EXPRESS: Multiplex Fluorescent Immunocytochemistry is a Complementary Test for the Diagnosis of Feline Infectious Peritonitis: A Diagnostic Clinical Trial

OBJECTIVES: Feline infectious peritonitis (FIP) is a common and treatable viral disease of cats for which there is no single, reliable antemortem diagnostic test. The goals of this report were to evaluate the diagnostic accuracy (sensitivity, specificity, and positive and negative predictive values) of multiplex fluorescent immunocytochemistry (MF-ICC) for the diagnosis of FIP in clinical patients under field conditions, and to directly compare that accuracy to the diagnostic performance of other commonly used assays. METHODS: Cats presenting with clinical signs compatible with FIP were enrolled at two academic veterinary teaching hospitals and tested using a combination of CBC, biochemistry, fluid analysis, cytology, MF-ICC, serology, and RT-PCR. Antibodies against both feline coronavirus and vimentin, as well as a nuclear stain, were used for the MF-ICC assay. Outcomes were determined by necropsy with histopathology and IHC, response to antiviral therapy, and/or clinical follow-up. RESULTS: 84 cases comprising 58 cats with FIP and 26 without FIP (control cases) were included in this study. Using a threshold of ≥ 1 dual-expressing mononuclear cell, MF-ICC was found to have 77% sensitivity, 81% specificity, 92% positive predictive value, and 53% negative predictive value in this cohort. Using a threshold of ≥ 2 dual-expressing mononuclear cells improved specificity but reduced both sensitivity and overall diagnostic accuracy. MF-ICC was more sensitive, but less specific, in effusion fluids over tissues. MF-ICC resulted in the highest overall diagnostic accuracy (78%) when compared with serology (75%), RT-PCR (69%), and serum A:G ratio (76%). CONCLUSIONS AND RELEVANCE: MF-ICC is an imperfect test but is considered complementary to other commonly used antemortem diagnostic assays for FIP. It can be used alongside RT-PCR and other minimally invasive tests to build a case for FIP in an individual patient.