Efficacy of beta-lactam antibiotics: integration of pharmacokinetics and pharmacodynamics.

The study of pharmacokinetics teaches us how drugs are distributed and eliminated, whereas, pharmacodynamics looks at the relationship between drug concentration and drug activity. The free, nonprotein-bound fraction of the serum concentration can be used as a surrogate marker for the levels at the site of infection. It is tempting to use tissue levels for this purpose, but their use is fraught with difficulties. The single pharmacodynamic parameter that correlates best with therapeutic efficacy for beta-lactam antibiotics is time that free serum levels stay above the minimum inhibitory concentration (MIC). Recent clinical studies seem to confirm the value of time above MIC in predicting clinical and bacteriological outcome.