PREVIEW
Measuring the serum concentration of pharmacologic agents may be helpful in confirming treatment efficacy and may help explain adverse events. However, timing of the last dose, concomitant drug use, serum half-life, preexisting conditions, and even recent physical activity can alter results. Errors in interpretation and in application of findings can amplify problems. The authors describe how to achieve optimal therapy with six commonly selected agents by practicing vigilance and educated integration of laboratory and clinical information.