The necessity of drug level monitoring in anticonvulsant drug therapy.

A survey of 'steady-state' serum levels of anticonvulsant drugs from 221 epileptic patients at a university hospital was conducted. Serum concentrations of phenobarbital, primidone, and diphenylhydantoin were determined by a gas chromatographic method. Sixty-five percent (130) of the patients receiving diphenylhydantoin had levels below the therapeutic range of 10-20mug/ml. Subtherapeutic levels appear to be due to inadequate dosage adjustment. Only 25% (33) of the patients receiving phenobarbital had levels below the therapeutic range. Serum levels of diphenylhydantoin or phenobarbital could not be predicted from dosage. Most patients received two or more drugs. Over 10% of the patients had potentially toxic levels of anticonvulsant drugs. High levels of diphenylhydantoin were easily recognized clinically but high levels of phenobarbital were not.