Falsely increased digoxin concentrations in samples from neonates and infants.

Several previous studies have suggested the presence of a digoxin-like immunoreactive substance in the serum of infants. In this study we examined eight different immunoassay kits to confirm the presence of such a substance. The kits were evaluated using serum from newborns and infants who were not receiving digoxin. We investigated the relationship of this digoxin-like immunoreactive substance to substances that are present in unusual concentrations in newborn sera. These substances included fatty acids, cholesterol, triglycerides, bilirubin, protein, and albumin. No relationship could be established. Since serum steroid concentrations are known to be increased in the newborn and since digoxin is essentially a steroid derivative, we looked at steroids separately and collectively to see whether they mimicked the immunoreactivity of digoxin. We found that pooled steroids did indeed mimic digoxin and might be implicated as the immunoreactive substances.