Avoid the incorrect forms Coomb and Coomb's. a test routinely performed in cross-matching blood or in the investigation of transfusion reaction: test for patient's serum is incubated with a suspension of donor erythrocytes; if specific antibodies are present, they become attached to the antigen in the donor cells; after a washing with saline, Coombs antihuman globulin is added; agglutination at this point indicates that antibodies present in the original test serum had indeed become attached to donor erythrocytes.