At a glance
Also known as
Why get tested?
To determine phenytoin concentration in the blood, to maintain a therapeutic level, and to detect phenytoin 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
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.
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 utero or during the birthing process. 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 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 since its development in 1938. It is still being prescribed but is beginning to be replaced by newer drugs. Phenytoin levels 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 can be complicated. The drug is metabolised by the liver and excreted in the urine. in the liver process phenytoin at a rate that will vary from patient to patient and that it is affected by a patient's age (children metabolise it more quickly, the elderly metabolise 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 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
- difficulty swallowing
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.
No test preparation is needed.