There is no single definitive test for FIP at this time, however the diagnosis of FIP should be relatively simple given its affinity for younger cats, its strong tendency to involve catteries and shelters, the typical physical and historical findings, and numerous characteristic laboratory abnormalities. Nonetheless, it remains one of the most difficult of diagnoses for many veterinarians. The truth is that veterinarians have little trouble in placing FIP high, or at the top, of their diagnostic list, but have great difficulty, and even reluctance, in confirming their diagnosis. This is probably because FIP is viewed as a death sentence, and they are reluctant to confer such a sentence without certain proof.

Although a definitive test result would assist decision making, a certain diagnosis can be based on cumulative odds rather than a single, simple, definitive test result. A young cat from a cattery or shelter with chronic uveitis and/or neurologic signs, high serum proteins, hyperglobulinemia and hypoalbuminemia, fluctuating antibiotic unresponsive fever, leukocytosis with a lymphopenia, and an anemia of chronic disease can have no other disease than dry FIP based on odds alone. Likewise, the same cat with similar history and laboratory findings, but with yellow-tinged, mucinous, inflammatory ascites is highly unlikely to have any other disease than wet FIP.

In an attempt to reach the elusive definitive diagnosis, veterinarians rely on dozens of tests that claim to highly correlate with the disease or to be diagnostic. However, the only good definitive way to diagnose FIP is to identify the virus in macrophages within lesions or ascetic/pleural fluid by a procedure called immunohistochemistry. PCR would work equally well on diseased tissues or fluids, but many of the current tests are improperly designed and conducted and frequently yield misleading results. In some cases, the proper fluid or tissues cannot be obtained pre-mortem. A necropsy should be done by a qualified veterinary pathologist on any cat that requires a proper diagnosis.