The KELA, ELISA, IFA, and virus neutralization tests find the presence of coronavirus antibodies in a cat. A positive test only means that the cat has been exposed to a coronavirus (not necessarily one that causes FIP) and has developed antibodies against it. A negative test indicates that the cat has not been exposed to a coronavirus.
The number or titer, given is the highest dilution of serum still providing a positive reaction. A lower titer means few antibodies in the blood. A higher titer means many antibodies. A healthy cat with a high titer is not necessarily more likely to develop FIP or to be a carrier of a FIP-causing coronavirus. It does not mean that the cat is protected against a future FIP infection!
Recently, two new tests have been developed, which can detect parts of the virus itself. The immunoperoxidase test can determine FIP more accurately than the traditional histopathological test because it finds the infected cells in the tissue. A biopsy of affected tissue is required for evaluation. Another antigen test uses polymerase chain reaction (PCR) to determine viral genetic material in tissue or body fluids. Although the test is promising, PCR currently only detects coronaviruses in general and not FIP-causing viruses.
Unfortunately, many laboratories use different antigen tests, which are prepared in different ways. Also, the interpretation of test results may differ. False results may occur for non-specific reasons unless the tests are carefully checked. The test can be difficult to interpret as it usually depends on a subjective decision of the person interpreting the test.