Sorry to hear that your vet suspects this. I have never heard of a false negative titre - I personally wouldn't test, I lost a cat to FIP, and I didn't - Ginger had another 3.5 years with me after I lost Tig to it, so he was proof that they dont necessarily develop it - and out of the two, he fit the 'criteria' better than she did. It does normally only affect young and old cats, how old is Simon.