Was on the Glasgow site, and just found this - thought it might be of interest
Why do I have to have my cat's FIV test confirmed?
In a recent, as yet unpublished, survey, 5.5% of FIV infected cats were missed by the tests used in the practice and 7% of practice FIV positive results were false positives, that is, the test had wrongly labelled a healthy cat as infected with feline immunodeficiency virus.