HIV testing

The standard HIV test looks for antibodies in a person's blood. When HIV (which is a virus) enters a person's body, special proteins are produced. These are called antibodies. Antibodies are the body's response to an infection. So if a person has antibodies to HIV in their blood, it means they have been infected with HIV. There are only two exceptions to this rule. Firstly, babies born to positive mothers retain their mother's antibodies for up to 18 months, which means they may test positive on an HIV antibody test, even if they are actually HIV negative. This is why babies born to positive mothers may receive a PCR test after birth. Secondly, some people who have taken part in HIV vaccine trials may have HIV antibodies even if they are not infected with the virus.

Most people develop detectable HIV antibodies within 6 to 12 weeks of infection. In very rare cases, it can take up to 6 months. It is exceedingly unlikely that someone would take longer than 6 months to develop antibodies.

Getting tested earlier than 3 months may result in an unclear test result, as an infected person may not yet have developed antibodies to HIV. The time between infection and the development of antibodies is called the window period. During the window period people infected with HIV will not yet have antibodies in their blood that can be detected by an HIV test. However, the person may already have high levels of HIV in their blood, sexual fluids or breast milk. Someone can transmit HIV to another person during the window period even though they do not test positive on an antibody test. So it is best to wait for at least 3 months after the last time you were at risk before taking the test, and abstain from unprotected sex or drug use with shared needles in the meantime. Some test centres may recommend testing again at 6 months if you're deemed to be at particularly high risk of infection.

It is also important that you are not exposed to further risk of getting infected with HIV during the window period. The test is only accurate if there are no other exposures between the time of possible exposure to HIV and testing.

A negative test at three months will almost always mean a person is not infected with HIV. If an individual's test is still negative at six months and they have not had unprotected sex or shared needles again in the meantime, it means that they do not have HIV, and will not therefore go on to develop AIDS.

The only way to know for sure whether you are infected with HIV is to have an HIV antibody test. It is not possible to tell from any symptoms.