ABSTRACT
Therapeutic drug monitoring (TDM) refers to the assessment of tissue drug concentrations to optimize the dosage for patients receiving drug therapy for treatment (prevention, cure, or alleviation of symptoms) of disease. In practice, the blood concentration is measured as a surrogate for tissue levels. The rationale for its use is that for any particular therapeutic effect there is an optimal range of tissue concentration for the drug; below this range the drug is less effective, while above the range the drug might be toxic. TDM may also be used to monitor patient compliance to a particular therapy. Perhaps surprisingly, however, despite the large number of drugs that are prescribed daily, very few are measured in routine practice as part of a TDM program. There may be a number of reasons why this is so and unless knowledge of a drug concentration fits certain criteria, its routine measurement may offer little help in the management of a patient. Paradoxically, it is of interest to consider when TDM may not be of value (Table 21.1) even though an assay of drug concentration may be readily available.
