When people become infected with HIV, they develop antibodies against the virus. This process of developing antibodies takes from about six weeks to a few months. If someone has become infected with HIV, antibody tests will be positive by six months after exposure to the virus.
The period before antibody is present is called the window period. HIV tests that detect antibody will be negative during this time, even if a person has actually become infected with the virus.
To test someone during the window period, a different test, called viral load, can be done to detect the virus in the blood. Viral load tests are positive in HIV-infected people during the window period because the level of virus is the blood in high at this point in the infection.
Viral load tests are commonly used to monitor the level of virus in the blood of people known to be infected with HIV. They have not been approved for use as diagnostic tests, but they can be used for this purpose for people who think they have recently been exposed to the HIV virus.
All approved tests for the diagnosis of HIV are based upon the detection of antibody directed against the HIV virus. The standard test for antibodies to HIV is called an enzyme immunoassay. If this test is positive, it can be confirmed with another test, called a Western Blot, which detects different antibodies.
If both of these tests are positive, it confirms that a person is infected with HIV. If these tests are negative by six months after someone has been exposed to HIV, it is safe to assume that the person did not become infected with the virus.