Phenytoin

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.

Anyone at any age can experience a seizure. Seizures can be caused by high fevers, decreased oxygen supply, head trauma, exposure to certain toxins and drugs, withdrawal from alcohol after heavy abuse, severe infections, extremely low blood sugar, metabolic disorders, and brain tumours. Seizures may also be seen in newborns as a result of head trauma that occurred in the womb or during birth. In many cases, the cause of seizures is not known and the frequency and severity in those affected varies from patient to patient and may change over time. Patients may experience a single seizure and never have another, may have occasional seizures, or may have recurrent seizures. In rare cases, a patient may have a seizure that starts and does not stop without prompt medical intervention. Most patients do not experience residual damage from seizures, but muscle contractions during attacks may cause acute injuries, and losses in consciousness can lead to falls. In some cases, recurrent seizures can eventually lead to progressive brain damage.

Phenytoin is prescribed to help prevent the recurrence of certain types of seizures. It has been widely used in the UK since it was developed in 1938. It is still being prescribed but is gradually being replaced by newer drugs. Blood levels of phenytoin must be maintained within a narrow therapeutic range. Too little and the patient will experience seizures; too much and the patient will experience symptoms associated with phenytoin toxicity, such as: loss of balance and falling, nystagmus (eye movement from side to side), confusion, slurred speech, tremors, and low blood pressure.

Keeping a constant level of phenytoin in the blood may be difficult to maintain. The drug is metabolized by the liver and excreted in the urine. Enzymes in the liver process phenytoin at a rate that varies from patient to patient and that is affected by a patient's age (children metabolize it more quickly, the elderly metabolize it more slowly) and also by the health of their liver. When the body has reached its maximum 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 patient 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. These may include:

Swelling of the gums and/or lymph nodes
Excess hair growth (hirsutism) 
Insomnia (not sleeping properly)
Nausea
Confusion
Difficulty swallowing
Tiredness
Rashes

In some cases, the severity of side effects may cause the 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.

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

No test preparation is needed.