Other names: Epanutin

Phenytoin is a form of medication, which is used to treat patients with seizure disorders, such as epilepsy; these disorders affect the function of the brain because they prevent the brain from controlling nerve activity and transmitting electrical impulses.

Seizures can be triggered by a range of different factors, including trauma, increased temperatures, exposure to toxins or certain drugs, severe infections, decreased oxygen supply and hypoglycaemia (low blood sugar).

When is the test used?

The test is used to measure the level of Phenytoin in the blood; it is used to check that levels are within the target range. If levels are too low, this may indicate that the dose is too low and the drug may not be working properly; if the levels are too high, this may indicate that the dose is too high and the patient may experience symptoms of toxicity. The test is usually carried out after the patient has started drug therapy, especially if the patient is suffering side-effects; possible side-effects include:

  • Confusion
  • Nausea
  • Tiredness
  • Skin irritation
  • Excessive hair growth
  • Insomnia and disturbed sleep patterns
  • Problems with swallowing

It is important that the drug is controlled carefully to prevent side-effects.

The test is repeated on a regular basis to ensure the patient is receiving the correct dose.

How is the test done?

The test is done by taking a sample of blood from the patient’s arm; a needle is inserted into a vein and the blood is drawn out and collected in a syringe. Once the sample has been collected, the blood will be bottled, labelled and sent away to the laboratory for analysis.

What do the test results mean?

The target range for Phenytoin are between 10.0 and 20.0 mg/L; if the test results show that the level of Phenytoin is within the target range, this means that the patient is probably receiving a suitable dose (if the patient is experiencing symptoms, the doctor may change the dose).

If levels are too high, this means that the dosage is too high and the patient may experience symptoms of toxicity; the doctor will probably lower the dose if this is the case.

If levels are too low, this indicates that the dose is too low and the patient may still be experiencing seizures. The doctor will probably adjust the dose if this is the case.

Phenytoin may not be suitable for everyone and the doctor may prescribe alternative drugs.

Specific Blood Tests

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