Seven different digoxin immunoassay kits compared with respect to interference by a digoxin-like immunoreactive substance in serum from premature and full-term infants.

Seven different digoxin immunoassay kits showed cross reactivity with an endogenous digoxin-like immunoreactive substance consistently present in serum of neonates, whether premature or full-term. The degree of interference, in decreasing order was: NML greater than New England Nuclear greater than Bio-Rad greater than Clinical Assays greater than Becton Dickinson greater than Serono greater than Syva (EMIT). More recently purchased NML kits showed less sensitivity to the substance, evidently reflecting lot-to-lot differences in antibody. A single baseline determination of the substance before digoxin is administered inadequately compensates for this interference, because the interferent concentrations can differ from day to day, with evidence that it may be most concentrated on the fourth to sixth postnatal day. Its concentration in the serum of neonates is unrelated to the concentration of dehydroepiandrostenedione sulfate, an indicator of fetal adrenal-cortical activity.