A physician's office-based digoxin assay (Seralyzer) evaluated for interference by endogenous digoxin-like immunoreactive factors

A digoxin test for a physician's office based-chemistry analyzer (Ames Seralyzer) was evaluated for possible interference by digoxin-like immunoreactive factors (DLIF). Sera from patients likely to have high concentrations of DLIF (renal and hepatic patients, pregnant women, and neonates) as well as from normal patients and umbilical cord blood were analysed by the Seralyzer digoxin immunoassay and by a fluorescence polarization digoxin immunoassay (Abbott TDx) known to detect DLIF. For all patients who were not taking digoxin (n = 85) only four patients (4.7 percent) measured apparent digoxin values greater than 0.2 ng per mL by the Seralyzer compared to 64 (75 percent) by the TDx analyzer. Measurements of DLIF from adrenal extracts demonstrated a 17-fold greater potency for detection of DLIF by the TDx (2.9 ng per mL) compared to the Seralyzer technique (0.18 ng per mL). However, recovery data suggest that the presence of digoxin reduces the potency of DLIF interference as a function of increasing digoxin concentrations especially for the TDx assay. This diminished DLIF crossreactivity in the presence of digoxin is one explanation for the comparable correlation observed for both non-renal and renal failure patients taking digoxin when measured by these two immunoassays.