Which PK/PD parameter matters for beta-lactam antibiotics?

• A. Cmax/MIC
• B. AUC/MIC
• C. T>MIC
• D. Peak-to-trough ratio

Answer: (C)

Beta-lactam antibiotics are time-dependent killers, so their effectiveness hinges on how long the drug level stays above the pathogen’s MIC. The most important PK/PD driver is the fraction of the dosing interval during which the concentration exceeds the MIC (T>MIC). Keeping drug levels above MIC for a sufficient portion of each interval enhances bacterial kill and clinical outcomes, which is why dosing strategies often use frequent dosing or extended/continuous infusions to maximize T>MIC. In contrast, simply achieving a high peak concentration or a large total exposure (Cmax/MIC or AUC/MIC) doesn’t predict as well how well beta-lactams will perform, so these are less informative for guiding beta-lactam therapy.