While I am very sorry you have to go through the emotions caused by this, do not fret.

I am no expert (but then again, most vets are not experts on this subject either )

There is no test that can tell you your cat has FIP! A test will tell you if there has been exposure to it, though. MANY, MANY, MANY (get the idea) will test positive for FIP if given the test. I would bet every cat in this house has been exposed at one time or other.) But MOST will never show symptoms. why? Because the virus must mutate inside the cat to actually bring on the onset of symptoms. Combine that with a cat's natural immunity and you have very few who will: a) have low immunity AND b) have the virus mutate.

If I were you I would forgo the expense of testing the other cats, because they most certainly have been exposed.

Best of luck keeping Miley healthy and happy, but don't lose too much sleep over it, OK???