Phenytoin (Dilantin, Diphenylhydantoin, Phosphenytoin)

Phenytoin alone or in conjunction with phenobarbital is used to treat grand mal seizures and tonic clonic seizures. Phenytoin has a notoriously high risk/benefit ratio. At serum levels of 30 ug/mL, the drug is toxic in up to 50% of patients. The first sign of toxicity is usually nystagmus, which occurs at levels of 20 ug/mL. Cerebellar ataxia, tremor, and hyperreflexia occur at about 30 ug/mL and confusion, lethargy, and coma at 40 ug/mL and higher. Overdoses are generally treated by drug withdrawal and gastric lavage with activated charcoal. Deaths from phenytoin toxicity are rare.

Drugs known to increase the serum level of phenytoin include amiodarone, cimetidine, fluconazole, isoniazid, and certain sulfonamides.

Serum trough values are recommended to establish and monitor phenytoin dosage regimens. Trough levels are preferred over peak levels. The recommended sampling time for trough levels is immediately preceding the next dose. Steady state levels are reached after 7 to 10 days of treatment.

Measurements can be adjusted for albumin binding using the following formula:

Adjusted concentration = measured phenytoin level / (0.2 x albumin concentration) + 0.1.

If the measured phenytoin level is 30 ug/mL and the albumin level is 3.0 g/dL, the adjusted phenytoin concentration is 43 ug/mL. If the albumin level is 4.6 mg/dL, the adjusted phyentoin concentration is 29 ug/mL.

Therapeutic range is 10-20 ug/mL. Many experts advise maintaining a serum level of 14 to 18 ug/mL. Levels >30 ug/mL are considered critical values.

Specimen requirement is one plain red top tube of blood.

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