Analytical performance of a monoclonal digoxin assay with increased specificity on the ACS:180

Digoxin metabolites cross-react in the Ciba Coming ACS digoxin assay in proportion to their bioactivity, but have greater (near 100%) crossreactivity in the Abbott TDx, Baxter Stratus, and Ciba Coming Magic RIA digoxin assays. We studied the analytical performance of the ACS digoxin assay and compared it with these other assays. Coefficients of variation ranged from 5.5% at 3.11 ng/ml to 8.8% at 0.57 ng/ml. Mean analytical recovery was 96.4%. Results on dilutions were linear in the range of 0.6-5.0 ng/ml. We observed no interference by hemoglobin, bilirubin, or triglycerides. Dihydrodigoxin and digitoxin had lower cross-reactivity in the ACS and Stratus assays than in the TDx and Magic assays. Digoxin-like immunoreactive factor (DLIF) in patients' sera was not detected in the ACS assay but was in the TDx, Stratus, and Magic assays. Digibind therapy seemingly did not affect digoxin results by ACS or Stratus, but did for up to 10 days after therapy for TDx and Magic. We compared digoxin results for 121 sera from 49 patients. Deming regression analysis was performed on the first specimen from each patient: ACS = 1.08(TDx) - 0.17 ng/ml (r = 0.961, Sy,x = 0.164); ACS = 1.16(Stratus) - 0.46 ng/ml (r = 0.973, Sy,x = 0.123); ACS = 1.00(Magic) - 0.20 ng/ml (r = 0.982, Sy,x = 0.110). Discrepant results (>2Sy,x from the regression line) were usually lower by the ACS assay (87%). Nine of 11 patients with discrepant results had renal insufficiency or hepatic disease, conditions commonly associated with increased DLIF. These observations may be explained by the improved specificity of the ACS digoxin assay.