Does this test have other names?
What is this test?
This test monitors the level of the seizure medicine phenytoin (Dilantin) in your
blood. Phenytoin is an anticonvulsant medicine given to control seizures.
Why do I need this test?
If you take phenytoin, your healthcare provider must monitor your blood to make sure
you are getting the correct dose. Too much can be toxic. Not enough leaves your seizures
You may have this test more often when you first start taking the medicine and then
regularly throughout treatment. You may need to repeat the test if the medicine doesn't
seem to control your seizures or if any of your other prescription medicines change.
What other tests might I have along with this test?
Your healthcare provider also might order other blood tests, including:
Complete blood count
Blood urea nitrogen and creatinine test to check kidney function
Liver function panel or other liver tests
Glucose test to measure your blood sugar, because phenytoin can cause your blood sugar
Blood tests to measure the sodium level in your blood
Tests for other medicine levels
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
Results are given in micrograms per milliliter (μ/mL). The normal therapeutic range
is 10 to 20 μ/mL. Your healthcare provider will figure out the best dose for you based
in part on how well your seizures are controlled and how you feel. High levels of
phenytoin in your blood can be toxic.
How is this test done?
This test requires a blood sample, which is drawn through a needle from a vein in
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Adjusting the dosage of any other medicine or taking a new medicine—prescription or
nonprescription—can affect the level of phenytoin in your blood.
Tell your healthcare provider if you are taking any of these medicines:
Amiodarone for an irregular heartbeat
Chlordiazepoxide or diazepam for anxiety
Dicumarol, a blood thinner
Disulfiram, to treat alcoholism
Estrogens for hormone replacement
Aspirin and medicines with salicylates
Sulfonamides to treat infections
Tolbutamide used for diabetes management
Famotidine for ulcers
Isoniazid, an antibiotic
Methylphenidate for attention deficit disorders
Phenothiazines for nausea
Trazodone for depression and sleep problems
Primidone or valproic acid for seizures
Fluconazole for yeast infections
Drinking alcohol also can raise the level of phenytoin in your blood. A change in
metabolism also can affect your phenytoin levels.
How do I get ready for this test?
You don't need to prepare for this test. Be sure your healthcare provider knows about
all medicines, herbs, vitamins, and supplements you are taking. This includes medicines
that don't need a prescription and any illicit drugs you may use.