Evaluation of fluorescence immunoassay for total and unbound serum concentrations of disopyramide

The accuracy of determining disopyramide concentrations in 80 serum samples by high-performance liquid chromatography (HPLC) and fluorescence immunoassay (FIA) on 2 consecutive days was similar. The precision of both methods was excellent. The interday precision of FIA was superior to HPLC at nominal concentrations of 1.5 and 3.0 mg/L, but was similar at 7.0 mg/L. Following equilibrium dialysis of 40 serum samples, the unbound concentration of disopyramide determined by a modified FIA method under-estimated by 16% the unbound concentration determined by a reference method. The discrepancy may be avoided by preparing standards in the appropriate buffer. Fluorescence of disopyramide optical isomers at similar concentrations were identical. It is concluded that FIA for determining plasma concentrations of disopyramide is accurate and precise, and may be modified to accurately determine unbound concentrations following separation of plasma water and protein.