Effect of therapeutic digoxin antibodies on digoxin assays

The introduction of therapeutic digoxin antibodies in the treatment of digoxin toxicity has resulted in spurious results in the many assays of cardioglycoside in serum of patients receiving this therapy. An evaluation of currently used assay methods was incorporated into a pilot Ligand Assay Survey Program of the College of American Pathologists to define which assays were unaltered by the presence of a therapeutic digoxin antibody (Digibind). Disparity of affinity constants of the therapeutic and kit antibodies is the most likely explanation for erroneous values. Specific methods are defined that appear to be free of this phenomenon.