Falsely elevated digoxin level of 45.9 ng/mL due to interference from human antimouse antibody

We report the case of a 77-year-old man who was admitted to the intensive care unit with a serum digoxin level of 45.9 ng/mL. The patient was hemodynamically stable throughout his hospital course and did not require antidigoxin antibody fragments. The elevated digoxin level was determined by subsequent testing to be falsely elevated by interference from human antimouse antibodies in his serum. A repeat digoxin measurement using an assay not affected by human antimouse antibodies indicated a level of 1.3 ng/mL. Newer digoxin assays are not affected by human antimouse antibody interference, but clinicians should be aware of possible human antimouse antibody interference with older digoxin assays and in other tests utilizing mouse monoclonal antibody reagents.