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The detection of HIV
Usually HIV infection is detected by an HIV antibody test. The first test to be done, usually on blood, but possibly on saliva, is an ELISA (Enzyme Linked Immunosorbent Assay). Since this test can sometimes be positive even when someone is not infected – a `false positive' result – a second test called the Western Blot is done. This can confirm an ELISA.
The amount of time between getting HIV infection and developing antibodies varies. The vast majority of people with HIV will produce antibodies by around 45 days after infection. However, in a small proportion it may take up to six months for antibodies to develop, and in a very few people with HIV infection it may take even longer. This is one reason why a lack of HIV antibodies does not always mean freedom from infection.
It is important to bear in mind that the HIV antibody test is not an 'AIDS test': there is no such thing. It is simply a test for one of the results of HIV infection. For instance, the fact that you can find antibodies to HIV in saliva does NOT mean that you can find virus there in any quantity. Nor does the test predict whether or not you will develop AIDS.
There are also a number of laboratory tests which can look for the virus or parts of the virus itself (antigen testing and PCR, Polymerase Chain Reaction, or viral load testing), or damage to the immune system, or other aspects of the body's response to the effects of the virus. These should not be confused with the HIV antibody test.
An antigen is a part of a virus that stimulates the production of antibodies. Someone who is antibody negative but antigen positive has the virus but has not yet produced antibodies.
For further information about HIV antibody testing see the chapter HIV testing.