How do HIV tests work?
There are two main ways that you can test for HIV. The first is by checking your blood for HIV antigens. Antigens are what surrounds the outer surface of a virus. They are part of the virus itself. When high levels of HIV antigens are found in your blood it means that you have HIV.
The second way to test is to look for HIV antibodies. Antibodies belong to your body’s immune response. They are the cells that the body produces to respond to and try to clear an infection. Having lots of HIV antibodies in your blood is a sign that your body has detected an HIV infection and is trying to fight it off.
Most tests will use a combination of these two approaches, looking for antibodies and antigens. The combined test is the standard test for HIV these days.
What is a window period?
Window periods describe the time it takes after infection for HIV to show up on a test. There will be a delay between the moment that HIV enters your body and the time when a test can reliably detect the virus in your blood (or oral fluids). In other words, HIV tests can't accurately tell you if you have HIV until the window period is over. How long the window period lasts depends on the type of test used.
Antibody tests can only reliably detect HIV three months after exposure. This is because your body needs time firstly to detect the HIV and then to start producing antibodies. The level of antibodies has to be high enough for it to be sure that they will show in the test. After three months 99% of people will have antibody levels high enough to be detected in tests.
Antigen tests have a shorter window period. They are able to detect HIV from four weeks after exposure. This window period allows for the time it takes HIV to make enough copies of itself to be detectable in the blood.
If you are taking PrEP or PEP the window period on your HIV test may be longer, as these medicines can delay the production of antibodies.