Monitoring drug therapy

It is important to monitor drug therapy because the effects of a particular drug regimen can vary significantly between individuals. Wherever possible, therapeutic effect should be monitored using a clinical endpoint (i.e. a measure that directly reflects how the patient feels, functions or survives). In practice, it is often not feasible to use a clinical endpoint to guide therapy, particularly for preventive treatments. The next best option is to use a surrogate endpoint: a measure that changes so as to predict whether the clinical endpoint will be achieved. For a few drugs, neither a clinical nor a surrogate endpoint is available. In these instances, if the drug has a narrow therapeutic index and there is a predictable relationship between its concentration and its effects, it may be appropriate to measure its concentration in the blood. This article discusses approaches to monitoring drug therapy using clinical and surrogate endpoints, and plasma concentration monitoring. Specific guidance is provided for plasma concentration monitoring of digoxin, gentamicin, vancomycin, phenytoin, lithium and theophylline.