Liquid chromatography and fluorescence spectroscopy compared with a homogeneous enzyme immunoassay technique for determining quinidine in serum.

Sera from 67 cardiac patients being treated with quinidine were analyzed for this drug by both a homogeneous enzyme immunoassay (EMIT) and a fluorescence procedure (double-extraction method). Of these samples, 47 were also analyzed for quinidine and its major metabolite in serum, (3s)-3-hydroxyquinidine, by liquid chromatography. Results by the EMIT procedure correlated well with those by both the chromatographic and fluorescence procedures, but EMIT gave, on the average, 25% higher values for quinidine than did the chromatographic procedure. The EMIT procedure is therefore not totally specific. (3s)-3-Hydroxyquinidine and dihydroquinidine (a contaminant present in standard quinidine formulations) each show cross reactivity in the EMIT assay for quinidine. Nevertheless, the EMIT assay for quinidine is acceptable for clinical use because these two compounds are also pharmacologically active.