Which PK/PD parameter best correlates with efficacy for beta-lactam antibiotics?

• A. AUC/MIC
• B. Cmax/MIC
• C. Time below MIC (T<MIC)
• D. Time above MIC (fT>MIC)

Answer: (D)

Beta-lactam antibiotics work best when their concentrations stay above the pathogen’s MIC for as long as possible within each dosing interval. This is because their killing is time-dependent rather than concentration-dependent, so elevating the peak level doesn’t add much to efficacy compared with keeping the drug above the MIC. The PK/PD parameter that best predicts success with beta-lactams is the time above MIC (often expressed as the fraction of the dosing interval where free drug concentration exceeds MIC, fT>MIC). Clinically, this means strategies that maximize the duration above MIC—such as using longer or continuous infusions or more frequent dosing—improve outcomes, especially for organisms with higher MICs. AUC/MIC and Cmax/MIC are more relevant for other antibiotic classes, not beta-lactams.