Clinical Pharmacology · MU Sofia

21. Principles of rational antimicrobial drug prescribing. Efficacy criteria. Algorithm for antibacterial drug choice.

Covers why antibiotic use is so often unnecessary or inappropriate, and the difference between empirical, targeted and prophylactic therapy. It then sets out the factors to weigh before choosing a drug, the efficacy criteria (spectrum, MIC, susceptibility categories, tissue penetration and PK/PD dosing indices), and a stepwise algorithm for selecting an antibiotic. A summary table of PK/PD killing patterns and drug classes is included.

Inside: 5 sections · 2 tables · 16 flashcards · 3 min

Topic 21 of 30 in the MU Sofia syllabus for Clinical Pharmacology (Medicine, Year 5). The free AceMed note covers it in 5 sections, about 312 words (3 min of reading), with 16 flashcards.

In this note

  1. Introduction
  2. Types of therapy
  3. Before choosing: three groups of factors
  4. Efficacy criteria
  5. Algorithm for choosing an antibiotic

Introduction

Up to half of all antibiotic use is unnecessary or inappropriate: antibiotics for viral infections, wrong drug, dose or duration, sales without prescription. This drives resistance, adverse reactions and cost. The minimum goal of antimicrobial therapy is clinical cure and prevention of complications at a reasonable cost; the maximum goal adds eradication of the pathogen.

Types of therapy

  • Empirical (≈ 75% of cases): started before the pathogen is known, based on the likely organisms at that site; in about half of cases no pathogen is ever found.
  • Targeted (aetiological): guided by culture and sensitivity (results take 36–48 h); switch to the narrowest effective drug (de-escalation).
  • Prophylactic: e.g. surgical prophylaxis.

Efficacy criteria

  1. Spectrum and potency: the pathogen must be susceptible; the MIC (minimal inhibitory concentration; MIC₉₀ inhibits 90% of strains) should be low. Susceptibility is reported as S (susceptible), I (susceptible, increased exposure) or R (resistant).
  2. Pharmacokinetics: the drug must reach the site of infection at concentrations above the MIC (e.g. CSF, bone, prostate, urine).
  3. PK/PD target, which decides the dosing regimen:
  1. Bactericidal (β-lactams, aminoglycosides, quinolones, glycopeptides, co-trimoxazole) is preferred in severe infections and in the immunocompromised; bacteriostatic (macrolides, tetracyclines, sulfonamides) relies on host defences.
  2. Duration: as short as effective (e.g. 3 days for uncomplicated cystitis in women with some agents, 5–7 days for most community-acquired pneumonia).

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