Variable cross-reactivity of digoxin metabolites in digoxin immunoassays

The authors investigated the cross-reactivity of the major known digoxin metabolites--digoxigenin, digoxigenin monodigitoxoside, digoxigenin bisdigitoxoside, and dihydrodigoxin--and of digitoxin in three 125I-radioimmunoassays and one enzyme immunoassay for digoxin. Digitoxin and dihydrodigoxin exhibit low cross-reactivity and nonparallel dilution responses for these assays. The cross-reactivities of the other three substances are significant for all assays studied with digoxigenin and monodigitoxoside having nonparallel and enhanced tracer displacement compared with digoxin itself. The authors demonstrate that because of nonparallel tracer displacement estimates of cross-reactivity calculated by the 50% displacement method fail to adequately predict the error induced in digoxin assays by digitoxin. They conclude that digoxin metabolites in serum are measured to various extents as the parent digoxin compound by all of the immunoassays they studied. In view of the varying biologic activity of digoxin metabolites and the large patient to patient variations in digoxin metabolism, the cross-reactivities the authors observe may help to explain the discrepancies in correlation of clinical response to measured serum digoxin values reported in other studies.