A CBC and Chemistry profile will show altered values due to the organs affected by FIV. A conclusive test is available and most veterinarians use a combo Idexx® FIV/FeLV Snap Test. The clinical signs of both diseases are similar so it is wise to run both tests at the same time. There are some issues though:
1. The snap test detects antibodies to FIV. Young kittens may also test positive since their mother passes short term passive immunity to them via nursing. Retest every 60 days till they are about 6 months of age. Cats that are vaccinated against FIV will test positive since vaccine antibodies (at the moment) can not be differentiated from actual infection antibodies.
2. A negative snap test means there are no antibodies present and can be assumed to be free of the disease...except in those cats just exposed to the virus and tested, there will be no antibodies. Most diseases require at least 2 months to develop an effective humoral immune response.
No lab test is ever 100% effective one way or another.