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Phenytoin Blood Test

Last updated Oct. 11, 2015

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Phenytoin, or Dilantin®, an anticonvulsant drug used to treat seizures.


What are other names for this test? (Equivalent Terms)

  • Dilantin Levels Blood Test

What is Phenytoin Blood Test? (Background Information)

  • Phenytoin, or Dilantin®, is an anticonvulsant drug used to treat seizures. Seizures are episodes of abnormal and excessive brain cell activity; they may last several seconds to even 5 minutes
  • Often, the cause of a seizure is unknown. A seizure may occur as result of a nervous system disorder, called epilepsy. They may also occur following a brain surgery
  • During a seizure, an individual may convulse or display other uncontrolled physical behaviors. Such seizures are said to be convulsive and convulsive seizures are the most common seizures
  • Alternatively, non-convulsive seizures do not exhibit this uncontrolled physical behavior. Instead, an individual experiencing this seizure type may momentarily lapse out of consciousness, effectively “zoning out”
  • Brain cells communicate to each other and to other cells, through electrical signals. During a seizure, brain cells are rendered excessively conductive. The brain cells that are over-conductive transfer electrical signals too freely. This results in the derailment of their communications system, causing either convulsive or non-convulsive effects of seizure
  • The drug phenytoin dampens the activities of brain cells. It works by reducing the sky-high conductivity of brain cells, to more functional levels
  • Like every other drug, phenytoin must be kept within a certain range, in order to prevent certain unwanted effects. If levels are too high, it may result in toxicity, marked by confusion and a drop in blood pressure. If the levels are too low, the drug may not successfully ward-off a seizure attack
  • A Phenytoin Blood Test is a test that assesses the levels of phenytoin in blood. It is used while adjusting the initial dose and to ensure that phenytoin levels stay within an acceptable range

What are the clinical indications for performing the Phenytoin Blood Test?

Following are the clinical indicators for performing a Phenytoin Blood Test:

  • To determine the initial dosage requirement in an individual with seizure, by measuring the levels of phenytoin in blood
  • To monitor phenytoin therapy; to ensure that the anticonvulsant drug is within acceptable levels

How is the specimen collected for Phenytoin Blood Test?

Following is the specimen collection process for Phenytoin Blood Test:

Sample required: Blood

Process: Insertion of a needle into an arm vein.

Preparation required: No special preparation is needed prior to the test.

What is the significance of the Phenytoin Blood Test result?

The significance of Phenytoin Blood Test is explained:

  • Increased phenytoin levels may indicate phenytoin toxicity, marked by:
    • Involuntary side-to-side eye movements (nystagmus)
    • Loss of consciousness
    • Confusion
    • Decreased blood pressure
  • Decreased phenytoin levels may indicate that more phenytoin is necessary to achieve a therapeutic dose

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and relevant useful information:

  • The point at which phenytoin reaches a toxic level is difficult to predict, since phenytoin is metabolized by enzymes that have a pre-set capacity, unique to every individual. This factor makes the Phenytoin Blood Test essential, when one is being treated using the anticonvulsant drug phenytoin

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

What are some useful resources for additional information?

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References and information sources used for the article:

Fischer, R. (2005). Epileptic seizures and epilepsy: Definitions proposed by the International League Against Epilepsy (ILAE) and the International Bureau for Epilepsy (IBE). Epilepsia46(4), 470-72

Keogh, K. (2010). Nursing Laboratory and Diagnostic Tests Demystified. New York City, NY: McGraw-Hill Medical.

Lab Tests Online (2013, June 24). Retrieved June 7, 2014 from http://labtestsonline.org/understanding/analytes/vma/

Martini, F., Nath, J. L., & Bartholomew, E. F. (2012). Fundamentals of anatomy & physiology (9th ed.). San Francisco: Benjamin Cummings.

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: June 8, 2014
Last updated: Oct. 11, 2015