Interference from digoxin-like immunoreactive substance(s) in commercial digoxin kit assay methods

We have examined the current state of interference by digoxin-like immunoreactive substance(s) (DLIS) in 10 commercially available digoxin assay methods, ie 5 radioimmunoassays (RIA) and 5 non-radioactive immunoassays. Fifty-five specimens of maternal venous blood (30 from third trimester pregnancy and 25 at delivery) and 32 cord samples from their offspring were tested (none of the subjects being medicated with digoxin). The results demonstrated a wide range of DLIS interference with values up to 1.1 micrograms.l-1 being obtained in the cord blood specimens. Of the methods tested the "Coat-a Count" RIA (Diagnostic Products Corporation) and the EMIT column method (Syva) run on the Cobas MIRA (Roche) consistently showed the least interference with respect to all 3 sources of specimens. The Delfia Method (LKB/Wallac) consistently showed the greatest crossreactivity with DLIS. The present study thus demonstrates that DLIS interference persists in several commercial digoxin methods and suggests that the use of data obtained from such methods may compromise patient management.