A 55-year-old patient with urinary sepsis caused by Pseudomonas aeruginosa is on gentamicin. The most important PK/PD parameter predicting gentamicin efficacy against P. aeruginosa is:

• A Time above minimum inhibitory concentration (%T > MIC) because aminoglycosides are time-dependent killers
• B AUC/MIC ratio without concentration dependence, similar to vancomycin against MRSA
• C Peak/MIC ratio (Cmax/MIC) because aminoglycosides show concentration-dependent killing with a prolonged post-antibiotic effect ✓
• D Trough concentration maintenance above 2 mcg/mL to ensure continuous bactericidal activity

Explanation

Aminoglycosides demonstrate concentration-dependent bactericidal activity — the higher the peak concentration relative to the MIC (Cmax/MIC ratio), the greater and more rapid the bacterial killing. Additionally, they produce a prolonged post-antibiotic effect (PAE), meaning bacterial suppression continues even after drug concentrations fall below the MIC. This is the pharmacological basis for once-daily dosing (high peak, extended PAE, low trough). Time-dependent killing (%T > MIC) characterises beta-lactams; AUC/MIC (>400) characterises vancomycin's pharmacodynamic target.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.