Development and clinical application of methods for detection of antineutrophil antibody in serum of the cat

Two techniques, leukoagglutination and indirect immunofluorescence, were adapted to test for the presence of antineutrophil antibody in cat serum. The leukoagglutination test was analogous to an indirect Coombs' test. The test was performed on freshly isolated cat blood neutrophils, with the test results read from stained smears (Wright's stain) made from sedimented antiserum-treated neutrophils. A positive test response was indicated by agglutinated neutrophils on the stained smear. The indirect immunofluorescence test was performed by incubating paraformaldehyde-fixed cat blood neutrophils with test serum, after which the neutrophils were stained with fluorescein isothiocyanate-tagged antiglobulin. A positive test response was a ring of fluorescence surrounding the cells, as viewed through a UV microscope. Serum samples (n = 55) from clinically neutropenic cats were tested for the presence of antineutrophil antibody by the indirect immunofluorescence technique. Ten positive-control sera (rabbit anti-cat neutrophil serum) and 10 negative-control sera (normal cat serum) were included. Only the positive control sera exhibited neutrophil fluorescence, indicative of antineutrophil antibody. None of the 55 samples of clinical origin showed any appreciable fluorescence.