Monitoring drug therapy

It is important to monitor drug therapy because the effects of a particular drug regimen may vary between individuals. Ideally, drug response is measured against the desired clinical outcome directly. However, this may not always be feasible, particularly for preventative therapies, in which case a surrogate endpoint may be substituted. For a few drugs, neither a clinical nor a surrogate endpoint can be used to guide therapy. In this case, if there is a predictable relationship between concentration and effect, it may be appropriate to measure the concentration of the drug in the blood. Drugs for which plasma concentration monitoring may be worthwhile include gentamicin, vancomycin, phenytoin, lithium, and theophylline. Measurement may be indicated when a suboptimal effect is suspected to be due to subtherapeutic concentrations; when it is difficult to distinguish drug toxicity from manifestations of the underlying disease; or when a change in the patient's condition means that measurement may guide dosage adjustment. Samples should generally be taken at steady state and their timing should always be recorded.