Comparison of 4 Direct Coombs' Test Methods with Polyclonal Antiglobulins in Anemic and Nonanemic Dogs for In-Clinic or Laboratory Use

BACKGROUND: Difficulties with the direct antiglobulin test (DAT) and its apparent lack of sensitivity and specificity for immune-mediated hemolytic anemia (IMHA) in dogs have raised skepticism regarding its diagnostic value. OBJECTIVE: To compare different DATs and other hematologic parameters in dogs. ANIMALS: Anticoagulated blood samples from 59 nonanemic and 46 anemic dogs (± IMHA) from a research colony and veterinary clinics. METHODS: Prospective observational study: Immunochromatographic strip, gel microcolumn, and capillary techniques were compared with standard microtiter DAT using 2 polyvalent antiglobulins. Spherocytosis, autoagglutination, osmotic fragility, and clinical data were assessed. RESULTS: Blood samples from all 59 nonanemic dogs were DAT-. Among 46 anemic dogs, 33 were suspected of IMHA, but only 20 were DAT+. Old and new DAT methods yielded comparable and consistent results even after storage of chilled blood samples for 1 week. Spherocytosis and autoagglutination (that did not persist after washing) were noted in 15 and 16 DAT+ dogs, respectively. The other 26 anemic dogs, including 21 previously transfused dogs and 4 with autoagglutination, tested DAT- by the other methods. Osmotic fragility was increased in 70% (19/27) of anemic and all 15 DAT+ dogs tested. Limited follow-up testing revealed DAT+ results for 3-70 days. CONCLUSIONS AND CLINICAL IMPORTANCE: The novel strip and capillary DAT methods are promising adjunct in-clinic tools. Despite prior immunosuppressive treatment and presence of autoagglutination, the DAT was positive in anemic dogs with IMHA. Transfusion did not cause false DAT+ results. Our results support DAT as a cornerstone in the diagnosis of canine IMHA.