Digoxin assay anomalies due to digoxin-specific Fab immunotherapy

We demonstrate that digoxin-specific Fab antibody fragments, used in the treatment of severe digitalis intoxication, cause a marked interference with both fluorescence excitation transfer immunoassays and radioimmunoassays for digoxin. In addition, we describe a rapid ultrafiltration method that avoids the Fab-induced interference and affords excellent accuracy and precision. This technique will provide a useful adjunct to the clinical evaluation of patients with digoxin toxicity who are recipients of antidigoxin immunotherapy by enabling a precise determination of their serum digoxin concentrations. By separating the free and protein-bound digoxin this technique also may be useful in evaluating serum digoxin concentrations in patients with abnormal plasma protein concentrations.