HIV tests rely on the presence of a detectable level of antibodies to the virus; the process of developing these is called seroconversion. Although seroconversion occurs in an average of about one month, according to HIVTest.org, a test at six weeks is not necessarily conclusive.
Although the average time of seroconversion is 25 days, the process may take between three and six months, according to HIVTest.org.
There is no way to know how long it will take for any given individual to develop a level of antibodies that can be detected by an HIV test.
An HIV test administered six weeks (or more) after the last possible exposure to the virus and which returns a positive result can be considered conclusive. However, a negative test at this time does not mean that a person is HIV-negative.
People should discuss their last possible exposure to the virus with a health-care or HIV-testing professional to determine whether or not retesting is appropriate.
RNA tests, which directly detect the presence of the virus, can reliably find the virus much sooner in infection. However, they are not commonly used as screening tests for HIV and are not available at all testing sites, according to HIVTest.org.