A patient is diagnosed with methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia. The microbiologist confirms vancomycin MIC of 1.5 mg/L (intermediate susceptibility). Which pharmacokinetic/pharmacodynamic (PK/PD) index is most predictive of vancomycin efficacy, and what target should be achieved?

• A Cmax/MIC ratio > 10; achieved by maximising peak concentration
• B AUC/MIC ratio of 400–600 mg·h/L; achieved by AUC-guided dosing ✓
• C Time above MIC > 50%; use continuous infusion
• D Trough concentration of 5–10 mg/L; maintain with fixed dosing

Explanation

Vancomycin exhibits concentration-independent (time-dependent) killing with an important post-antibiotic effect; its optimal PK/PD index is the AUC24/MIC ratio. Current ASHP/IDSA/SIDP guidelines (2020) recommend AUC24/MIC of 400–600 mg·h/L as the target, using Bayesian-guided AUC monitoring rather than trough-only monitoring, which was associated with nephrotoxicity without improving outcomes. An AUC/MIC > 600 is associated with nephrotoxicity, while < 400 predicts treatment failure. Cmax/MIC is the index for aminoglycosides.

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

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Written and medically reviewed by the StethoPrep medical team.