Whereas the classic wet form of FIP is usually easy to diagnose, the dry form of FIP can present a difficult diagnostic challenge. Although there is no single sufficiently reliable test to confirm an ante-mortem diagnosis of FIP, a combination of clinical signs, routine laboratory testing, evaluation of body fluids, serology, and PCR testing can give a high clinical index of suspicion for FIP, with confirmation of the diagnosis obtained at necropsy.

The most common signs in young cats are non-antibiotic responsive fever, lethargy, and stunted growth. Cats with the wet form of FIP develop ascites and/or pleural effusion that may cause abdominal distention or tachypnea. The dry form of FIP is associated with granulomas in the kidney, liver, lymph nodes, central nervous system, and eyes. Cats with dry FIP often present with ocular or neurologic signs.

Hematologic changes are non-specific. Many cats with FIP have anemia of chronic disease, neutrophilic leukocytosis, and lymphopenia. About 2/3 of cats have hyperglobulinemia with polyclonal gammopathy on serum protein electrophoresis. Hyperbilirubinemia and liver enzyme elevation are often seen.

Body cavity effusions are usually yellow and viscous. They typically have a high protein content (5-12 g/dL) and low-to-moderate cellularity, with non-degenerative neutrophils predominating. Protein electrophoresis of effusion can also be helpful. FIP effusions typically have >50% total globulin and >32% gamma globulin, but is not specific for FIP.

A positive FCV titer indicates past or present infection with any coronavirus, but is not diagnostic of FIP. Cats with FIP tend to have high FCV titers, but there is much overlap between cats with FIP and healthy cats. High FCV titers in conjunction with appropriate clinical signs are supportive of FIP, but FCV titers are not predictive of viral shedding of FECV.

This test detects antibodies against the FIP 7b protein, which have been postulated to increase wih mutations of the coronavirus 3c gene, and reflect a key event in the pathogenesis of FIP. Although initial data looked very promising, field-testing has shown that not all cats with positive titers have or develop FIP, and that some cats with acute FIP or immunosuppression have negative titers.

The utility of PCR testing in the diagnosis of FIP is not well established. PCR testing for coronavirus in blood is neither sensitive nor specific for FIP. Although FECV does not cause systemic disease, coronavirus can be found systemically in some cats infected with FECV. PCR testing of body cavity effusions, aqueous humor or CSF, may he helpful in confirming a diagnosis of FIP, but results must be interpreted in light of clinical signs and other laboratory data. PCR testing of feces may be helpful in identifying shedders of FECV, but multiple tests are probably required as some cats shed FECV transiently or intermittently.

Immunohistochemistry of a suspect lesion or fluid with coronavirus antigen detected within infected macrophages remains the gold standard for determining an ante-mortem diagnosis (especially in dry FIP cases). The characteristic biopsy finding is a disseminated pyogranulomatous and necrotic change around small veins.