Ginny & Elaine have given excellent info. This subject is difficult to explain.
The test for Corona is just a place to start and a positive test doesn't mean that a cat has FIP.
Many cats are exposed to Corona. Very few develop FIP. They must be genetically predisposed.
Hopefully your vet will help you figure out how to proceed. There are ways to try to treat it IF it is FIP but there are no treatments that "cure" it YET!
I hope and pray that this discussion ends up being completely unnecessary--but I thought I would share a bit of my experience.
You mentioned that Monday they were testing fluids. Do you mean a blood test or would they be testing the fluids in the belly?
I never had the fluid withdrawn from my kitty King's belly because the vet was certain he had wet FIP --because of his swollen belly, high Corona titre, his fever, lethargy etc...and the fact that his half sister had died of FIP just a few months earlier..And I just "knew" it was FIP the first day he started acting really lethargic.
I just didn't want to put King through the ordeal. It is a personal decision how much you want to do in the way of testing and treating.
Some people may disagree with me about this and feel that "everything" must be done before helping the cat "cross the bridge". But unfortunately, with FIP, the outcome is always the same in the end. And definitive diagnosis is only made with a necropsy.
King's necropsy showed he had "Wet FIP" and more tests and treatments were not going to change that. I feel good about the decision I made not to prolong his suffering. There were a lot of things to consider with King--symptoms, Corona test results, a family history of the same illness, that all helped me decide what to do. Without the family history part, it would have been much harder to proceed.
When it happed to Mariah several months earlier, I didn't know what FIP was and she had the "dry" form. It took longer to figure out what was going on and unfortunately, she progressed further with the illness and suffered for it. I hate that but I can't change it now.
Try to remain strong, we are here for you and Franco. I will hope that he does not have FIP. Big hugs for you both. Please contact me by pm if you need anything at all. xxoo