At a Glance
Why Get Tested?
To determine phenytoin concentration in the blood, 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
A blood sample drawn from a vein in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
This test measures the amount of phenytoin in the blood. Phenytoin is a drug that is used to treat some seizure disorders (also called epilepsy). Seizure disorders affect the brain's ability to transmit electrical impulses and to regulate nerve activity. During a seizure, a patient may experience changes in consciousness, alterations in sight, smell, and taste, and may experience uncontrolled muscular convulsions of 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 at any age can experience a seizure. In many cases, the cause of seizures is not known and the frequency and severity in those affected 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. Levels of phenytoin in the blood must be maintained within a narrow therapeutic range. If levels are too low, the affected person will experience seizures; too high and the person may experience symptoms associated with phenytoin toxicity, such as 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. The drug is metabolized by the liver and excreted in the urine. Enzymes in the liver process phenytoin at a rate that will vary from person to person and is affected by age (children metabolize it more quickly; the elderly metabolize it more slowly) and by the health of their liver. When the body has reached its capacity to process phenytoin, small increases in the dose can cause large increases in blood concentrations, increasing the severity of side effects and causing phenytoin toxicity. 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, then they may have an excess of active phenytoin. Adding to the complexity is the fact that phenytoin often interacts with other drugs, increasing or decreasing the other medication's effectiveness and/or increasing or decreasing phenytoin's effectiveness.
The total effect can be unpredictable. Dosages of phenytoin 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. Doctors must also evaluate their patient for side effects and adverse reactions during initial dosage adjustment and over time. In some cases, the severity of side effects may cause patient and doctor to seek another anti-seizure medication.
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.
Ask a Laboratory Scientist
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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
(Dec 30, 2008). von Winckelmann S, Spriet I, Willems L. Therapeutic Drug Monitoring of Phenytoin in Critically Ill Patients. MedScape. Available online at http://www.medscape.com/viewarticle/583862 through http://www.medscape.com. Accessed Sept 2010.
(Updated July 15, 2009). Miller C. Toxicity, Phenytoin. Medscape eMedicine. Available online at http://emedicine.medscape.com/article/816447-overview through http://emedicine.medscape.com. Accessed September 2010.
(Updated May 1, 2009). Phenytoin. MedlinePlus Drug Information [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/druginfo/meds/a682022.html. Accessed September 2010.
(Updated February 3, 2009). Duldner J, Phenytoin Overdose. Medline Plus Drug Information. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/002533.htm. Accessed September 2010.
Malaty W, Stiglemen S. Antiepileptic Drug Monitoring. Am Fam Physician. 2008 Aug 1;78(3):385-386. Available online at http://www.aafp.org/afp/2008/0801/p385.html through http://www.aafp.org. 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 http://www.medscape.com.
Beckmann, Charles R, et al. Obstetrics and Gynecology. 4th Ed. Baltimore: Lippincott Williams & Wilkins, 2002.
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 http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a682022.html.
Whetstone, W. (2005 May 10, Revised). Phenytoin Overdose. MedlinePlus Drug Information [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a682022.html.
Wilner, A. (2004 October 11). The Epilepsy Continuum: From Age to Age. Medscape [On-line CME]. Available online at http://www.medscape.com/viewarticle/489615 through http://www.medscape.com.
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 http://www.medscape.com/viewarticle/530483 through http://www.medscape.com.
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 http://www.annals.org/cgi/reprint/144/6/421.pdf through http://www.annals.org.
(© 2006). Phenytoin, Free & Total. ARUP's Laboratory Test Directory [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0090141.jsp through http://www.aruplab.com/.
Morantz, C. and Torrey, B. (2005 January 15). Guidelines for Prescribing Antiepileptic Drugs. American Family Physician: v71(2) [On-line information]. Available online at http://www.aafp.org/afp/20050115/practice.html through http://www.aafp.org.
(© 2006). Phenytoin. Epilepsy.com [On-line information]. Available online at http://www.epilepsy.com/medications/a_phenytoin_intro.html through http://www.epilepsy.com.
(2003 February 1, revised). Seizure Disorders. Merck Manual Home Edition [On-line information]. Available online at http://www.merck.com/mmhe/print/sec06/ch085/ch085a.html through http://www.merck.com.
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 http://www.medscape.com/medline/abstract/16175137 through http://www.medscape.com.
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 http://www.medscape.com/medline/abstract/12476936 through http://www.medscape.com.