Usually HIV infection is detected by an HIV antibody test. This detects proteins called antibodies which the body manufactures as part of its immune response to HIV.

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. Other kids of test are becoming available.

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 handful of people with HIV infection it has taken even longer or has not happened at all. So a lack of HIV antibodies does not always mean freedom from infection, particularly if the person has been recently infected.

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 and does not 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 viral load testing or PCR, Polymerase Chain Reaction, 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.