Digoxin-like immunoreactivity in normal human plasma and urine, as detected by a solid-phase radioimmunoassay

Reports on the presence of digoxin-like immunoreactive substance(s) (DLIS) in the plasma and urine of several animal species and a few human test subjects prompted us to undertake to confirm the presence of DLIS in plasma and urine of normal persons and to investigate some characteristics of this antibody-DLIS binding. For this purpose we used a modified radioimmunoassay kit involving antidigoxin antibody-coated test tubes and 125I-labeled digoxin to measure DLIS in urine and concentrated plasma of ostensibly healthy subjects. In 12 separate experiments with plasma the mean sensitivity was 5.12 (SD 1.11) pg per tube, expressed as digoxin. There was no significant cross reactivity with human serum albumin in concentrations up to 200 g/L. The mean DLIS value (in digoxin equivalents) for plasma from 24 normal subjects was 33.58 (SD 14.24) ng/L. Its mean concentration in urine from five subjects was 315.00 (SD 91.38) ng/L.