What is an HIV test?

What is an HIV test?

HIV is usually diagnosed by a blood test, known as an HIV antibody test or an HIV test. This test looks for antibodies formed by the immune system if HIV is present.

If HIV antibodies are found, the test is referred to as positive. The person is HIV antibody positive, often referred to as HIV-positive. If antibodies are not found, the test is negative and the person is HIV antibody negative or HIV-negative.

There is a gap, or 'window period', between infection and the formation of antibodies. This can last from a few weeks to three months. During this window period,the HIV antibody test will be negative even if HIV is present. That is why people who think they may have been infected need to wait for at least three months before they have a test. Some testing centres suggest a test after six months, to be absolutely sure.

HIV tests in the UK are highly accurate. A positive result is confirmed using a different technique. Negative tests are accurate so long as they are not done during the window period. Very occasionally the result may be unclear or 'equivocal'. When this happens a second test a few weeks later will give a clear result. See the booklet on the Terrence Higgin's website Testing Issues for further information about HIV tests.

Testing babies for HIV

Babies are born with their mothers' antibodies, which clear once the baby has developed its own immune system. Babies born to women with HIV have HIV antibodies in their blood from the mother for the first 18 months of life.An HIV antibody test on the baby's blood would show the mother's antibodies, and would not tell the doctor whether the baby had HIV. Some hospitals are now able to perform tests on much younger babies of HIV-positive women, to discover whether the baby itself is infected with HIV.