Methods for eliminating interferences in digoxin immunoassays caused by digoxin-like factors.

Endogenous, circulating digoxin-like immunoreactive factors (DLIF) are known to cross react with many antisera used in digoxin assays, complicating the quantification of serum digoxin. We have explored ways to decrease or remove the interference from DLIF in such assays. Prolonging the assay incubation from 30 to 60 min decreased apparent digoxin concentrations (attributable to DLIF) by an average of 68% in the digoxin radioimmunoassays studied. The serum protein-binding of DLIF was also exploited to remove them from serum. Ultrafiltration, performed as part of a simple centrifugation step, removed approximately 90% of the DLIF. Analytical-recovery studies with true digoxin also demonstrated ultrafiltration to be an adequate method (greater than 95% of digoxin was recovered) for routine clinical use. Heat- or acid-precipitation of protein removed DLIF less effectively. Appropriate incubation times and ultrafiltration can dramatically minimize or eliminate DLIF interference in digoxin immunoassays.