The HIV antibody test is a blood test to find out if the person has come into contact with the Human Immunodeficiency Virus or HIV.
If a person has come into contact with the virus their immune system will produce antibodies to fight the infection; it is these antibodies that are detected in the test. Usually, antibodies are produced between 2 and 8 weeks following infection with HIV, but it can take longer for the tests to detect them (see window period below) in some people.
If the test is positive another test will be done to confirm the result. When the result is confirmed it means that the person has HIV - i.e., they are HIV-positive.
The p24 antigen is a protein that is part of the HIV virus. It is produced in high quantities early in the process of HIV infection becoming established in a person’s body and a p24 antigen test will detect HIV infection at an earlier stage than an HIV antibody test.
In Australia, most laboratories currently use tests which test for both HIV antibodies and p24 antigen.
The window period of an HIV test refers to the time it can take from when someone becomes infected until a test can detect the infection. This can vary depending on the test used (e.g. antibody test or antibody and antigen test), but can be up to three months.