If the initial HIV screening shows a negative result, most people do not need to be retested because a negative HIV antibody test basically rules out the possibility of AIDS. However, a positive HIV antibody test does not identify a person with AIDS, because the false positive rate is high, and a positive result requires further testing, i.e., a confirmatory test called a complementary test. In addition, there is a window period for HIV infection, which is as short as 2 weeks and may be as long as 3 months. If the antibody test shows negative within 1-2 weeks after the event of possible HIV infection, in this case a re-test is needed because the corresponding antibodies during the window period are usually negative. If the result of the initial screening is negative after the window period, there is no need for retesting.