There is a "window period" which is the time it takes the body to produce antibodies after HIV infection has begun. For the vast majority of those who will test positive, antibodies to HIV will develop within 4-6 weeks after exposure. Some will take a little longer to develop antibodies. To make certain that you receive a reliable test result, it's necessary to wait at least three months (13 weeks) after your last possible exposure to the virus before being tested.
Getting tested before three months may result in an unclear result or a false negative. Some testing centers may recommend testing again at six months. All but less than 1% of those who are going to seroconvert will do so within three months (seroconversion is the development of detectable antibodies to HIV in the blood as a result of infection.) It's extremely rare for seroconversion to take more than six months to develop detectable antibodies.
No diagnostic test will ever be 100% reliable, but if you test negative at the appropriate time (i.e., 13 weeks after possible exposure to the virus), you can consider that to be a dependable confirmation that you are HIV negative.
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