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.
Usually, a total phenytoin test is used to monitor levels. In the blood, phenytoin is highly bound to serum proteins. Only the portion of phenytoin that is unbound or "free" is pharmacologically active. Under normal conditions, the balance between bound and unbound phenytoin in the blood is relatively stable, so measuring the total phenytoin (bound plus unbound) is appropriate for monitoring therapeutic levels. However, in certain conditions and disease states, that balance can be upset, the percentage of free or active phenytoin can increase, and the person may experience symptoms of toxicity even though their total phenytoin result falls within therapeutic range. In such cases, health practitioners may order a free phenytoin test to monitor the person's levels.
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. Toxic symptoms can affect the mouth, cardiovascular and nervous systems, and the eyes and include:
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.
In general, when phenytoin results are in the therapeutic range, the person is not having recurrent seizures and is not experiencing significant side effects, then the dosage of phenytoin is adequate. People should not increase, decrease, or stop taking their medication without consulting with their health care provider as it can increase their risk of having a seizure and may affect other medications that they are taking. Dosage determinations and adjustments must be evaluated on a case-by-case basis.
People who take phenytoin long-term may develop vitamin D deficiency, numbness in their extremities due to nerve damage (peripheral neuropathy), acne, excessive hair on their face or body, a thickening of facial features, or weakness of their bones. Rarely, a person may develop a severe rash and skin condition that requires hospitalization.
Women who use phenytoin during pregnancy are at an increased risk of having babies born with several types of birth defects. Women who want to become pregnant should talk to their health practitioner.
Phenytoin is sometimes prescribed for other conditions such as trigeminal neuralgia, which causes episodes of sharp pain along the jaw, and other causes of nerve pain. This use must also be monitored.
Phenytoin is not effective for every kind of seizure and will not work for every person.
A large variety of prescribed drugs, over-the-counter medications, and supplements can increase, decrease, or interfere with the concentration of phenytoin in the blood and its effects. Drugs whose effects are reduced by phenytoin include corticosteroids such as prednisone, estrogens, warfarin, and some antidepressants. Phenytoin also can reduce the effectiveness of oral contraceptives and other forms of birth control such as Depo-Provera or implants. Drugs that may increase phenytoin blood levels and toxicity include alcohol, diazepam, certain antidepressants, and omeprazole. The oral absorption of phenytoin can be reduced by antacids containing magnesium, calcium carbonate, calcium salts, or enteral feeding products. Many more drugs interact with phenytoin, so tell your health practitioner about any prescription or over-the-counter drugs, supplements, or herbal products you take.
This article was last reviewed on February 20, 2014. | This article was last modified on February 20, 2014.
The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.
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