDM; nephro/ototoxic |
| Carbapenems (meropenem, imipenem, ertapenem) | "Last resort" for ESBL/MDR Gram-negatives; preserve for severe infections |
| Aminoglycosides (gentamicin, amikacin) | Gram-negative synergy; once-daily dosing; nephro/ototoxic; requires TDM; non-oliguric AKI |
The lecture directs students to the IMPACT website for Hong Kong-specific resistance data.
| Resistant Organism | Key Facts for HK | Clinical Implication |
|---|---|---|
| MRSA | Most common MDR Gram-positive in hospital-acquired infections | Use vancomycin or linezolid; contact precautions |
| ESBL-producing Enterobacteriaceae | Rising in both hospital and community | Carbapenem for serious infections; nitrofurantoin may still work for uncomplicated UTI |
| Carbapenem-resistant Enterobacteriaceae (CRE) | Emerging threat — very limited treatment options | Colistin, ceftazidime-avibactam, meropenem-vaborbactam |
| VRE | Enterococcus resistant to vancomycin | Linezolid, daptomycin |
| Candida auris | Most common MDR yeast in hospital settings | Contact precautions; echinocandins |
| C. difficile | Most common cause of infectious diarrhoea in hospital (antibiotic-associated) | Stop offending antibiotic; oral vancomycin or fidaxomicin |
Past Paper Link
The 2024 Fourth Summative SAQ Q12 asked: (a) Most common MDR Gram-positive by contact → MRSA; (b) Most common MDR Gram-negative by contact → ESBL-producing Enterobacteriaceae or CRE; (c) Most common MDR yeast by contact → Candida auris; (d) Most common bacterial cause of infectious diarrhoea by contact → C. difficile. [3]
This frequently appears in exams and is directly relevant to antibiotic choice:
Triple therapy (if no penicillin allergy): PPI (e.g., omeprazole) + Amoxicillin + Clarithromycin for 14 days [2][8]
- If penicillin allergy: PPI + Clarithromycin + Metronidazole
- Falling efficacy due to clarithromycin resistance → now 14 days (was 7 days)
- Factors for eradication failure: patient compliance, prior antibiotic exposure, H. pylori antibiotic resistance, degree of acid suppression, regimen duration [8]
| High-Risk Patients | At-Risk Dental Procedures |
|---|---|
| Prosthetic valve (including transcatheter) | Manipulation of gingival/periapical region |
| Previous infective endocarditis | Perforation of oral mucosa |
| Cyanotic congenital heart disease | |
| Congenital heart disease repaired with prosthetic material (up to 6 months post-op, or lifelong if residual defect) |
Regimen [9]:
- No penicillin allergy: Amoxicillin PO 1 hour before or Ampicillin IV ≤30 min before
- Penicillin allergy: Clindamycin PO 1 hour before or Azithromycin/Clarithromycin PO 1 hour before
Clinical Approach: Systematic Framework for Antibiotic Prescribing
- Drug allergies — type of reaction (rash vs. anaphylaxis); timing
- Previous antibiotic use — within last 3 months → risk of resistance
- Travel history — MDR organisms, tropical infections
- Recent hospitalization — hospital-acquired pathogens
- Immunosuppression — HIV, chemotherapy, biologics, transplant
- G6PD status — especially in HK (high prevalence in Southern Chinese)
- Pregnancy/breastfeeding
- Baseline renal and hepatic function
- Indwelling devices — catheters, prosthetic valves, joints → biofilm infections
- Blood cultures (at least 2 sets from different sites) — for any patient with fever + suspected bacteraemia/sepsis
- Urine culture — for UTI
- Sputum culture — for pneumonia (if productive cough)
- Wound swab — for skin/soft tissue infections
- CSF — for meningitis (after CT if indicated)
- Other site-specific — ascitic fluid (SBP), joint aspirate (septic arthritis), peritoneal fluid
Golden Rule
Always try to obtain cultures BEFORE starting antibiotics. However, NEVER delay antibiotics in a critically ill/septic patient for the sake of cultures. Start empirical therapy immediately and send cultures as soon as practical.
Special Populations
- Definition: Neutrophils < 0.5 × 10⁹/L + single temperature ≥ 38.3°C or sustained ≥ 38.0°C for 1 hour
- Medical emergency: Start empirical broad-spectrum antibiotics within 1 hour
- First-line: Piperacillin-tazobactam or meropenem (must cover Pseudomonas)
- Add vancomycin if: catheter-related, skin/soft tissue, MRSA risk, hemodynamic instability
- Add antifungal if: persistent fever > 4–7 days despite antibiotics [10]
Mycoplasma pneumoniae peaks in school-age children (2–11 years) — this is a local HK finding. [1]
- Acute otitis media: Observation option in children > 2 years with mild unilateral disease; if treating → amoxicillin first-line
- Neonatal sepsis: Ampicillin + gentamicin (covers GBS, E. coli, Listeria)
- Dose calculation: Always weight-based (mg/kg) in children
Exam Intelligence
-
"A 25-year-old with sore throat, cough, runny nose, fever 38°C — calculate the Centor score and advise on management." → Score = 1 → No antibiotics.
-
"A 60-year-old with COPD presents with productive cough, fever, CXR shows consolidation. Name the most likely organism and empirical antibiotic." → S. pneumoniae (consider H. influenzae); amoxicillin-clavulanate or ceftriaxone + macrolide.
-
"A young woman with uncomplicated cystitis and G6PD deficiency. Which commonly used UTI antibiotic is contraindicated?" → Nitrofurantoin.
-
"Name the components of H. pylori triple therapy." → PPI + amoxicillin + clarithromycin × 14 days.
-
"A 7-year-old with community-acquired pneumonia. What organism has the highest incidence in this age group in Hong Kong?" → Mycoplasma pneumoniae.
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"When are antibiotics indicated for acute rhinosinusitis?" → Persistent > 10 days, double-sickening, severe (high fever + purulent discharge ≥ 3 days).
-
"Name 3 characteristic features of Scarlet fever." → Strawberry tongue, sandpaper rash, Pastia's lines.
-
"What are the indications for IE prophylaxis before dental procedures?" → Prosthetic valve, previous IE, cyanotic CHD, repaired CHD with prosthetic material.
| Trap | Correct Approach |
|---|---|
| Prescribing antibiotics for green sputum in acute bronchitis | No antibiotics — sputum colour ≠ bacterial infection |
| Using nitrofurantoin for pyelonephritis | Wrong — only concentrates in bladder, not renal parenchyma |
| Treating all sore throats with antibiotics | Use Centor score; most are viral |
| Giving amoxicillin for pharyngitis without considering EBV | If EBV suspected (lymphadenopathy, splenomegaly, atypical lymphocytes) → avoid amoxicillin (rash) |
| Forgetting to check G6PD before nitrofurantoin | Essential in HK population (high G6PD prevalence in Southern Chinese) |
| Using fluoroquinolones as first-line for uncomplicated UTI | Reserve for pyelonephritis; too broad for simple cystitis |
| Confusing prophylaxis with treatment | Prophylaxis = prevention before contamination; treatment = existing infection |
| Related Lecture | Connection |
|---|---|
| GC 098: Antibiotic Prophylaxis | Surgical prophylaxis principles; IE prophylaxis; timing, duration, drug choice |
| GC 099: Antimicrobial Resistance | MRSA, ESBL, CRE, VRE — understand why empirical choices matter for stewardship |
| GC 100: Defense Against Microbes | Host immunity determines who gets infected and how severely — immunocompromised hosts need broader coverage |
| GC 101: Diagnosis of Infections | Proper specimen collection BEFORE antibiotics; interpretation of culture and sensitivity |
| GC 104: Infection Outbreak/Infection Control | Contact precautions for MDR organisms; hand hygiene; isolation |
| GC 071: It is Red and Painful | Skin/soft tissue infection management — cellulitis, abscess, necrotizing fasciitis |
| GC 141/142: Paediatric Cough/Diarrhoea | Mycoplasma in children; viral vs. bacterial gastroenteritis |
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SAQ: "A 22-year-old woman presents with dysuria and urinary frequency. She has known G6PD deficiency. (a) What is the most likely diagnosis? (b) Name one commonly used antibiotic for this condition that is contraindicated in her case, and explain why."
- Markscheme: (a) Acute uncomplicated cystitis / lower UTI; (b) Nitrofurantoin — it is an oxidant drug that causes haemolytic anaemia in G6PD-deficient patients.
-
MCQ: "A 25-year-old man presents with 2-day history of sore throat, no exudate, cough, runny nose, and fever 38°C. His Centor score is:" → 1 (only fever scores; cough is present so absence-of-cough criterion is NOT met).
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SAQ: "List 3 characteristic physical signs of Scarlet fever." → Strawberry tongue, sandpaper-textured rash, Pastia's lines.
-
MCQ: "Which age group has the highest incidence of Mycoplasma pneumoniae infection in Hong Kong?" → School-age children (2–11 years).
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SAQ: "Name 3 situations in which antibiotics are indicated for acute rhinosinusitis." → Persistent symptoms > 10 days; double-sickening; severe symptoms (high fever ≥ 39°C + purulent nasal discharge ≥ 3 consecutive days).
High Yield Summary
Key Takeaways for the Exam:
-
Empirical antibiotic choice is based on: clinical syndrome → predicted pathogen → patient factors (allergy, renal function, G6PD, pregnancy) → local resistance patterns.
-
Most URTIs (common cold, acute bronchitis, most pharyngitis, most acute rhinosinusitis) are viral — DO NOT prescribe antibiotics.
-
Use the Centor/McIsaac score for pharyngitis: Score 0–1 = no antibiotics; 2–3 = test; ≥4 = treat. GAS pharyngitis → Penicillin V or amoxicillin × 10 days.
-
Nitrofurantoin: first-line for uncomplicated cystitis BUT contraindicated in G6PD deficiency (haemolysis) and renal impairment (eGFR < 30–45). NOT for pyelonephritis.
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Mycoplasma pneumoniae peaks in school-age children (2–11 years) in HK. S. pneumoniae is the most common bacterial pneumonia cause across ALL ages.
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Scarlet fever (GAS): Strawberry tongue + sandpaper rash + Pastia's lines.
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EBV causes pharyngitis/tonsillitis — avoid amoxicillin (rash risk).
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Always send cultures BEFORE antibiotics when possible, but NEVER delay treatment in sepsis.
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Antibiotic stewardship: delayed prescribing, clinical prediction rules (Centor), patient education, de-escalation at 48–72 hours.
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IE prophylaxis: amoxicillin PO 1 hour before dental procedures in high-risk patients (prosthetic valves, previous IE, cyanotic CHD).
Active Recall - Practical Issues in Antibiotic Use
[1] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf (Learning objectives, MCQs 1-5, reading material references) [2] Past papers: 2022 Fourth Summative Minicase.pdf (Q1 — H. pylori triple therapy components) [3] Past papers: 2024 Fourth Summative SAQ.pdf (Q12 — MDR organisms in hospital-acquired infections) [4] Lecture slides: GC 071. It is red and painful.pdf (Skin/soft tissue infection management) [5] Lecture slides: GC 098. Antibiotic prophylaxis.pdf (Surgical prophylaxis, IE prophylaxis) [6] Lecture slides: CFB WCS27_Surgical Infection.pdf (Surgical infection principles) [7] Lecture slides: GC 099. Antimicrobial resistance.pdf (Resistance mechanisms, IMPACT HK data) [8] Senior notes: Block A - Upper abdominal pain_ peptic ulcer; pancreatitis and gallstone.pdf (H. pylori eradication failure factors) [9] Senior notes: Adrian Lui Pediatrics Notes.pdf p241 (IE prophylaxis regimens, ESC guidelines) [10] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (Neutropenic fever management) [11] Lecture slides: GC 141. A child with cough acute and chronic cough in children.pdf (Paediatric respiratory infections)
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.
GC107 Protect Yourself And Your Patients
GC107 Protect Yourself And Your Patients is a General Medical Council good clinical care guidance topic emphasizing infection control, safe practices, and risk management to safeguard both healthcare professionals and patients from harm.