High-pressure liquid chromatography and enzyme immunoassay compared with gas chromatography for determing phenytoin.

We compared a gas-chromatographic method for determination of phenytoin with a high-pressure liquid chromatographic technique and with enzyme immunoassay by three instrumental procedures. More than 100 sera from patients being treated with this drug were assayed by all these techniques. The coefficient of variation was the lowest (4.0%) with liquid chromatography, but all methods gave a CV of less than 10%. The correlation coefficients for all methods exceeded 0.97 when compared to gas chromatography. Operation costs varied with the number of tests per batch, reagent costs, and operator labor costs. All assays gave comparable values for the therapeutic range, so it would be plausible to use more than one method in a situation where (e.g.) satellite laboratories handle different quantities of assays. In any of these techniques, interferences from carbamazepine, mephenytoin, phenobarbital, and primidone were negligible.