Inaccuracies in digoxin measurement

Six commercial digoxin immunoassay kits were evaluated for their accuracy of calibration and their extent of interference by digoxin-like immunoreactive substance (DLIS). Calibration accuracy was investigated with digoxin reference standards in pooled human serum. The Abbott and Becton Dickinson kits underestimate while the other kits overestimate digoxin concentration. The magnitude of this bias generally increases with increasing concentration of digoxin. Sera from digoxin-free patient populations with potential DLIS interference--pregnant women, newborns, hypertensives, and uremics--were analyzed with each kit. Healthy subjects not on digoxin therapy served as controls. Groups with DLIS interference, as exemplified by a significant difference of p less than 0.05 from controls, are: Abbott--newborns and pregnant women; Becton Dickinson--newborns and pregnant women; Dade--no difference; Dupont--newborns, uremics, pregnant women, and hypertensives; Kallestad--newborns; and Syva--newborns. The limitations of each individual digoxin method should be realized for DLIS interference and bias, and patient results from that method should be interpreted accordingly.