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Also known as: [Often referred to by brand name. See MedlinePlus Medical Drug Information]
Formal name: Phenytoin, total and free

At a Glance

Why Get Tested?

To determine the level of the drug phenytoin in your blood in order to maintain a therapeutic level and to detect potential for toxicity

When to Get Tested?

At regular intervals to monitor; as needed to detect low or toxic concentrations

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

Phenytoin is a drug that is used to treat some seizure disorders (see Epilepsy), including complex partial seizures (psychomotor seizures) and seizures that occurr during or after neurosurgery. Phenytoin may be used alone or with phenobarbital or other anticonvulsants. This test measures the amount of phenytoin in the blood.

Seizure disorders affect the brain's ability to transmit electrical impulses and to regulate nerve activity. During a seizure, someone may experience changes in consciousness, alterations in sight, smell, and taste, and may experience uncontrolled muscular convulsions in one or more parts of the body. Phenytoin works by reducing the electrical conductance among brain cells, blocking excessive electrochemical activity occurring in the brain during a seizure.

Anyone can experience a seizure at any age. In many cases, the cause of seizures is not known. The frequency and severity varies from person to person and may change over time. People may experience a single seizure and never have another, may have occasional seizures, or may have recurrent seizures. In rare cases, a person may have a seizure that starts and does not stop without prompt medical intervention.

Phenytoin is prescribed to help prevent the recurrence of certain types of seizures. It has been widely used in the United States since its development in 1938. It is still being prescribed but is beginning to be replaced by newer drugs.

The level of phenytoin in the blood must be maintained within a narrow therapeutic range. If levels are too low, the affected person may experience seizures; if they are too high, the person may experience symptoms associated with phenytoin toxicity. These may include loss of balance and falling, involuntary eye movement from side to side (nystagmus), confusion, slurred speech, tremors, and low blood pressure.

Maintaining a therapeutic level of phenytoin in the blood can be a challenge. Enzymes in the liver process phenytoin at rates that differ from person to person and are affected by age (children metabolize it more quickly while the elderly metabolize it more slowly) and by the health of the liver. When the body has reached its capacity to process phenytoin, small increases in the dose can cause large increases in blood concentration, increasing the severity of side effects and causing phenytoin toxicity.

Phenytoin's total effect can be unpredictable. Dosages must be adjusted slowly until a steady concentration in the blood is reached. The actual amount of drug that it takes to reach this steady state will vary from person to person and may change over time. Health practitioners must also watch for side effects and adverse reactions during initial dosage adjustment and over time. In some cases, the severity of side effects may make it necessary to consider another anti-seizure medication.

Most phenytoin is bound to protein in the bloodstream; it is the unbound "free" portion that is active. If a person has a lower than normal amount of protein in their blood, such as low albumin in liver failure, then the person may have an excess of active phenytoin. Phenytoin can also interact with other drugs and increase or decrease the other medications' effectiveness. it is important to always discuss with a health practitioner all medications being taken to determine if drug interactions are a possibility.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Phenytoin. Medline Plus. Available online at Last Revised 05/01/2009. Accessed February 13, 2014.

Dilantin. Available online at through Last updated February 7, 2014. Accessed February 13, 2014.

Charlene Miller et al. Phenytoin Toxicity Clinical Presentation. Medscape. Available online at Last updated November 9, 2012. Accessed February 13, 2014.

Phenytoin (Phenytek, Dilantin). NYU Langone Medical Center. Available online at through Accessed February 13, 2014.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

(2003 April 1, Revised). Phenytoin Oral. MedlinePlus Drug Information [On-line information]. Available online at

Whetstone, W. (2005 May 10, Revised). Phenytoin Overdose. MedlinePlus Drug Information [On-line information]. Available online at

Wilner, A. (2004 October 11). The Epilepsy Continuum: From Age to Age. Medscape [On-line CME]. Available online at through

Pack, A. (2006 April 28). Therapy Insight: Clinical Management of Pregnant Women With Epilepsy. Medscape from Nat Clin Pract Neurol. 2006;2(4):190-200 [On-line information]. Available online at through

Holloway, R. and Jozefowicz, R. (2006 March). Update in Neurology. Ann Intern Med 2006:144:421-426 [On-line journal]. PDF available for download at through

(© 2006). Phenytoin, Free & Total. ARUP's Laboratory Test Directory [On-line information]. Available online at through

Morantz, C. and Torrey, B. (2005 January 15). Guidelines for Prescribing Antiepileptic Drugs. American Family Physician: v71(2) [On-line information]. Available online at through

(© 2006). Phenytoin. [On-line information]. Available online at through

(2003 February 1, revised). Seizure Disorders. Merck Manual Home Edition [On-line information]. Available online at through

McMillin GA; Juenke J; Dasgupta A ( Oct. 2005). Effect of ultrafiltrate volume on determination of free phenytoin concentration. Medscape from Ther Drug Monit. 2005; 27(5):630-3 (ISSN: 0163-4356). Available online at through

Dasgupta A (Oct. 2002). Clinical utility of free drug monitoring. Medscape from Clin Chem Lab Med. 2002; 40(10):986-93 (ISSN: 1434-6621). Available online at through

(Dec 30, 2008). von Winckelmann S, Spriet I, Willems L. Therapeutic Drug Monitoring of Phenytoin in Critically Ill Patients. MedScape. Available online at through Accessed Sept 2010.

(Updated July 15, 2009). Miller C. Toxicity, Phenytoin. Medscape eMedicine. Available online at through Accessed September 2010.

(Updated May 1, 2009). Phenytoin. MedlinePlus Drug Information [On-line information]. Available online at Accessed September 2010.

(Updated February 3, 2009). Duldner J, Phenytoin Overdose. Medline Plus Drug Information. Available online at Accessed September 2010.

Malaty W, Stiglemen S. Antiepileptic Drug Monitoring. Am Fam Physician. 2008 Aug 1;78(3):385-386. Available online at through Accessed September 2010.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. St. Louis: Elsevier Saunders; 2006 Pp 1252, 2313.

Carl GF, Smith ML. Phenytoin-folate interactions: differing effects of the sodium salt and the free acid of phenytoin. Epilepsia 1992; 33(2):372-5. Available online at

Beckmann, Charles R, et al. Obstetrics and Gynecology. 4th Ed. Baltimore: Lippincott Williams & Wilkins, 2002.