Actually, they can determine if a cat has FIP before they die. It's done more easily in cats who have the "wet" version of the virus--which is what it seems that your cat has. My vet thinks that my cat has the dry version--which unfortunately is not so easy to diagnose before death. They could confirm it if they were to do surgeries to biopsy the granulomas. But that is costly and will only confirm whether or not he has it. If it could help cure it, I'd do it in a heartbeat. But we all know that's not going to happen.
According to peteducation.com "The only way to be absolutely sure of an FIPV infection is to biopsy affected tissues and have them examined by a veterinary pathologist. As a result, most often the diagnosis is made after the cat has died, a postmortem examination has been performed and tissues have been examined." However, if you read further, there are ways to diagnose FIP, but they are expensive and extensive, especially if your feline has the dry version.
If they text for the corona virus itself (FCoV), it is likely that it will come up as a false positive or false negative. But there are tests that can confirm it.
Here is the
How is it diagnosed? excerpt:
When a cat is exposed to FCoV, four things can happen, depending on a number of factors including age, health status, and strength of the cat's cellular immune system. The strain and dose of the virus can also influence the outcome.
Mammals' immune systems can be divided into two parts: the antibody-producing part, and the part in which cells kill invaders through direct contact or chemicals they produce. It is this second part of the immune system, the cellular immune system which plays a very important role in determining the result of exposure to FCoV.
If a cat's cellular immunity is very strong, the cat can usually fight off the infection.
If a cat's cellular immunity is moderately strong, the cat may be unable to kill all the virus, but is able to keep it in check. This results in a "latent" infection. If the cat is severely stressed or becomes ill from other diseases, the latent infection can be reactivated and the cat can develop FIP.
If a cat's cellular immunity is relatively weak, the virus continues to multiply slowly, FIPV becomes the predominant virus and FIP develops. In this form of disease, called "dry FIP" nodular lesions called granulomas slowly develop in one or multiple places in the body.
If the cellular immune system is very weak, the virus can multiply virtually uncontrolled. A "wet" form of FIP develops. In this form, large amounts of fluid accumulate in the chest and abdomen due to damage to blood vessels and subsequent leaking of fluid and protein into the surrounding tissues.
The damage to the body from FIPV is not so much due to the virus itself, but to the body's response to it. Complexes of FIPV and antibodies the cat produces against it are deposited on the walls of blood vessels. Macrophages, which are cells that eat cellular debris and foreign material, consume the virus and the virus replicates inside these cells. These macrophages are also deposited along blood vessels and in tissues. When they accumulate in large numbers they can form granulomas.
Works Cited:
http://www.peteducation.com/article.cfm ... 16&aid=212
Good luck with everything! We're still hoping that the doctors' hunches are wrong, but only time will tell.
[/i]