Therapeutic drug monitoring (TDM) is a branch of clinical chemistry and clinical pharmacology that specializes in the measurement of medication concentrations in blood. Its main focus is on drugs with a narrow therapeutic range, i.e. drugs that can easily be under- or overdosed. In other words, TDM refers to the individualization of drug dosage by maintaining plasma or blood drug concentrations within a targeted therapeutic range or window.
The basic assumptions underlying therapeutic drug monitoring are that drug metabolism varies from patient to patient and that the plasma level of a drug is more closely related to the drugs therapeutic effect or toxicity than is the dosage. By combining the knowledge of pharmaceutics, pharmacokinetics and pharmacodynamics, TDM enables the assessment of the efficacy and safety of a particular medication in a variety of clinical settings.