Analytic performance of two automated nonpretreatment digoxin immunoassays

The analytic performance of two automated nonpretreatment digoxin methods, AxSYM Digoxin II and Vitros digoxin immunoassays, was assessed. Both assays had analytic sensitivities of less than 0.2 mu g/L, were linear from digox in concentrations of 0.5 to 4.0 mu g/L, and showed acceptable precision, wi th a maximum total coefficient of variation (CV) of 8.9% and 6.4% for the A xSYM and Vitros, respectively. Comparison of the two methods using samples from patients receiving digoxin gave the following relationship: Vitros = 0 .91 x AxSYM + 0.23 (r = 0.97, S-y,S-x = 0.12). Digoxinlike immunoreactive f actor (DLIF) crossreactivity was examined in specimens from patients who ha d hepatic disease, renal insufficiency, had undergone cardiac surgery, and in neonatal cord blood samples. Minimal crossreactivity was observed for mo st samples and the average crossreactivity for each group of samples was co mparable for the two methods. The recovery of digoxin added to samples from each group of DLIF was similar, except for that from cord blood samples, f or which recovery was significantly lower with the AxSYM method. Titration of a digoxin-spiked serum pool with digoxin-immune Fab showed a similar dec rease in the measured digoxin concentration for both methods. Overall, the analytic performance characteristics of these two methods were comparable.