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.
Phenytoin is ordered frequently when a person begins phenytoin treatment and when/if a person’s medications change (other drugs are started, stopped, or changed) or if there is a change in the status of their health. Once stable blood concentrations in the therapeutic range have been achieved, phenytoin is monitored at regular intervals to ensure that it remains in this range.
The test may be ordered when a person’s condition does not appear to be responding to phenytoin (e.g, he or she continues to have seizures) to determine whether the concentration is too low, the medication is ineffective, and/or to determine if the person is complying with therapy (taking the phenytoin regularly). The test may also be ordered when a person experiences side effects and/or exhibits symptoms that the health practitioner suspects may be due to 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%.
Within these ranges, most people will respond to the drug without symptoms of toxicity. Response and side effects will be individual, however. Some people will experience seizures at the low end of the therapeutic range and some people will experience excessive side effects at the upper end. People should work closely with their health practitioner to find the dosage and concentration that works the best for them.
- Sample of blood serum
- We perform the test daily