Objectives: Comparison of a new monoclonal digoxin assay with three polyclonal digoxin assays for their cross-reactivity to digoxin-like immunoreactive factors (DLIF) and digoxin metabolites. Design and Methods: Sixty-six nondigitalized patient samples from 5 different groups: neonates, women in 3rd trimester pregnancy, and patients with liver or renal diseases, or undergoing organ transplants, and 139 samples from digoxin-treated patients of 4 categories (hospital sick, liver, renal, and outpatients) were compared in 4 different digoxin assays: (a) ACS(TM) Digoxin (ACS) developed for the automated chemiluminescent Ciba Corning ACS 180® system, (b) Baxter Stratus(TM) (Stratus, a fluoroimmunoassay), (c) Ciba-Corning Magic(TM) (Magic, a radioimmunoassay), and (d) an in-house radioimmunoassay (RIA). The ACS(TM) and RIA were also compared for their cross-reactivity to four principal digoxin metabolites. Results and Conclusion: Among the nondigitalized specimens, no significant DLIF interference was found for all 4 assays among the pregnant women or liver and transplant patients. However, the neonates registered high DLIF interference with Magic(TM) and RIA, but none for ACS(TM) or Stratus(TM). DLIF interference in renal samples was highest in the Magic assay and lowest in RIA. Among the specimens with digoxin, a higher number of discrepant samples were found from the sick patients than from outpatients. In 75% of such discrepant samples, the ACS(TM) result was less than other assay results, suggesting DLIF as the probable cause. The two assays differed most in their cross-reactivity to the deglycated metabolites, digoxigenin and its mono-digitoxoside.
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