Q and A


What is the window period in fourth generation tests?


How do doctors claim conclusive results at 28 days while test manufacturers state: ‘4th generation tests shorten window period 1 to 2 weeks’.

So, in my opinion, it should be conclusive at 10-11 days, not on the 28th day.

What is your opinion? What am I missing?




Thank you for your question.

The fourth generation tests look for both antibodies AND antigens. Antigens are proteins on the surface of the HIV particle. Antibodies are produced by the immune system in response to the HIV antigens.

Antigens are able to be detected usually about two weeks after infection. HIV tests look for p24 (protein 24) which is is detectable an average or 14 days after infection. However, testing for p24 is not accurate enough on it’s own to be the basis of an HIV test.

Antibodies are not usually produced until 3-4 weeks after an infection depending on the speed and strength of the individual’s immune system. About 95% of people produce either antibodies or p24 by day 28.

Individual response to HIV can vary considerably. There are odd cases where responses are much quicker and other cases where everything takes much longer.

In order to be effective as a test it needs to be accurate at picking up genuine infections and also accurate at ruling out infections when they are not present.

These two factors are complicated and guidelines are usually conservative to allow for the extremes.

Information on this website is provided by treatment advocates and offered as a guide only. Decisions about your treatment should always be taken in consultation with your doctor.


  1. Lekova

    I had an unprotected sex with my girlfriend on the 1/8/2016. I tested her with a 4th generation test kit (Determine) and she turns negative though she said not to have had sex 4 months before the test. It’s been 9 weeks from then and my 26/9/2016 hiv test with Determine turns negative but am still afraid though the window period for the kit says 4 weeks, please help me out. Thanks.

  2. I’m sorry but i-Base no longer answers individual questions about risks of HIV transmission or HIV testing.

    This service is based on information relating to treatment of people who are HIV positive.

    Information online
    Most testing questions are already answered on the FAQ page:

    Our guide to HIV Testing and risks of sexual transmission guide has more detailed and easy to read information about these topics. It can be read online at this link:

    Worried about HIV?
    If you are worried about a possible infection contact your local testing centre.
    In the UK you can find your nearest clinic using the NHS website:

    Understanding HIV test results:
    If you do not understand results from an HIV test, please contact the centre where you were tested.

    Worried about symptoms?
    If you are concerned about any symptoms please contact a doctor.

  3. gary mullaney

    do they use the 4th generation test in the uk ie gum clinics

  4. If your friend has had 3 negative HIV tests then she does not have HIV. It is possible that she has been lucky and was not infected through the anal sex with her partner. If he did not ejaculate then the risk of transmission is much lower. Your friend has nothing to worry about.

  5. henna

    A friend of mine had 2 or 3 incomplete anal sex acts. Her partner did not ejaculate in her anus and the penis was not inserted fully in to the anus. Later she came to know that her partner was HIV positive. She got herself tested and the test result was negative. She has been worried ever since she knew that her partner was HIV positive. She tested after three months and then after 6months. Both tests were negative. She again tested after 2 years and was negative. She didn’t have sex after that. She went through many websites to find out about the window period. She is anxious to know more about whether or not she is infected please.

  6. The window period for picking up antibodies as well as antigens with a fourth generation test is 28 days. The test can pick up antigens 3-4 weeks after an infection which means it is shortening the window period. For more information please follow this link.

  7. Ali

    Dear Charlotte,

    I am sorry, I meant 10-11 weeks.

    That is, I read your post regarding the “different tests and window period”

    3rd generation test: you say: “it is conclusive at 12 weeks”
    4th generation test: you say: “it shortens the window period 1-2 weeks”

    So, I am asking that: why do you state that the window period for 4th generation tests is 28 days. It should be: 12-1: 11 weeks.

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