Drugs may have metabolites that may be active, toxic or present for some time. Measuring these metabolites may be indicated under certain circumstances.
A drug metabolite should be measured if both of the following are present:
(1) there is a reliable method to quantitate the metabolite
(2) one or more of the following:
(2a) the metabolite is toxic AND the patient is showing signs of toxicity
(2b) the metabolite is active AND the patient is not responding as expected
(2c) the metabolite has a longer half-life than the parent drug AND there is a need to document exposure
(2d) the metabolite is specific to the parent drug AND there is a need to document intake by the patient with metabolic conversion (rather than in vitro contamination)
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