False-positive reactions associated with anti-mouse activity in serotests for feline leukemia virus antigen

False-positive reactions were observed in commercial test kits designed to detect FeLV infections in cats. The false-positive reactions were associated with heterophilic antibody in cat serum directed primarily against mouse immunoglobulins. Of 2,830 sera tested, 579 were considered FeLV positive or equivocal via ELISA; of these, 10 were found that did not have FeLV antigen but did have anti-mouse antibody. With 95% confidence, 0.14% to 0.57% of cats would have such false-positive reactions in ELISA, because one in 283 (0.35%) cats had anti-mouse antibodies.