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Understanding HIV test

Understanding HIV test

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Dr.Deyn Natthakhet Yaemim MD, Last reviewed 28 January 2020

Q: WINDOW PERIOD: What is the window period for an HIV test?

A: 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 be very infectious but still test HIV negative.

The window period for a 4th generation antigen/antibody test is two weeks. At this time 95% of infections will be detected (see Figure 7). There is a three month window period after exposure, for the confirmatory result to detect more than 99.9% of infections.

 

Q: 3rd and 4th generation HIV tests?

HIV testing in BC usually starts with a 4th generation test that looks for antibodies and the p24 antigen in the blood. If this test is reactive, additional confirmatory testing is done, including Western Blot or Nucleic Acid Amplification Testing (NAAT). The 4th generation HIV test has a reported average window period of 14 - 21 days. Since both 3rd generation and 4th generation HIV tests depend on individual immune responses, the amount of time needed for a person to develop enough antibodies can vary.

 

The range of times it can take to respond to HIV infection is shown below in Figure 8 on the left side.

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.

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