Evaluation of five immunoassays for the determination of digoxin in serum

Five immunoassays for the determination of digoxin have been evaluated (Digoxin II, Abbott; Cedia Digoxin XL, Microgenics; Coat-a-Count Digoxin, Diagnostic Procedure Corporation, DPC; "On-line" Digoxin, Roche Diagnostic Systems; EMIT 2000 Digoxin, Syva). Four of them required no sample pre-treatment. The methods included a radioimmunoassay, fluoroimmunoassay, two enzyme-immunoassays and a turbidimetric immunoassay: the last three mentioned were adapted to the Cobas Mira Plus. The intra- and inter-assay precision was lower than 9%, except for Microgenics. The calibration stability fluctuated from 120 days for Abbott to 27 days for the Roche test, 7 days for the Syva assay and 2 days for Microgenics. The DPC test was not assayed for calibration stability. The interference from "digoxin-like immunoreactive factor(s)" differed according to the assay. The highest interference was seen with Abbott and Microgenics, and the lowest with the DPC test. The comparison among all the methods offered values of "r" higher than 0.95 except Microgenics and Syva assays where "r" was 0.896. The results obtained with Roche and Microgenics were higher than 12% of the remaining assays.