Skip to main content


URMC / Encyclopedia / Content


Does this test have other names?

Dilantin test

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 uncontrolled.

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 to rise

  • Blood tests to measure the sodium level in your blood

  • Tests for other medicine levels

  • Urinalysis

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 provider.

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 your arm.

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. 

Medical Reviewers:

  • Freeborn, Donna, PhD, CNM, FNP
  • Snyder, Mandy, APRN