Originally Posted by FamilytimeRags
If the test is thought to be wrong, or you would like to confirm the positive results, there are 2 other less sensitive tests that may be used. IFA (immunofluorescent anitbody assay) or the Western blotting test. Either test rarely gives a false positive.
You're right that the test doesn't give a false positive, but the end result is the same - a vaccinated cat will still test positive. It's a true positive because the test has accurately measured antibody levels as it was intended to do, but it is not reflective of the cat's infection or vaccination status.
Infection with FIV rarely causes antigen levels high enough to be detected, so the only tests available - ELISA, IFA, and Western blotting - all check antibody levels. Vaccination causes detectable antibody levels - that is, after all, the whole purpose of vaccination. So the end result is that FIV vaccinated cats will test positive on all available tests because all check for antibodies.
A kitten under 6 months of age who tests positive should be re-tested at 6 months of age because kittens born to infected mothers carry their mother's antibodies, derived from nursing. It is rare for FIV infection to be spread from mother to kitten. Kittens who initially test positive will almost always test negative by 6 months of age. You can re-test earlier than 6 months if you want because usually it is in everyone's best interests for the kittens to be given a clean bill of health as early as possible so they can be better candidates for adoption. Often - but not always - kittens will test negative well before the 6 month mark. A negative test at any point can be considered conclusive, but a positive test indicates re-testing at a later point.
Incidentally, the FIV vaccine is not known to have any efficacy at all against viral subtype B, which is the subtype found in the eastern US. Grrr.