We've had kittens test positive, only to turn out negative later on.

One of my fosters, Piglet, tested positive 2 or 3 xs when the family who originally had him tested him at her vet's office. I had him from the time he was 1 1/2 to 2 1/2yrs old. We retested him 3xs. He was negative 3xs in a row .

From what I was told by the vet's office was that it was possible to test a false positive, but NOT possible to test a false negative. Same with human HIV tests. You can't test negative and not be negative. If the antibodies are there, they will be picked up by the test, even if it's light.