Plasma Levels of Anticonvulsants

Summary: The measurement of plasma concentrations of anticonvulsant drugs is becoming increasingly important in managing the patient with epilepsy. It provides a guide to the adequacy of therapy, may explain symptoms due to otherwise unsuspected drug overdosage, and indicates when the dose of an anticonvulsant is approaching its limit of tolerance. The plasma concentration of an anticonvulsant reflects the drug's concentration at its sites of biological activity, and this determines the extent of the biological activity of the drug.
The present paper reviews the methods available for measuring plasma levels of the more commonly used anticonvulsants and discusses briefly their metabolism, since some e.g. primidone, methylphenobarbitone, trimethadione, are converted to biologically-active metabolites which should be measured. Where possible, biological activity is correlated with the plasma levels of individual drugs, and oral dosages with plasma levels, and the time-course of the plasma levels after single and repeated doses is indicated. For phenytoin, and to a lesserextent for phenobarbitone and its congeners, information is available on the plasma protein binding of the drugs, and on the effects of other drugs and disease on the plasma levels of the primary anticonvulsant.