The window period is time between potential exposure to HIV infection and the point when the test will give an accurate result.
During the window period a person can be infected with HIV and infectious but have a negative HIV test.
The window period for a 4th generation antigen/antibody test is four weeks. At this time this test will detect 95% of infections. After a three month window period after exposure, the confirmatory test will detect more than 99.97% of infections.
The earliest marker is HIV viral load. This is in the first weeks after infection (usually from 1 to 6 weeks after exposure). A high viral load is related to seroconversion symptoms. The first HIV protein (antigen) that can be measured is p24 (from 1 to 8 weeks after exposure).
Viral load and p24 tests are not accurate for diagnosing early HIV if the results are negative.
An HIV antibody response can be detected as early as two weeks in a few people and in more than 99.9% of people by 12 weeks. An antibody test at 4 weeks will detect 95% of infections.
Antibody testing at 4 weeks can give you a good indication of your HIV status, but you need a test at 12 weeks after the exposure to be considered HIV negative.