A fourth or fifth generation ELISA assay for HIV antigens and antibodies is considered to be conclusive for negative results 90 days after your last exposure to HIV. Most people with HIV will test positive by 21 days after exposure. Almost all people with HIV will test positive by 6 weeks after exposure. ELISA assays are very, very sensitive. The tests themselves are not the limiting factor. It really all depends on the levels of antigens and antibodies in your blood. Antigens show up first and are present in direct proportion to viral load. The more virus in your blood, the more antigens in your blood. Given that antigen levels are generally very high during early seroconversion, ELISA assays are very good at detecting infections in the earliest stages. As antigen levels drop off after seroconversion, antibody production ramps up. By 6 weeks, almost everyone with HIV has detectable levels of antibodies in their blood. In very rare circumstances, people need longer than that to be detectable. By 90 days, though, you can trust negative results. If your blood doesn't have HIV antibodies in it by then, it almost certainly is never going to have. There have been a handful of delayed seroconversion cases reported in medical literature, but they are extraordinary. You're more likely to win a multi-state lottery than to be one of those cases.