Clinical definition of the phenytoin test
Phenytoin is an antiepileptic drug and 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. Seizures are abnormal electrical discharges in the brain that temporarily disrupt the normal transmission of messages to the body. They produce alterations in consciousness, cause odd rhythmic or repetitive movements, jerking or twitching of limbs, and/or can lead to convulsions (a sudden uncontrollable attack) that contract muscles throughout the body. This test measures the amount of phenytoin in the blood. In the blood phenytoin is highly bound to serum proteins. The unbound or free is pharmacologically active. Phenytoin works by reducing the electrical conductance among brain cells, blocking excessive electrochemical activity occurring in the brain during a seizure. Phenytoin levels have to be monitored, as very low  levels of phenytoin can lead to seizures in an affected person and high levels of phenytoin result in phenytoin toxicity.

Why is the phenytoin test done?
Phenytoin test done to determine the phenytoin concentration in the blood, to maintain a therapeutic level, and to detect potential for toxicity. Phenytoin is used to control seizures and to treat seizures that may begin during or after surgery to the brain or nervous system

What are the common signs/symptoms when the phenytoin test is done?

  • Slurred speech
  • Tremors
  • Hirsutism
  • Low blood pressure
  • Swelling of the gums and/or lymph nodes
  • Insomnia
  • Nausea
  • Confusion
  • Difficulty swallowing
  • Fatigue
  • Rashes
  • changes in consciousness
  • Alterations in sight, smell, and taste
  • Uncontrolled muscular convulsions
  • Loss of balance and falling
  • Involuntary eye movement from side to side (nystagmus)

Who should do the phenytoin test (Target population)?
Persons experiencing the above mentioned  symptoms.

What should I do before the phenytoin test?

Specimen type

Specimen collection procedure

Preparatory instructions before the test *

Serum (Blood Sample)


Collection of blood from a vein, usually from the arm.

No Fasting Required.

No other special preparations required.

*Subjects suffering from any illness or on oral or injectable medications are advised to consult their physician prior to requesting any tests or procedures.

How do I interpret my phenytoin test results?

Reference Range*


10 to 20 µg/ml

Therapeutic range

Above 20 µg/ml

Toxic range

‘*A Reference range is a set of values which helps the healthcare professional to interpret a medical test. It may vary with age, gender, and other factors. Reference ranges may also vary between labs, in value & units depending on instruments used and method of establishment of reference ranges’

Diseases/conditions related to the phenytoin test

  • Nystagmus
  • Hypoalbuminemia
  • Kidney failure
  • Liver disease

Other tests related to the phenytoin test

  • Therapeutic Drug Monitoring
  • Emergency and Overdose Drug Testing
  • Folic acid level

Synonyms : Dilantin, Epanutin, Eptoin