Competitive nephelometric immunoassay of carbamazepine and its epoxide metabolite in patient blood plasma.

We have prepared anti-carbamazepine antiserum and developed a competitive nephelometric immunoassay for the determination of carbamazepine and its epoxide metabolite in patient blood plasma. The antiserum was raised by immunization of a rabbit with a carbamazepine (bovine serum albumin) conjugate. A carbamazepine-(human serum albumin) conjugate was used as an assay reagent. Carbamazepine and its epoxide inhibited competitively and almost equally the immunoprecipitation of the carbamazepine-(human serum albumin) conjugate. Therefore carbamazepine and its epoxide could be determined by the measurement of the scattered light from the immunoprecipitate in assay solution on a laser nephelometer. Patient plasma specimens were analyzed, and the values correlated well to those determined by the high-performance liquid chromatography. The epoxide concentrations were considerably lower than therapeutic carbamazepine concentrations, which was certified by the chromatographic results. This immunoassay was rapid (incubation time: within 20 min), simple and precise (coefficient of variation: less than 6%), and required as little as 6 microliters of plasma, and seemed suitable for routine monitoring of carbamazepine.