Specific interaction between antidigoxin antibodies and digoxin-like immunoreactive substances in cord serum

Digoxin-like immunoreactive substances (DLIS) have been quantified by two different digoxin radioimmunoassays (RIA) in 47 cord sera. The mean DLIS value (in digoxin equivalents) ranged from 0.960 (SD 0.184) to 0.181 (SD 0.104) nmol/L between the two different kits and different lot numbers of the reagents. One of the RIA methods showed an obvious lot-to-lot effect. The use of a longer incubation interval and a higher incubation temperature markedly decreased cross reactions with DLIS. The effect of modifying the incubation conditions in RIA is similar to that described for assays of steroids because the dissociation rates of the immunocomplex play a critical role. Data suggest a specific interaction between DLIS and digoxin antibodies. Control of the incubation conditions is recommended, to decrease or increase the amount of the DLIS in cord serum specimens.