Comparison of four test kits for feline leukemia virus antigen

The sensitivity and specificity of 4 commercial FeLV ELISA kits, using blood, were compared with results of virus isolation from blood and immunofluorescent antibody (IFA) testing on blood. Significant differences were not found among the 4 ELISA kits. Marked decrease in sensitivity of the ELISA kits was detected when virus isolation was used as the standard of positivity rather than the IFA test. Virus isolation was a more sensitive indicator of early infection, with marked discrepancy among results obtained by virus isolation, ELISA, and the IFA test. Results became progressively more concordant as infection became fully established. Cats FeLV-positive by virus isolation alone were more likely to eliminate viremia. All cats FeLV-positive by IFA testing remained persistently viremic. Virus isolation, ELISA, and IFA testing appear to differ in their prognostic value. The use of blood rather than serum for the ELISA resulted in several discordant results. Six cats were FeLV-positive by ELISA when blood was tested but were FeLV-negative when serum was tested. Positive ELISA results were obtained for 4 of these cats when serum was tested, using extended incubation to increase sensitivity. It is possible that blood may actually be more sensitive than serum for use of the ELISA method.