The distribution of plasma phenytoin levels in epileptic patients.

Plasma phenytoin levels were estimated in 111 adult epileptic patients. A wide scatter of values was found, with over half outside the therapeutic limits of 10 to 20 μg/ml. Phenytoin tolerance tests carried out in five patients revealed a possible failure to metabolize the drug adequately in three of them. Factors affecting the plasma phenytoin level are discussed. The importance of this assay is seen in view of the number of patients potentially at risk from toxic blood levels in the absence of accepted clinical manifestations of acute or subacute intoxication.