Diagnosis of FIP
A diagnosis of FIP usually is based on the cat's clinical signs and symptoms, x-rays, routine laboratory tests, and an evaluation of the fluid in the abdominal or chest cavity. Some cases, however, are difficult to diagnose because the symptoms vary greatly and are similar to those of other diseases. In all cases, microscopic examination of a tissue sample (biopsy) is the only way to confirm a diagnosis of FIP.
Tests to Diagnose FIP
FIP is notoriously difficult to diagnose through standard laboratory blood tests, and many veterinarians simply do not bother with them. Those who do, generally use blood tests to look for an elevated level of gamma globulin proteins or a low level of albumin in the blood.
Tests that detect the presence of coronavirus antibodies in a cat cannot distinguish between antibodies for FIP, FECV, or any other feline coronavirus (FCoV). A positive result only indicates that there has been exposure to some type of coronavirus, and a negative result does not rule out FIP.
It is possible for a cat that has been vaccinated to have enough antibodies to produce a positive test. False positive readings may occur in up to 30 percent of tests. Results from one laboratory will not necessarily correspond with those from another, and labs do not use the same terms to describe results. There are no clinical standards for laboratories, no regulatory body to oversee them, and no requirements for validation of test results.
For these reasons, pet owners are advised against placing too much emphasis on test results.