Measuring serum phenytoin is used to monitor drug levels during treatment, monitor patient compliance with treatment, and assess any drug toxicity.
Phenytoin (Epanutin, Dilantin) is a hydantoin derivative used as an anticonvulsant drug. Phenytoin is widely distributed throughout the body, metabolized by the liver, and excreted in the bile and urine, with a half-life of approximately 22 hours when administrated orally. Five to 6 days of treatment are required to reach steady-state levels in adults and 2-5 days in children. Because phenytoin is highly protein-bound, free phenytoin levels are useful only in patients with abnormal plasma protein binding of the drug. Such cases are observed during pregnancy, in the elderly, and in patients with hypoproteinemia or hyperalbuminemia.
Possible Interpretations of Abnormal Values
- Increase: Phenytoin abuse, phenytoin overdose, kidney disease. Drugs that may increase phenytoin levels: allopurinol, amiodarone, anticoagulants (oral), benzodiazepines, chloramphenicol, chlordiazepoxide, cimetidine, diazepam, H2 antagonists, disulfiram, estrogens, ethyl alcohol (acute), ethosuximide, glutamine (in children), fluoxetine, isoniazid, methsuximide, methylphenidate, phenacemide, phenothiazines, phenylbutazone, propoxyphene, salicylates, sulfonamides, thiazides, trazodone, tolbutamide, trimethoprim, vinblastine sulfate.
- Decrease: (Below therapeutic range) Inadequate treatment, non-compliance. Drugs that may alter (speed up) the metabolism of phenytoin, leading to a decrease in its serum levels: carbamazepine, diazoxide, ethyl alcohol (chronic intake), folic acid, loxapine, methotrexate, sulfonylureas, theophylline, reserpine, sucralfate, drugs that contain calcium.
- Drugs that may either increase or decrease phenytoin serum levels: Phenobarbital, sodium valproate, valproic acid.