Here is the info from the SAME entity listed above, for Feline AIDS.

Link: http://www.vin.com/mainpub/feline/aafpfivdx.htm

Summary:The blood test is quite accurate, but false positives and occasionally false negatives do occur. In healthy, low-risk populations FIV is quite uncommon, and this leads to an increase in the relative number of false positive results. The blood test may also falsely identify recently infected cats as negative. To be absolutely certain, cats must be tested 1-3 months after their last known exposure.The test can not be accurately interpreted in young kittens.
There is currently no test available to distinguish between infected cats and cats that have been vaccinated for FIV.
If resources allow, cats testing positive by the ELISA test should be retested by sending the appropriate sample to a laboratory for IFA or western blot analysis. In high-volume shelters, holding cats for retesting may not always be practical. At the very least, the ELISA test should be repeated to ensure that correct technique was used

Maternal antibodies may interfere with FIV testing in kittens. The literature is in disagreement as to when FIV screening may be performed: maternal antibody interference can rarely occur up to 4 months but has disappeared by 6 months. As mentioned, even in adult cats infection may take up to 1-3 months to develop, so a final test should be performed 3 months after the last known exposure to be absolutely certain. Samples should be tested individually, pooling samples significantly decreases test accuracy.

There is a vaccine available for FIV, although it is not 100% effective in preventing infection. FIV vaccinated cats will test positive for FIV on all currently available tests.