Quinidine assays: enzyme immunoassay versus high performance liquid chromatography

Plasma quinidine results determined by enzyme multiplied immunoassay (EMIT) (Syva) were compared with results from a quinidine-specific high performance liquid chromatographic (HPLC) assay. During a clinical study, 16 patients with stable ventricular arrhythmias were treated with three oral quinidine preparations given during three consecutive 3-day periods. Seventeen plasma samples were drawn from each patient at steady-state during each period. Each specimen was divided into two portions, one for assay by EMIT and the other for assay by HPLC. EMIT assays were done on a Syva Autolab 6000 System using Syva quinidine kits and bilevel Ortho Diagnostics controls. The overall mean (+/- SD) quinidine concentrations by EMIT and HPLC were 2.16 +/- 0.58 and 1.81 +/- 0.60, respectively, n = 816, with a mean overall EMIT/HPLC ratio of 1.23 +/- 0.18. Mean ratios in individual patients ranged from 1.01 to 1.56; the average of mean individual ratios was 1.23 +/- 0.13. The EMIT assay, which also reports dihydroquinidine and small amounts of quinidine metabolites as quinidine, reported quinidine values that averaged 1.2-fold greater than results from a quinidine-specific HPLC method.