Definitive diagnosis: There isn’t one. There are zero tests for FIP that are 0% false positive and 0% false negative.
Wet or Dry FIP: It’s the same diseases caused by the same virus but FIP manifests in two, more or less, distinct forms: Wet and Dry. My understanding is that in cases of Dry FIP, the cat’s immune system is fighting the disease. In cases of Wet FIP, the immune system is not fighting.
Wet FIP diagnosis: With Wet FIP, there will be an accumulation of fluid (usually yellow) in the belly and/or the chest. Wet FIP is relatively easy to diagnose. Fluid can be sampled and viewed visually. If it’s yellow, you should assume your cat has FIP and start treatment ASAP. Wet FIP has a short timeline (hours or days, maybe a few weeks). The cat’s that are not cured by treatment are the ones that don’t start soon enough. Cat’s that are still with us after the first 7 days of treatment very rarely don’t make it. Wet FIP diagnosis can be further supported by the other typical FIP tests (see below).
Dry FIP: Dry FIP very often never gets a definitive diagnosis. Vets will often say “I think your cat cat FIP or Lymphoma”. Your cat will usually present with weight loss, possibly fever, and possibly lack of energy. Dry FIP can have a long timeline. My cat had dry FIP for at least a year before diagnosis and then another 6 months while on Prednisolone before starting actual treatment. Imaging, such as Ultrasounds and X-rays, and help diagnose FIP. Very often imaging will show swollen organs and lymph nodes which can be caused by FIP. CBC and Chemistry blood tests usually (but not always) show characteristic changes. Dry FIP cats will often have fever.
Ocular and Neurological FIP: Again, it’s not a different disease. With Ocular FIP, the virus has entered the eye and causes symptoms that you can see and can affect your cat’s vision. Usually, you will see the eye turn a cloudy red/brown (Uveitis). With Neurological FIP, the virus has entered the brain and is causing neurological symptoms. The main one is paralysis of the rear legs but there are many other symptoms. Both the eyes and the brain are protected by osmotic barriers. These barriers often but not always keep the FIP virus out. If your cat has Ocular or Neurological FIP, it can still be treated but higher dosing is required. In most cases, all the symptoms are reversible and resolve with in two weeks. The full treatment is till 84 days.
Should I treat? Without treatment, FIP is 100% fatal. There is little downside to starting treatment even if it turns out your cat doesn’t have FIP. In many cases, the treatment is the test (see below). You can always stop the treatment if your vet 100% rules out FIP somehow.
CBC and Chemistry blood tests: All cats suspected of having FIP should get CBC and Full Chemistry blood tests. Results are usually available the next day. Some vets have “in house” machines that can do partial CBC and Chemistry testing with instant results. Most (but not all) cats with FIP will have changes in the CBC and Chemistry levels that are consistent with FIP. See my Reading Blood Tests page for details. CBC & Chem can help with diagnosis but also will help you track treatment progress. We advise folks to re-test at 6 and 12 weeks but you need a baseline at or before treatment starts for comparison.
Fever test: If you cat has fever from FIP, it makes for the basis of a very good test. You simply start treatment. If you cat has FIP, the fever will usually be gone the day after the first shot.
Diagnostic GS: basically this is the same as the fever test but looking at other symptoms. You simply start FIP treatment. If symptoms resolve, FIP is confirmed and you just continue the treatment.
PCR: PCR testing is useful if positive but worthless if negative. This is because it has a high false-negative rate. It is fairly expensive and takes several days to get results back. It is rarely worth waiting for PCR results to decide on treatment.
Immunohistochemistry (IHC) Staining: IHC, like PCR, is definitive if positive but not definitive if negative. IHC requires a surgical biopsy so is rather invasive. It is also expensive and takes days to get the results back.
Corona Virus Titer: This test is pretty much worthless. Most cats are exposed to Feline Enteric Corona Virus (FECV) as kittens. FECV spreads easily in shared litter boxes. The test measures the amount of FECV antibodies’ in the blood. Most cats will test positive and the test does not differentiate between benign FECV and FIP. There is a weak correlation between FIP and very high titer levels.
Rivalta test: Rivalta is actually a pretty good test. It is simple and inexpensive. It only applies to Wet FIP cases because the test involves putting a drop of the fluid into dilute vinegar. You can literally do it in your kitchen if your Vet will give you some extracted fluid. Rivalta is very good at ruling out FIP (96%). If Rivalta is negative, it is unlikely your cat has FIP. Rivalta testing, for some reason, seems to be popular outside of USA. Few if any USA Vets perform Rivalta “in house” (although it’s easy) nor do they usually suggest having a lab do it.