Comparison and interpretation of diagnostic tests for feline immunodeficiency virus infection

Abstract

Summary Feline sera were submitted to the Cornell Feline Health Center (n = 497) or to the New York State Diagnostic Laboratory (n = 1,565) for feline immunodeficiency virus (fiv) testing. Some sera (n = 166) were submitted for confirmation of previous fiv-positive results; 151 of these sera had been tested at the referring veterinary practice or laboratory, using an in-house elisa. Excluding the samples submitted for confirmation, a total of 173 samples (9.1%) were fiv-positive; 11.6% of the clinically ill or high-risk cats and 0.49% of the healthy, low risk cats were positive for fiv antibody. A commercially available elisa for detection of antibody to fiv was evaluated in relation to the immunofluorescent antibody (ifa) test and the immunoblot assay. The elisa was interpreted according to the manufacturer's instructions, with the ratio of sample optical density to positive control optical density (s/p) determining a positive or negative result. The elisa results based on the s/p interpretation were compared with a kinetics-based (kela) interpretation of the elisa. The kela values were reported as positive, negative, or equivocal. Using the immunoblot as the standard, elisa (s/p interpretation) had sensitivity of 0.93 and specificity of 0.98, whereas the ifa test had sensitivity of 0.95 and specificity of 0.98. However, the sensitivity and specificity of the elisa (s/p interpretation) were markedly reduced for sample results falling in the kela equivocal range, indicating that equivocal results were valid interpretations for some sera. A high number (22.5%) of the samples submitted for confirmation of a positive result from use of the in-house elisa were determined to be negative for fiv antibody. Operator error or incorrect interpretation of the in-house elisa were thought to be the cause of most of these false-positive test results.