Q: One of our cats had a blood test for feline infectious peritonitis (FIP) and the antibody titer was 1:100. A month later it was 1:400, and she is to be tested again in another month. She is not ill, and our veterinarian is not sure of her prognosis. Does the cat have FIP, and is there a danger to our other cats?
A: FIP is one of the most frequently misdiagnosed diseases of cats because of the prevailing over-reliance on nothing but the results of serologic blood tests. FIP is caused by a coronavirus, but there are other coronaviruses that affect cats, and one of these causes only a mild diarrheal disease. A cat that has had this diarrhea can give a false positive reading on the FIP blood antibody test. For that reason, a diagnosis of FIP based on that test alone is not guaranteed to be accurate.
I do not think your cat has FIP, because the titer is very low, and unless it keeps rising and she develops signs of illness, I would not worry about her. Nor would I worry about your other two cats; they already have been exposed to whatever is ailing the first cat.