Lack of apparent effect of assay methodology on the pharmacokinetics of digoxin

The purpose of this study was to determine if serum digoxin concentration data using three different automated immunoassay methods would produce similar pharmacokinetic values in normal volunteer subjects. Area under the curve (AUC), steady-state volume of distribution/bioavailability ratio (Vd/F), terminal elimination rate constant (beta), clearance/bioavailability ratio (CL/F), maximum digoxin concentration (Cmax), minimum digoxin concentration (Cmin), and time of peak (Tp) were evaluated. Ten healthy volunteers received digoxin capsules 0.2 mg daily for 10 days. On day 10, 16 serial blood samples were collected over a 24-h dosing interval and analyzed by radioimmunoassay (RIA) (Concept 4, Micromedic Systems), fluorescence polarization immunoassay (FPIA) (TDx, Abbott Laboratories), and affinity column-mediated immunoassay (ACMIA), (aca, duPont Instruments). When comparing RIA and FPIA, the mean of the percent differences for AUC, Vd/F, beta, and CL/F were 9, 4, 10, and 6%, respectively. The mean of the percent differences were 2, 3, 44, and 6%, respectively, when comparing RIA and ACMIA. However, none of these differences were statistically significant. Although a trend toward higher Cmax values by RIA was noted, there was no statistical difference in Cmax, Cmin, and Tp. Orthogonal regression of all serum digoxin concentrations showed that FPIA = 0.76 RIA + 0.19, r = 0.967 (p less than 0.001); and ACMIA = 0.92 RIA + 0.04, r = 0.943 (p less than 0.001). At serum digoxin concentrations less than 1 ng/ml, FPIA overestimated RIA results (p less than 0.005), while ACMIA was approximately equal to the RIA results.(ABSTRACT TRUNCATED AT 250 WORDS)