This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
There is a test which will look for the presence of coronavirus antibodies in your cat's blood. If your cat has been exposed to a coronavirus, ANY coronavirus, its immune system will build up antibodies to it, and the titer tests for the level of those antibodies in the blood. But it does not distinguish between antibodies made specifically against FIP, or FECV, or any other coronavirus. A positive titer means only that your cat has created antibodies (therefore been exposed to) SOME form of coronavirus.
The higher the titer, the more antibodies the cat has created. As if there were not enough problems with the coronavirus test, there is no uniformity between different labs. One cannot compare results from one lab to another. Some labs just specify positive or negative if the results are above or below a given titer (often these labs do not even specify the titer). There are no standards for setting up a lab, there is no regulatory body that oversees them, and no requirement for validation of test results. It is also possible for a cat which has received the vaccine (more below) to have enough antibodies to appear on the titer test. To top it all off, false positives occur in up to 30% of the tests. In sum: DO NOT PLACE MUCH CREDENCE IN THE TITER TEST, AND UNDER NO CIRCUMSTANCES SHOULD A CAT BE EUTHANIZED BASED SOLELY ON THE RESULTS OF THE TITER TEST.
There are some clinical indicators which your vet may discuss with you if s/he suspects that a cat has FIP, particularly if it is showing likely symptoms. Some blood tests can help your vet pinpoint FIP as a cause for your cat's condition, this includes looking for a high amount of gamma globulin proteins and a low amount of albumin proteins in the blood. There has also been talk of a polymerase chain reaction (PCR) test, in the hopes that it can tell the difference between FIP and other coronaviruses. Significant scientific studies have yet to be concluded on this method. However, given that the most common way of a cat coming down with FIP is via the mutation of FECV, this test may have little or no value in the great majority of cases.