Therapeutic Drug Monitoring (TDM)

TDM is the use of drug measurements in body fluids as an aid to the management of patients receiving drug
therapy

What drugs require TDM?

For some drugs the desired effect can be assessed clinically eg the dose of an antihypertensive can be altered according to the patient's blood pressure.

For other drugs the dose can be assessed by a laboratory marker eg the dose of a statin is assessed by
measuring cholesterol, the dose of thyroxine according to the TSH, the dose of warfarin according to INR.

For most drugs there is a standard dose that will produce a satisfactory response in the majority of patients
eg penicillins but in a few there is poor correlation between dose and plasma concentration and so treatment
has to be individualised according to plasma concentrations of the drug eg phenytoin.

For some drugs the margin between desirable and toxic concentrations is small eg digoxin and hence
monitoring is required to achieve effective concentrations without toxicity.

So a drug is likely to require TDM if any one of the following -

Lack of good clinical markers of effect
There is a poor correlation between dose and effect
Narrow concentration interval between therapeutic and toxic effects
Good correlation between plasma concentration and effect

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