Radioimmunoassay of digoxin in serum using monoclonal antibodies and assessment of interference by digoxin-like immunoreactive substances.

We used 7 monoclonal antibodies (MoAbs) and one polyclonal antibody to develop radioimmunoassays (RIAs) for digoxin in serum or plasma. These RIAs were tested for measuring apparent digoxin concentrations in serum from patients receiving the drug, from normal individuals, and in cord blood plasma. We found that two MoAbs cross-reacted significantly with substances in cord blood. The magnitude of cross-reactivity was dependent on the incubation time and temperature. Under equilibrium conditions, one antibody gave apparent digoxin values in cord blood plasma averaging 2.15 ng/ml. We suggest that this cross-reactivity is partially due to progesterone and 17-hydroxyprogesterone in cord blood plasma. The antibody that shows high cross-reactivity with digoxin-like immunoreactive substances may prove a useful tool for studies dealing with characterization of the cross-reacting compounds.