Coombs Testing: A Diagnostic Step for Anemic Cats

Anemia is a common medical problem in cats. There are many medical causes, including but not limited to infections, neoplasia, drug reactions, immune-mediated diseases, and idiopathic immune hemolytic anemia (IMHA). In this study, Coombs testing at 4˚ C and 37˚ C and polymerase chain reaction (PCR) testing for 3 species of hemoplasma, feline immunodeficiency virus (FIV), and feline leukemia virus (FeLV) were performed on 60 cats with anemia and 60 cats without anemia. Of the 120 cats, a definitive diagnosis was not determined in 14 cats and 1 was found to be healthy. Thirteen cats were PCR-positive for Candidatus Mycoplasma haemominutum (3 of 13 were anemic), 2 cats were FIV positive (1 of 2 were anemic), and 8 were FeLV positive (2 of 8 were anemic). Of the 120 cats, 13 cats (all anemic) had persistent autoagglutination in phosphate-buffered saline at 4˚ C; Coombs titers could not be determined. Nine of these cats had IMHA, 2 had neoplasia, and 2 had infectious diseases. Persistent autoagglutination was not seen at 37˚ C. Of the remaining 47 anemic cats for which Coombs testing was possible, 19 (40%) had a positive test result at 4˚ C or 37˚ C. Of the 60 nonanemic cats, 12 (20%) had a positive test result at 4˚ C or 37˚ C. Persistent autoagglutination at 4˚ C was significantly more common in anemic cats and in cats with autoimmune hemolytic anemia (AIHA); a positive Coombs result at 37˚ C was significantly more common in cats with AIHA. Positive titers on Coombs tests were more common in 40% of anemic cats but were also found in 20% of cats without anemia.

Commentary: This study elucidates some important points about AIHA in cats. AIHA occurs with some frequency, and a Coombs test at 4˚C (“cold” agglutination) is a good diagnostic test for AIHA in anemic cats. The presence of nonanemic, sick cats with a 20% incidence of Coombs positivity demonstrates low Coombs sensitivity in this population. Additional diagnostic methods should be used to support an AIHA diagnosis if test results and clinical signs are not concordant. There was a 19% incidence of infectious causes of illness in this population; 11% of all cats tested positive for Mycoplasma species, 1.7% tested positive for FIV, and 6.7% tested positive for FeLV. Positive Coombs tests may occur with infectious disease, neoplasia, AIHA, and, interestingly, with pancreatitis. Autoagglutination was identified in cats with AIHA, neoplasia, and infectious disease. If a sick, anemic cat is not autoagglutinating, a Coombs test is a reasonable diagnostic step to ascertain a definitive diagnosis of either primary or secondary feline AIHA.—Lisa Powell, DVM, Diplomate ACVECC

Coombs, haemoplasma, and retrovirus testing in feline anaemia. Tasker S, Murray JK, Knowles TG, Day MJ. J SMALL ANIM PRACT 52:191-199, 2010.