Rational Therapeutic Drug Monitoring

FOR MANY drugs, the measurement of concentrations in serum or plasma has become widely available and accepted as an important component of clinical decision making. While these drug levels often do allow more objective monitoring and titration of therapy, the information also has the potential to be valueless or even misleading. Laboratories sometimes report that a serum concentration is in the "toxic" range, when the patient is doing well and has no evidence of toxic effects. Or, conversely, the drug is not detectable in serum. Such discrepancies between measured serum drug concentrations and observed clinical drug effects may occur for numerous reasons. This article will review some principles and problems associated with therapeutic drug monitoring. RATIONALE FOR MONITORING SERUM DRUG LEVELS For a serum drug concentration to be potentially useful for purposes of therapeutic monitoring, at least two requisites must be fulfilled.¹First, the serum drug concentration must reflect