Phenytoin (diphenylhydantoin) has been used extensively for seizure control in patients having both grand mal epilepsy (major motor), cortical focal seizures, and temporal lobe epilepsy. Serum level monitoring of the drug is essential in order to achieve maximal seizure control while maintaining minimal blood levels. Estimation of phenytoin levels may be useful when establishing a dosage for new patients, monitoring compliance or assessing for toxicity.
Because of individual variation in absorption and metabolism, optimum levels may vary. Cross-reactivity of the phenytoin assay with fosphenytoin means that samples should be collected 2 hours post-IV fosphenytoin dosage and 4 hours post-IM dosage
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