Once HIV enters the body, the immune system starts to produce antibodies-chemicals that are part of the immune system that recognize invaders like bacteria and viruses and mobilize the body’s attempt to fight infection. In the case of HIV, these antibodies cannot fight off the infection, but their presence is used to tell whether a person has HIV in his or her body. In other words, most HIV tests look for HIV antibodies rather than looking for HIV itself. There are tests that look for HIVs genetic material directly, but these are not in widespread use.
The most common HIV tests use blood to detect HIV infection. Tests using saliva or urine are also available. Some tests take a few days for results, but rapid HIV tests can give results in about 20 minutes. All positive HIV tests must be followed up by another test to confirm the positive result. The results of this confirmatory test can take a few days to a few weeks.