Therapeutic Drug Monitoring - TDM

TDM is the use of drug measurements in body fluids (usually blood)  as an aid to the management of patients receiving drug  therapy. It is required for those drugs that -


Lack a good clinical marker of effect

Have a poor correlation between dose and effect

Have a narrow concentration interval between therapeutic and toxic effects

Have a good correlation between plasma concentration and effect


In primary care the drugs that are commonly measured as part of TDM are

Digoxin
Lithium
Theophylline
Phenytoin
Carbamazepine




Note that -

The timing of the sample in relation to the dose is critical in interpreting the result

Evidence of toxicity may occur within the target range

Toxicity may be made more likely to occur with some drugs of there is electrolyte disturbance, use of other drugs, intercurrent illness.

Some patients may benefit from levels below the target range and others above the target range.