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Therapeutic drug sampling


Information for Therapeutic drug sampling


Drug Recommended Sampling Time
PHENYTOIN Pre-dose preferred*
PHENOBARBITONE Pre-dose preferred*
LAMOTRAGINE Pre-dose preferred*
DIGOXIN At least 6hrs Post-dose
LITHIUM 12hrs Post-dose (as near to)

A steady state trough/pre-dose concentration (usually collected just before next dose) is the best and most reproducible parameter for monitoring as it’s the least variable point in the dosing interval.

Random tests may be requested if toxicity is suspected. Sampling at peak levels (timing varies depending on drug) or at the time of specific symptoms may detect such toxicity. To be effective, peak levels should be below toxic concentrations and pre-dose levels should remain in the therapeutic range.

*For drugs with long half-lives such as phenytoin and phenobarbitone, samples can be collected at any point in the dosage interval since fluctuations between peak and pre-dose concentrations at steady state are relatively small, however for consistency, pre-dose may be preferred

For meaningful interpretation it is important to know the length of time that has elapsed since the last dose, therefore time of last dose should be noted on the form by requestor, patient or phlebotomist.

When making comparative measurements, it is advisable that sampling time be consistent (ideally: pre-dose level).

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