This is the answer from the Glasgow site :
FIV tests
There are many tests available for FIV. All tests involve taking a sample of blood from the cat. Usually, the sample is then tested for antibodies to FIV: most FIV infected cats have antibodies. Your veterinary surgeon may perform the test in his or her practice laboratory, so that you have a result very quickly, or may send the blood off to a commercial laboratory such as Companion.
The gold standard test is the immunofluorescent antibody test (IFAT) used by Companion at the University of Glasgow Veterinary School and all cats tested positive by one of the quick in-practice tests should be confirmed by this test. The IFAT has an internal negative control to avoid false positive results, and if the control shows that something is wrong, a further test called a western blot is performed for no further charge.
Some cats that are infected with FIV are negative by antibody tests. There are three possible explanations for this finding: sometimes the cats are in the first 12 weeks after infection and have not yet made antibodies; sometimes because they are dying and are no longer able to make antibodies; or the antibodies they do have are bound to virus within their body and cannot bind to virus in the test. In such cases, virus can be grown by a specialist laboratory and virus isolation is a further diagnostic test which is sometimes done on cats which are suspected of being infected with FIV, but who give a negative antibody test.
As stated above, FIV infected cats have a deficiency of CD4+ cells, therefore have a decreased CD4+/CD8+ ratio (the CD8+ is another kind of white blood cell). It is possible that a decline in CD4+/CD8+ ratio is a poor prognostic sign for a cat, and this hypothesis is being investigated further by scientists.
So I think yes is the answer to your question Mark.
