The method for determining the blood level of ethosuximide was first described by Hansen in 1963. Referring to his method, we determined the plasma level of ethosuximide by the method for measuring phenytoin and phenobarbital proposed by Saitoh and others. This method proved to be much reproducible. In the test, 2ml of phosphoric acid buffer and 11 ml of dichloroethane were added to 0.5ml of plasma. The solution was well shaken and centrifuged at 2500 rpm. After 3ml of 1N-NaOH was added to 9ml of dichloroethane layer, the mixed solution was well shaken and centrifuged at 2500 rpm. The 1N-NaOH layer was transferred to a microcell and its absorbancy at 218 nm was measured.
The standard curve showed a straight line passing through the origin of the coordinate axes. The average recovery rate was 97.8±1.4%. Thus, by the method of Saitoh and others it became possible to measure at the same time the levels of phenytoin, phenobarbital and ethosuximide.