Multiplex fluorescent immunocytochemistry is a complementary test for the diagnosis of feline infectious peritonitis: a diagnostic clinical trial.

ObjectivesFeline infectious peritonitis (FIP) is a common and treatable viral disease of cats for which there is no single, reliable ante-mortem diagnostic test. The goals of this study 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.MethodsCats presenting with clinical signs compatible with FIP were enrolled at two academic veterinary teaching hospitals and tested using a combination of complete blood count, 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 immunohistochemistry, response to antiviral therapy and/or clinical follow-up.ResultsA total of 84 cases comprising 58 cats with FIP and 26 without FIP (control cases) were included in this study. Using a threshold of one dual-expressing mononuclear cell or more, 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 two or more 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 albumin:globulin ratio (76%).Conclusions and relevanceMF-ICC is an imperfect test but is considered complementary to other commonly used ante-mortem 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.