The phenytoin test is used to measure and monitor the amount of phenytoin in the blood and to determine whether drug concentrations are in the therapeutic range. It may be ordered every few days when a person first begins taking phenytoin to help adjust the dose to the desired blood level. The test is then ordered at regular intervals and as needed to monitor blood concentrations. One or more phenytoin tests may be ordered if a person starts or stops taking additional medications (to judge their effect, if any, on phenytoin levels) and may be ordered if the person has a seizure or if a health practitioner suspects toxicity.
The therapeutic ranges for adults taking phenytoin have been established at 10.0-20.0 mcg/mL for total phenytoin (bound plus unbound) and 1.0-2.0 mcg/mL for free phenytoin (unbound only). The range for percent free phenytoin is 8-14%.
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