Ask the Vets if they can run PCR to tell whether it's false posi

http://franklintnvet.com/positive-fiv-test/

"Unfortunately, we need to run another test. This test is specialized test called a PCR (Polymerase Chain Reaction) Test.

This test is more specific and will truly identify those cats under most conditions who are truly positive for FIV. This test searches out and identifies the specific DNA viral particles of FIV. If no DNA viral particles are present, then the test will be negative. This test, again, is very specific for FIV. If the PCR FIV test comes back negative, then your kitty most likely does not have FIV and the ELISA test mentioned prior would be termed as a “false positive” and the cat does not have FIV."