Serum Phenytoin (Dilantin)

Specimen Required

3-5cc Clotted Blood or Serum

Fasting Required

Purpose of the Test

To determine phenytoin concentration in the blood, to maintain an appropriate level, and to detect phenytoin toxicity

When this test is required

Phenytoin is requested frequently when a patient is starting phenytoin treatment and when/if a patient’s treatment changes (other drugs are started, stopped, or changed). Once stable blood concentrations in the therapeutic range have been achieved, then phenytoin is monitored at regular intervals to ensure that it remains in this range.The test may be requested when a patient’s condition does not appear to be responding to phenytoin and they continue to have seizures, this is to determine whether concentrations are too low and the medication is ineffective or to determine if the patient is not taking the phenytoin regularly. It may also be requested when a patient experiences a troublesome level of side effects and/or exhibits symptoms that the doctor suspects may be due to toxicity.

What the Test Detects

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…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 nodesExcess hair growth (hirsutism) Insomnia (not sleeping properly)NauseaConfusionDifficulty swallowingTirednessRashes 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. See MoreSee Less

Preparation for the Test

None

Sample Requirements

A blood sample taken from a vein in your arm

Additional Notes

Patients who take phenytoin long-term may develop vitamin D deficiency, bone deformation related to vitamin D deficiency (osteomalacia), weakness and numbness in extremities (peripheral neuropathy), acne, and a thickening of facial features. Rarely, a patient may develop a severe rash and skin condition that requires hospitalisation.Women who use phenytoin during pregnancy are at an increased risk of several birth defects. Women who wish to become pregnant should talk to their doctors prior to conception.Phenytoin is sometimes prescribed for other conditions, such as to help treat trigeminal neuralgia (which causes episodes of sharp pain along the jaw) and other causes of nerve pain. This use must also be monitored.Various prescribed drugs, over-the-counter medications, and supplements can increase, decrease, or interfere with the concentrations of phenytoin in the blood. Drugs that can increase phenytoin in the body include: diazepam, carbamazepine (can raise or lower), alcohol, aspirin (large doses), chloramphenicol, oestrogen, isoniazid, omeprazole, trimethoprim, and warfarin. Drugs that can decrease phenytoin include: antacids (when taken with phenytoin), folic acid, chronic alcohol abuse, rifampicin, and methotrexate. There are also a variety of other drugs that are affected by phenytoin. Patients should talk to their doctors about all of the drugs and supplements that they are taking and about the medication(s) that are right for them. Phenytoin is not effective for every kind of seizure and will not work for every patient.

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