Most HIV tests are antibody tests that measure the antibodies your body makes against HIV. It can take some time for the immune system to produce enough antibodies for the antibody test to detect, and this time period can vary from person to person. This time period is commonly referred to as the "window period." Most people will develop detectable antibodies within two to eight weeks (the average is 25 days). Even so, there is a chance that some individuals will take longer to develop detectable antibodies.
Therefore, if the initial negative HIV test was conducted within the first three months after possible exposure, repeat testing should be considered more than three months after the exposure occurred to account for the possibility of a false-negative result. 97% of persons will develop antibodies in the first three months following the time of their infections. In very rare cases, it can take up to six months to develop antibodies to HIV.