Specific Interaction between Antidigoxin Antibodies and Digoxin-Like Immunoreactive Substances in Cord Serum

Digoxin-like immunoreactive substances (DLIS) have been quantified by two Tfferent digoxin radioimmunoassays (AlA) in 47 cord sera. The mean DLIS value (in digoxin equiva-lents) ranged from 0.960 (SD 0.184) to 0.181 (SD 0.104) nmol/L between the two different kits and different lot num-bers 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 de-creased cross reactions with DLIS. The effect of modifying the incubation conditions in RIA is similar to that described forassays 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