This questions was recently addressed by Michael J. Postelnick, Senior Infectious Diseas Pharmacist of Northwestern Memorial Hospital in Chicago, Illinois in an Ask-the-Experts post on the Medscape website. Postelnick gives a good explanation of how various pharmacokinetic and pharmacodynamic principles are related to antibiotic choice for infectious pathogens. According to Postelnick “research into antimicrobial pharmacokinetics and pharmacodynamics has established surrogate relationships between the 2 that correlate with outcomes such [as] bacterial eradication or clinical cure. These relationships include the ratio of Cmax to the MIC, time above the MIC (defined as the amount of time during the dosing interval that the antimicrobial concentration in the blood or at the site of infection remains above the MIC of the organism), and the ratio of the AUC to the MIC. For concentration-dependent antimicrobial agents such as fluoroquinolones and aminoglycosides, Cmax/MIC or AUC/MIC most closely correlates with clinical and microbiological outcomes. For time-dependent antimicrobial agents such as beta-lactams, the percentage of time during the dosing interval that the drug concentration remains above the MIC of the organism is the measure that most closely predicts outcomes.” In other words, selection of an antibiotic requires more than the MIC. Knowledge of the organism in addition to pharmacokinetic and pharmacodynamic principles of the drug is necessary to make an informed choice.