Endogenous
digoxin-like immunoreactive factors (DLIF) may crossreact with antidigoxin antibody and falsely elevate immunoassay results. Recently, a new
enzyme-linked
immunosorbent chemiluminescent assay for
digoxin has been available for use on the ADVIA IMS (Integrated Modular System) 800i analyzer (Bayer Diagnostics). We studied potential interference of DLIF with this new
digoxin assay. We analyzed 30 serum specimens from patients who have pathologic conditions that may increase serum DLIF concentrations. These patients were never exposed to
digoxin or other agents that may lead to a measurable
digoxin concentration. We also analyzed 10 specimens from neonates, 10 cord blood specimens, and 10 amniotic fluid specimens. Apparent
digoxin concentrations were measured using the new
enzyme-linked
immunosorbent digoxin assay (IMS-
Digoxin), a fluorescence polarization immunoassay (FPIA), and also a chemiluminescent immunoassay (CLIA, run on ACS:180(R) system from Bayer Diagnostics). We observed measurable apparent
digoxin levels with the FPIA in 4 uremic patients (range 0.21-0.36 ng/mL,
digoxin equivalent), 7 patients with
liver disease (range 0.21-0.72 ng/mL), and 3 patients in the third trimester of pregnancy (0.22-0.66 ng/mL). We also observed measurable DLIF concentrations with the FPIA in 2 neonates (0.22 and 0.36 ng/mL), 5 cord blood specimens (range 0.21-1.18 ng/mL), and 5 amniotic fluid specimens (0.21-0.50 ng/mL). None of these DLIF-positive specimens showed any measurable
digoxin concentration using the IMS-
Digoxin or the CLIA assay. When serum specimens containing elevated concentrations of DLIF but no
digoxin (as measured by FPIA) were supplemented with known concentrations of
digoxin, we observed falsely elevated
digoxin concentrations, as expected, only by the FPIA. In contrast, we observed a good agreement between the target and observed concentrations when the new IMS-
Digoxin or the CLIA assay was used. We conclude that the IMS-
Digoxin assay is free from interference of DLIF.