In Vivo Diagnosis of Feline Infectious Peritonitis by Comparison of Protein Content, Cytology, and Direct Immunofluorescence Test on Peritoneal and Pleural Effusions

The diagnosis of feline infectious peritonitis (FIP) is hampered by the characteristic etiopathogenesis of the disease.9 The coronavirus responsible for the disease (FIPV) originated by a mutation in the widespread feline enteric coronavirus (FECV).9,14 These coronaviruses are both phenotypically and genotypically identical, as demonstrated by polymerase chain reaction (PCR).14 Both feline coronaviruses (FCoVs) are able to trigger the production of antibodies. High antiFCoV titers are also detectable in healthy cats in FECVendemic catteries,9,10 and the formation of immune complexes could results in negative serology in symptomatic cats.8 Furthermore, false-positive results occur from cross-reactivity with coronaviruses of other species, such as canine coronavirus (CCV) or transimissible gastroenteritis virus (TGEV), or with extraneous noncoronavirus proteins that are copurified with the FCoV and included in commercially available diagnostic enzyme-linked immunosorbent assay kits.1 Hematology, serum chemistry, and serum protein electrophoresis give only a strongly suggestive but nonpathognomonic pattern of FIP.9,12,13 In the effusive forms, diagnosis could be improved by cytology3 and protein analysis of the fluid.9,11–13 Although FCoV RNA has been demonstrated in the plasma of healthy cats in FECV-endemic catteries,6 FCoVs detected from areas other than the intestinal lumen should be interpreted as FIPV, particularly if the viruses are detected in lesions15 or effusions.2 In this study, the results obtained using 3 different diagnostic methods (anti-FCoV direct immunofluorescence test, cytology, and protein analysis) for 110 cats with effusions over a 5-year period were compared to determine the utility of these methods for diagnosing FIP from analysis of the effusion alone. Cats with clinically detectable effusion were examined (Table 1), although in some cases clinical and laboratory data (serum chemistry, ultrasonography, macroscopic appearance of the effusion) suggested diseases other than FIP. Approximately 2 ml of effusion was withdrawn from each cat before death (72 cats) or within 2 hours after death (38 cats). The sampled fluid was put in a tube containing ethylenediamine tetraacetic acid. The protein content of the effusions of 51 cats was evaluated in a discrete autoanalyzera by the biuret method.b Fifty to 100 ml of fluid was cytocentrifugedc within 15 hours at 130 3 g for 10 minutes. Two