Therapeutic drug monitoring (TDM) strategies are implemented on the assumption that clinical effects correlate better with drug concentrations in serum rather than with dose.
TDM is often employed when using drugs with a narrow therapeutic window, where the concentrations producing the desired beneficial effects are relatively close to the concentrations which produce toxic results.
Monitoring can also be beneficial in situations where drug toxicity is difficult to distinguish from underlying disease.
Therapeutic Drug Monitoring (TDM) is applicable for substances where is it assumed that metabolism will vary from patient to patient and that the plasma level of drug is more closely related to therapeutic effect or toxicity than the dose.
Pharmacokinetics can be complex and are dependent upon bioavailability, volume of distribution and distribution phase, clearance, half-life and protein binding. All of these can be affected by changes in health status, for example renal failure. Monitoring of therapeutic drugs is often applied to drugs with a narrow therapeutic window, where the concentrations producing therapeutic affect are not largely different from those causing toxicity. TDM can also be important where drug toxicity may be difficult to distinguish from the patients underlying disease.