Most people who are infected with HIV will test positive within one month of being infected. The period of time after infection, before the HIV test turns positive is called the window period. When a person becomes infected with HIV, the body makes antibodies to fight HIV. When enough antibodies are developed, the HIV antibody test will be positive. Each person’s body responds to HIV infection a little, differently, so the window period varies slightly from person to person. Most people infected with HIV will develop enough antibodies to be detected by HIV antibody tests four weeks after the exposure (transmission). Virtually all cases of HIV infection can be detected by three months.
During the window period, a person with HIV infection can pass it to others, even if his or her HIV antibody test is negative. In fact, during this period, the person may have very high levels of the virus and be most likely to infect others.
If the HIV antibody test is negative, a person can be sure that he or she does not have HIV only if he or she did not engage in any HIV risk behaviors (having unprotected sex or sharing needles) during the past three months.
A PCR (Polymerase Chain Reaction) test looks for HIV directly instead of detecting antibodies. This test can find HIV infection soon after the person is infected. Doctors may suggest an HIV PCR test if a person has symptoms suggestive of HIV infection (fever, rash, swollen lymph nodes, etc.) and reports high risk behaviors in the past few weeks. It is used to find HIV infection in newborns of mothers known to be infected with HIV. Sometimes doctors use a PCR test to measure viral load. The viral load test shows the amount of HIV in the blood of someone who is already known to be infected.