'Just a repeat' - When drug monitoring is indicated.

BACKGROUND: Therapeutic drug monitoring, the measurement of plasma or blood concentrations of a medication to assist the management of patients, is commonly performed by general practitioners and specialists alike. However, established therapeutic ranges are only available for a limited number of medications.

OBJECTIVE: This article outlines the basics of therapeutic drug monitoring,including the drugs for which monitoring is suitable and when, how and why it should be performed in general practice.

DISCUSSION: Therapeutic drug monitoring is generally only indicated when medications have specific characteristics (eg. a narrow therapeutic index), where there is an established therapeutic range, where the consequences of undertreatment cannot be recognised clinically and can be serious (eg. seizure) and/or if toxicity is suspected. Commonly used medications where therapeutic drug monitoring is indicated include some antiepileptic drugs (eg. phenytoin, carbamazepine), lithium and digoxin. For the majority of medications, therapeutic drug monitoring is unlikely to assist management and should not be performed.