Monitoring drug therapy

The effects of drugs vary significantly between individuals, and monitoring is essential to ensure the desired effect is achieved. Wherever possible, therapeutic effects should be monitored directly, using a measure that captures how the patient feels, functions or survives (a ‘clinical endpoint’). 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 predicts 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 the efficacy of drug therapy using clinical and surrogate endpoints and plasma concentration monitoring. Specific guidance is provided for plasma concentration monitoring of digoxin, gentamicin, vancomycin, phenytoin, lamotrigine, lithium and theophylline.