Clinical Pharmacy · Short notes

Therapeutics of Infectious Disease: Short Notes & Cheat Sheet

Antibiotic principles and stewardship, PK/PD dosing, and first-line treatment of common infections including XDR typhoid in Pakistan.

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Short notes

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Antibiotic stewardship in practice

Stewardship is about the right drug, dose, route and duration, not simply using fewer antibiotics.

Principles

Send cultures before starting antibiotics when possible, start empirical therapy based on the likely organism and local resistance, then narrow it when results arrive (de-escalation).
Time-dependent killers (β-lactams, vancomycin) work best when concentrations stay above the MIC for long periods, so dose frequently or infuse for longer.
Concentration-dependent killers (aminoglycosides, fluoroquinolones) work best with high peaks, so give once daily.
Review IV therapy at 48–72 hours and switch to oral when the patient is improving, can swallow and absorbs normally.
Surgical prophylaxis (usually cefazolin) is given within 60 minutes before incision and rarely continued beyond 24 hours.

Common infections

Community-acquired pneumonia: amoxicillin for mild cases; add a macrolide for atypical cover; co-amoxiclav or ceftriaxone plus a macrolide when severe.
Uncomplicated cystitis in women: nitrofurantoin or fosfomycin. Pyelonephritis: ceftriaxone or ciprofloxacin. Treat asymptomatic bacteriuria only in pregnancy.
Bacterial meningitis: ceftriaxone (plus vancomycin where resistant pneumococci occur; add ampicillin for Listeria in neonates, elderly and immunocompromised) and dexamethasone with the first dose.
Cellulitis: cloxacillin/flucloxacillin or cephalexin. MRSA: clindamycin, doxycycline, co-trimoxazole or IV vancomycin.
C. difficile: oral vancomycin or fidaxomicin.
Syphilis: benzathine penicillin 2.4 million units IM. Gonorrhoea: ceftriaxone IM. Chlamydia: doxycycline for 7 days.

Pakistan-relevant infections

Extensively drug-resistant (XDR) typhoid emerged in Sindh in 2016. It resists chloramphenicol, ampicillin, co-trimoxazole, fluoroquinolones and third-generation cephalosporins.
XDR typhoid is treated with oral azithromycin (uncomplicated) or IV meropenem (complicated). Typhoid conjugate vaccine is now part of routine immunisation.
Drug-sensitive TB: 2 months of HRZE then 4 months of HR. MDR-TB uses bedaquiline-based all-oral regimens.
Malaria: P. vivax — chloroquine then primaquine for radical cure (after G6PD testing). Uncomplicated P. falciparum — an artemisinin combination (ACT). Severe malaria — IV artesunate.
Dengue: supportive care and paracetamol; avoid aspirin and NSAIDs because of bleeding risk.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Infection → first choice

Infection	Usual first choice
Mild CAP	Amoxicillin
Atypical pneumonia	Clarithromycin / azithromycin / doxycycline
Uncomplicated cystitis	Nitrofurantoin, fosfomycin
Meningitis (empirical)	Ceftriaxone ± vancomycin/ampicillin + dexamethasone
XDR typhoid	Azithromycin; meropenem if complicated
Syphilis	Benzathine penicillin G
C. difficile	Oral vancomycin / fidaxomicin
Surgical prophylaxis	Cefazolin ≤ 60 min before incision
Uncomplicated falciparum	ACT (artemether–lumefantrine)
Severe malaria	IV artesunate

PK/PD targets

Antibiotic class	Best predictor
β-lactams	Time above MIC
Vancomycin	AUC/MIC
Aminoglycosides	Peak/MIC (Cmax/MIC)
Fluoroquinolones	AUC/MIC and Cmax/MIC

Memory aids

Start smart, then focus: Empirical broad therapy after cultures, then de-escalate to targeted therapy at 48–72 hours.

Exam traps

Ceftriaxone does not work in XDR typhoid, because the strain is resistant to third-generation cephalosporins.
Asymptomatic bacteriuria is treated in pregnancy but not in most other adults.
Aminoglycosides are given once daily because their killing is concentration-dependent.
Primaquine must not be given before checking G6PD status.

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