Phenytoin Test

Phenytoin test is ordered to epilepsy patients at regular intervals for monitoring the right dose required for treatment. The test may be ordered one hour after intravenous administration and 24 hours after an oral dose.

Additionally, doctors order this test to people who do not respond well to phenytoin therapy (such patients keep on getting seizures despite taking the drug), or to those who show symptoms of phenytoin toxicity.

The symptoms of phenytoin toxicity are as follows:

• Dizziness
• Confusion
• Fever
• Low blood pressure
• Slurred speech
• Muscle rigidity
• Occasional seizures
• Swollen gums
• Tremors
• Sleepiness
• Unsteadiness
• Coma
• Nystagmus (eye movement from side to side)

The levels of phenytoin in the blood can vary significantly due to the following:

• Type of formulation prescribed
• Enzyme saturation for the metabolism of phenytoin, leading to a considerable increase (and toxicity) in the concentration of this drug in the blood for a small increase in the dose.
• Decreased albumin, leading to decreased albumin-bound phenytoin and increased free active phenytoin
• Interactions with other medicines

Proper monitoring of blood phenytoin levels is therefore needed for effective and safe use of the drug.

You do not need any special preparation for this test. Phenytoin levels can be affected if you start consuming any new drug (apart from phenytoin) or modify the dose of the medicines that you had been taking initially. These include the following prescription and non-prescription drugs:

• Phenothiazines for nausea 
• Oestrogens in hormone replacement therapy
• Disulfiram for the treatment of alcohol dependence
• Dicumarol, an anticoagulant
• Amiodarone, anti-arrhythmic drug
• Chlordiazepoxide for treatment of anxiety
• Medicines with salicylates and aspirin
• Tolbutamide for diabetes
• Sulphonamides for the treatment of infections
• Isoniazid, an antibiotic
• Famotidine for treatment of gastric ulcers
• Trazodone, an antidepressant
• Valproic acid and primidone for seizures
• Fluconazole, for the treatment of yeast infections
• Methylphenidate for treatment of attention-deficit disorders

Drinking alcohol also increases the level of phenytoin in the blood. Consult your healthcare provider to check if there are any possible drug interactions before starting any new drug therapy.

Your doctor will use a sterile needle to withdraw a small amount of blood from a vein in your arm. It is a short procedure that doesn’t take more than a few minutes. 

Some risks associated with blood testing include:

• Bleeding
• Bruising
• Dizziness
• Chances of developing infections

Normal results:

The therapeutic blood levels of total phenytoin are in the range of 10-20 mcg/mL for children and adults and 8-15 mcg/mL for neonates. 

The doctor will decide the doses as per the conditions of the person with epilepsy, and the reference range is just a guide.

Abnormal results:

Any values above or below the normal range are considered abnormal. At values below the normal range, a person may have seizures. 

If the drug levels are above the therapeutic range (high), it may indicate toxicity.

However, the test results may show slight variation depending on certain factors as follows:

• Your age
• Your medical history
• Your gender
• The method used to perform the test

Discuss the test results with your doctor. He/she will determine the right dose of phenytoin for the therapy depending on how you respond to the treatment.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.