CROSS-REACTIVITY OF TDX AND OPUS IMMUNOASSAY SYSTEMS FOR SERUM DIGOXIN DETERMINATION

The properties of the widely used TDX Analyzer and recently developed OPUS Immunoassay System were compared using 403 serum specimens taken from patients who did or did not take digoxin. Of the 210 specimens fr om patients not treated with digoxin, a false-positive digoxin concent ration was detected in 15 specimens (7%) by TDX and in only 2 specimen s (1%) by OPUS because of the cross-reactivity with structurally simil ar drugs. Potassium canrenoate, digitoxin, deslanoside, and methyldigo xin exhibited masked concentration-dependent cross-reactivity in the T DX assay method, whereas deslanoside and methyldigoxin only showed cro ss-reactivity with the antibody used in the OPUS method. Although a po or correlation was observed between these two methods for the determin ation of 193 samples from patients treated with digoxin, the correlati on was remarkably improved (r = 0.914) and the slope approximated unit y when excluded the data from patients who were treated concurrently w ith the cross-reactive compounds. Tn routine TDM of digoxin, the autho rs experienced two cases in which cross-reactivity of the assay system caused a clinical problem. Concurrent administration of intravenous c anrenoate apparently interfered with the digoxin assay by TDX, but thi s problem was solved by using the OPUS system. The authors found OPUS more useful for monitoring serum digoxin concentrations in patients be cause of its superior specificity.