Therapeutic drug monitoring

Therapeutic drug monitoring of concentrations of drugs in body fluids, usually plasma, can be used
during treatment and for diagnostic purposes. The selection of drugs for therapeutic drug monitoring is important as the concentrations of many drugs are not clearly related to their effects. For selected drugs therapeutic drug monitoring aims to enhance drug efficacy, reduce toxicity or assist with diagnosis. Despite its apparent advantages, it has inherent limitations. Some large hospitals have services which provide support with drug monitoring and interpretation of results.
Drug concentration at the site of action cannot be routinely measured, but the desired or adverse effects may correlate better with plasma or blood concentrations than they do with dose.
Therapeutic drug monitoring involves not only measuring drug concentrations, but also the clinical interpretation of the result. This requires knowledge of the pharmacokinetics, sampling time, drug history and the patient's clinical condition.

Which drugs should be monitored

* having narrow therapeutic target range.
* significant pharmacokinetic variability.
* a reasonable relationship between plasma concentrations and clinical effects.
* established target concentration range.
* availability of cost-effective drug assay.

Drug assays are costly, so the reason for monitoring and the additional information to be gained (if any) should be carefully considered. For some drugs, therapeutic drug monitoring helps to increase efficacy (vancomycin), to decrease toxicity (paracetamol) and to assist diagnosis (salicylates). Routine monitoring is not advocated for most drugs. Only clinically meaningful tests should be performed.