According to AIDS.gov, an antigen test shows the presence of HIV within one to three weeks of infection. A polymerase chain reaction, or PCR, test detects HIV in the blood two or three weeks after an individual is infected.
Antibody tests are not as reliable in the earliest stages of infection, though they are the most common tests used, notes AIDS.gov. The U.S. Food and Drug Administration indicates that it takes about three months for the immune systems of most people with HIV to develop detectable antibodies. AIDS.gov states that the enzyme immunoassay, EIA, test or rapid HIV antibody test measures HIV antibodies in blood, saliva or urine.