This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Unfortunately, yes. Although false negatives are not very common, they do occur, especially if you are dealing with a young kitten. Sometimes the cat has been recently exposed to FeLV, so the antibodies have not yet had enough time to build up enough of a response to appear on the test. To be absolutely sure a cat is not FeLV+ you should test it twice, a few weeks apart (the cat should remain isolated from other cats during this period, too, otherwise there is little point in doing a second test). The likelihood of getting a false negative is increased depending on the nature of the test. The ELISA test will show more false negatives than the IFA test. Some mail-order catalogues now offer FeLV testing kits using saliva or tears as the medium. These are more likely to trigger a false response than the ELISA test which uses blood. Fortunately, the false negative rate on the ELISA test (which is what most vet offices use for standard FeLV tests) is low enough that most people don't bother with the second test.