DILANTIN, TOTAL (Phenytoin)

LAB31

PHENYTOIN-TOTAL

Serum from a 7 mL red top tube

1.0 mL

Refrigerate

Set Up:Daily Report Available:1 day

Phenytoin, total

ug/mL

Therapeutic: 10 - 20 Toxic concentration: >30

Phenytoin (diphenylhydantoin or Dilantin®) is one of the most prescribed anti-convulsant drugs for the treatment of epilepsy, particularly grand mal epilepsy (major motor), cortical focal seizures and temporal lobe epilepsy. Oral phenytoin preparations are most commonly used and the absorption in the small intestine is reasonably complete (85 to 95%). Phenytoin in circulation is approximately 87 to 94% bound to plasma protein, mainly albumin, with relatively low binding constant. In the liver, phenytoin (about 70 to 80%) is metabolized by parahydroxylation to 5-(hydroxyphenyl)-5-phenylhydantoin (HPPH), an inactive metabolite. HPPH (90%) is then conjugated with glucuronic acid and excreted as 5-hydroxyphenylhydantoin into the urine. Approximately 5% is excreted in the urine unchanged in form. The basic mechanism of phenytoin action in seizure control has not specifically been explained. Phenytoin may impair transmitter release from presynaptic terminals, and thus restrict the spread of epileptic discharge. Monitoring phenytoin concentrations in serum is essential during therapy by providing physicians with an index for adjusting dosage. The need for monitoring phenytoin concentrations is also due to the narrow therapeutic index and wide variability in individual rates of drug absorption, metabolism, and clearance. The toxicity of phenytoin is often dose-related and affects mainly the central nervous system. Most common toxic symptoms are nystagmus, ataxia, drowsiness, diplopia, blurred vision and convulsions. Chronic treatment of phenytoin may lead to hyperplasia of gums, anemia, and osteomalacia. Other toxic effects, not necessarily related to drug dose, include skin eruptions, lymphadenopathy, hepatitis, blood dyscrasias, and systemic lupus erythematosus.