Endogenous substance in newborn infants causing false positive digoxin measurements.

We report the detection of a digoxin-like immunoequivalent substance in the plasma of neonates and infants and in amniotic fluid. Time-course studies in individual infants indicate that the substance probably is produced endogenously and is not exclusively retained by the infants from external sources. Digoxin recovery studies demonstrated that the presence of this material caused falsely elevated digoxin values. Our results cast considerable doubt on the reliability and clinical utility of digoxin radioimmunoassay measurements on the serum or plasma of neonatal and infant patients.