FIP testing & roundworm
The FIP test doesn't say anything. According to studies 90% of cats that have access to outside and 30% of cats that live at home have been exposed to some form of coronavirus (in most cases NOT the FIP mutation) thus are "FIP" positive according to the ELISA test. If the test is negative it also does not mean anything as it does not detect the coronaviruses during their incubation period. The test has only one meaningful outcome. If it is negative, vets usually perform the more precise PCR test. If the PCR is positive than it again does not mean anything (just presence of some form of the virus, no one can say if it is the FIP form or not). If the PCR test is negative, your cat is the rock star with no coronavirus. Because of the above vets in UK stopped using the ELISA test as it is quite useless.
Our cat was positive to general coronavirus. Nobody can tell if it is FIP or one of the many other forms (which causes no serious issues).
If your cat licks the brick or eats the cat litter you should double check the parasites! This is the direction that we are now following as we have detected the worms (ascaris is one of the most frequent roundworm). Unfortunately all the misleading treatment that the cat has received in past two weeks (ATB, corticoids, steroids etc.) plus the worms (roundworms excrement many toxic stuff when they are dying) lead to issues with his liver and he started turning yellow yesterday (this is unfortunately very serious). Our only hope is that all his issues were caused by the worms and when they are gone, he'll start doing better. It seemed to me that he was a bit more agile today morning but it might be a false hope. I'll keep you posted.