It takes time from the time of HIV infection to the appearance of HIV-related antibodies, and this time of appearance of antibodies is actually called the window period. This window period can be within 3-12 weeks, but generally speaking it is around 3 weeks, and rarely it is longer, close to 12 weeks. So after being infected with HIV, if one is tested for HIV antibodies for more than 12 weeks, they are able to determine or rule out that a person is infected with HIV. However, if during this period, it is likely that it will not be possible to determine, so this is the time to resort to some other means, such as checking for other components of HIV, the most commonly used being the p24 antigen. This antigen can be produced just after HIV infection and before the appearance of antibodies, and in combination with antibodies, the test can improve the detection rate of HIV. However, this test also has a certain rate of false positives, so a positive result is not immediately certain that the person is infected with HIV, and a confirmatory test is needed.