Diagnosis of FIP may be difficult and frustrating. There are no specific tests which are reliable in all cases. Although organ biopsy is the most reliable, this requires surgery. For obvious reasons, surgery may not be advisable in a sick cat. The following tests are usually used on cats with suspicious clinical signs.
- Coronavirus Test This test detects antibodies to any coronavirus. Antibodies are the circulating defense agents of the immune system. There are two coronaviruses that affect the cat: the FIP virus and the enteric coronavirus. If positive, this test indicates that one or both of those viruses WAS or IS present in the cat. However, we do not know which virus is reacting to the test. Since antibodies may persist even if the virus is no longer present, a positive test can be misleading in some cases. Also, terminally ill cats may have their antibodies "tied up" when large amounts of the FIP virus are present. This can result in a false negative test result. Therefore, this test must be interpreted in conjunction with results of other tests. These tests are listed below.
- Polymerase Chain Reaction (PC) This new test is more specific for the FIP virus than the coronavirus antibody test; however, it is still just a test for the presence of the FIP virus. A positive test means the virus is present, but does not necessarily mean the disease is present. This test is also subject to some false negative results.
- Serum Protetin Levels If the total serum protein is elevated AND the A:G ratio (ratio of two different blood proteins ) is less than normal, FIP becomes a more likely diagnosis. A few other diseases may also cause this, but these are also very severe and usually fatal. These findings occur in 50 percent of the cases of FIP.
- White Blood Cell Count If the white blood cell count is greater than 25,000 cells/ul, FIP becomes a stronger possibility. However, several other diseases may cause this and some of these are not fatal. Also, many cases of FIP have a normal white blood cell count (less than 18,000 cells/ul).
- Abdominal Chest Fluid Presence If fluid is present, this is a very meaningful test. If the characteristics of the fluid are appropriate and the cat has the correct clinical signs, a diagnosis can be made with greater assurance. Unfortunately, this fluid is not present in the dry form of FIP.
- Fine Needle Aspiration of the Liver or Kidneys A few cells may be aspirated from the liver or kidney without stressing the cat (i.e., with a local anesthetic in the skin). FIP produces a particular inflammatory pattern in these organs which, although not diagnostic, is strongly suggestive for the disease. This helps to rule out other diseases.
- Radiographs (X-Rays) of the Chest or Abdomen Radiographs serve to identify enlargements in organs and the presence of fluid in the chest or abdomen. They are helpful but not diagnostic and are used to decide which other tests are appropriate.
- A Combination of Three Blood Tests Cats with the combination of a low lymphocyte (a white blood cell) count, a high blood globulin (protein) level, and a positive coronavirus antibody test have been shown to have a 94 percent chance of having FIP.
- Organ Biopsy Organ biopsy is the only test which is diagnostic of FIP. However, it is not always possible since the organ involved may be the eye or the brain.
A case workup in the absence of organ biopsy often includes several or all of the above tests. Strongly suggestive findings with several tests often provides the basis for a presumptive diagnosis of FIP.
