Results of spin immunoassay for simultaneous measurement of phenytoin and phenobarbital in serum compared with those of liquid chromatography.

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Abstract
A spin immunoassay (SIA) is decribed in which the 2 antiepileptic agents, phenytoin (PHT) and phenobarbital (PB), are determined simultaneously is a single measurement. The method involves labeling the two with 15N- and 14N-nitroxide, respectively, whose ESR peaks do not overlap each other. With 2 .mu.l of serum, the smallest detectable concentration for either drug was 1.25 mg/l. PHT and PB levels in Q-PAK therapeutic drug-monitoring control sera were determined by SIA. The values obtained by SIA were in good agreement with those presented by the manufacturer. Sera from rabbits after administration of PHT and/or PB were analyzed by SIA and high-performance liquid chromatography. There was a good correlation between results obtained by the 2 methods.