Predictive performance study of two digoxin assays in subjects with various degrees of renal function

This prospective study was conducted to compare the predictive performance of fluorescence polarization immunoassay (FPIA, Abbott TDx Digoxin II) and radioimmunoassay (RTA, Kallestad Labs) with combined low-pressure liquid ch romatography/RIA (LPLC/RIA) digoxin assay in measuring 15-17 serum digoxin concentrations (SDC) obtained after a single 10 mug/kg intravenous digoxin dose in patients with various degrees of renal function and at different SD C ranges. Eighteen men and women were stratified into 3 age- and gender-mat ched groups based upon renal function [N = 6 in each, group I(Cl-cr < 10 mL /min), group II (Cl-cr = 10-50 mL/min), and group III (Cl-cr > 50 mL/min)]. Serum digoxin concentrations were measured at time zero; at 0.25, 0.5, 0.7 5, 1, 2, 3, 4, 6, 8, and 12 hours; and at 2, 3, 4, and 5-7 days after the d igoxin dose, using the three different digoxin assays. TDx Digoxin II was u nbiased [mean error -0.09 (95% CI -0.19, 0.01)] and RIA biased [mean error -0.29 (95% CI -0.36, -0.21)] to over-predict SDC by 14.2%. In group I patie nts, the analysis revealed a bias to over-predict SDC by 6.0% for TDx Digox in II [ mean error -0.16 (95% CI -0.29, -0.07)] and an unbiased performance by RIA. In groups IT and III, both TDx Digoxin II and RIA showed biased pe rformance, the mean magnitude of bias was low (< 20%). For intermediate SDC range (> 0.5 ng/mL and less than or equal to 3.0 ng/mL), TDx Digoxin II wa s unbiased in predicting SDC, whereas RIA was biased to under-predict SDC [ mean error 0.13 (95% CI 0.10, 0.16)] by 9.9%. The magnitude of bias observe d in all cases was less than 20%. Both assays, TDx Digoxin II and RIA, impr ecisely measured SDC for all samples combined, different groups and SDC ran ges. In all time-paired samples, TDx Digoxin II (FPIA) performed better tha n the RIA. In conclusion, the magnitude of bias observed with either assay at different groups and SDC ranges was not likely to be clinically relevant . Therefore, either assay may be used to measure SDC in clinical practice.