I sure hope that Chloe doesn't have FIP. I was reading more about it and it seems like the FIP tests aren't always conclusive.

The antibody tests for FIPV also test for antibodies to FECV, i.e. any
a) cat exposed to any feline corona virus may test "positive" or even "strongly positive".
b) cats given FIPV vaccine may also test positive.

The antibody tests for FIPV may be negative in FIP-positive cats, because
a) The immune system components may actually be involved in the progression of the disease and be "consumed" in the disease process.
b) Early in disease—there is not enough time to develop antibodies.
Some animals are immune-suppressed from concurrent diseases (e.g. feline AIDS).
c) Antibody levels fluctuate up and down, seemingly in random fashion, in either FIPV or FECV infected cats. There is NO SPECIFIC PATTERN!
d) More recent research concerning the disease, including the use/misuse of serum serology to diagnose FIP is presented here.

For more information click http://www.newmanveterinary.com/fip.html.