Interference of endogenous digoxin-like immunoreactive factors in serum digoxin measurement is minimized in a new turbidimetric digoxin immunoassay on ADVIA 1650 analyzer.

Endogenous digoxin-like immunoreactive factors (DLIF) cross-react with antidigoxin antibody and falsely elevate or lower measured serum digoxin concentrations, depending on the assay design. Recently, Bayer Diagnostics released a turbidimetric assay for digoxin on the ADVIA 1650 analyzer. We studied potential interference of DLIF with this new digoxin assay. We analyzed 40 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 five specimens from autopsy and five specimens from neonates. Apparent digoxin concentrations were measured using the new turbidimetric digoxin assay, the fluorescence polarization immunoassay (FPIA, Abbott Laboratories, Abbott Park, IL), and also the chemiluminescent immunoassay (CLIA, Bayer Diagnostics). We observed measurable apparent digoxin levels with the FPIA in 5 uremic patients (range 0.24-0.86 ng/mL), 6 patients with liver disease (range 0.21-0.72 ng/mL), in 3 patients in the third trimester of pregnancy (0.21-26 ng/mL), and in 3 neonates (range 0.21-0.46 ng/mL). Four out of 5 autopsy specimens showed measurable apparent digoxin concentrations (0.23-0.81 ng/mL). In contrast, only 1 specimen (a uremic patient) showed an apparent digoxin concentration of 0.26 ng/mL with the turbidimetric digoxin immunoassay (FPIA value 0.86 ng/mL, CLIA value 0.32 ng/mL). Because DLIF is absent in the protein-free ultrafiltrate, we also measured free digoxin concentrations in DLIF-positive patients to ensure that the apparent digoxin concentrations were caused by DLIF. We observed no apparent digoxin concentrations in the protein-free ultrafiltrate in any DLIF-positive specimens. When serum specimens containing elevated concentrations of DLIF but no digoxin were supplemented with a known concentration of digoxin, we observed falsely elevated digoxin concentrations by the FPIA, as expected. In contrast, we observed a good agreement between the target and observed concentrations when the new turbidimetric assay was used. We conclude that DLIF has minimal effect on serum digoxin measurements by the new turbidimetric assay.