Effects of digoxin immune Fab (ovine) on digoxin immunoassays

Digoxin immune Fab (ovine) (Digibind) is a preparation of Fab fragments of antidigoxin antibodies that is used as an antidote for severe digitalis toxicity. To assess the effects of its presence on digoxin immunoassays, specimens were prepared that simulated typical samples from treated patients and were distributed to a number of laboratories for analysis as unknowns. The antidote interfered to a varying extent with all assays reported. The results suggest that most competitive immunoassays will yield results that bear little relation to concentrations of total digoxin or free digoxin (i.e., digoxin not bound to the antidote). The aca, Dimension, and CEDIA sequential immunoassays moderately overestimated the free digoxin concentration. Free digoxin levels appeared to be only slightly overestimated by the Aria, Stratus, and Syva assays. The TDx Digoxin II assay yielded slight to moderate underestimates of the total digoxin concentration.