Marked digoxin-like immunoreactive factor interference with an enzyme immunoassay

A case in which digoxin-like immunoreactive factors (DLIF) interfered with an enzyme immunoassay in a patient with renal insufficiency is reported. A 79-year-old woman was found to have a serum digoxin concentration (SDC) determined by enzyme immunoassay of 5.0 ng/ml. Although all subsequent SDC determined by the enzyme immunoassay system were elevated, identical samples run on a fluorescence polarization immunoassay revealed SDC within the therapeutic range. Marked DLIF-related assay interference has been reported to occur with some digoxin assays; however, the enzyme immunoassay methods have never been reported to cross-react to the magnitude seen in this case.