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It is difficult to make a definitive diagnosis of FIP in a living cat, however, there are certain clinical signs that are very consistent with the disease process. Unfortunately, it is a poor prognosis and very few cats survive, even with excellent supportive care. While your cats certainly are consistent with findings in cats with FIP, other cause stop rule out would include liver failure, hemolytic anemia, other viral infections and cancer such as lymphoma. Besides a positive corona virus titer (which does not tell about active infection only exposure to the virus and not all cats exposed to the virus will become ill), the biggest blood marker is a high globulins. A test called protein electrophoresis can tell us if the globulins are a result of immune stimulation (i.e. from infection) or from cancer cells that produce identical proteins that are all of the same molecular weight. Another finding that almost certainly points to FIP is a yellow abdominal effusion that is viscous and stringy. Other findings include inclusions in the anterior chamber of the eye or granulomas on the internal organs, in the abdomen or in the chest. Since FIP comes in 2 forms- a "dry" form or one that is characterized by granuloma formation and a "wet" form- one that is characterized by abdominal, plural or multicavitary effusion, and the disease is believed to be caused by a mutated virus that lots of cats are exposed to but don't always get sick from diagnosis is tricky.
If you can share some of the specific blood work results you are referring to, perhaps I can help to explain why they are consistent with FIP or not.