What is a 4th gen HIV test?

The AFA Anonymous Testing Services have been providing simple, quick and affordable HIV testing utilising 3rd generation test kits for both blood and oral fluids. In an effort to improve our services, the AFA anonymous testing clinic and mobile testing services will be introducing the 4th generation test kits.

What’s the difference between 4th and 3rd gen test kits?

Fourth-generation tests kits look for both antibodies AND antigens.

  • P24 Antigens are proteins on the surface of the HIV particle.
  • Antibodies are produced by the immune system in response to the HIV antigens. They fit together like a lock and key.


Immune response of HIV infection

3rd generation tests take 6-8 weeks and only tests for antibodies.

4th generation tests are accurate 14 days after exposure, because this is when the p24 antigen becomes high enough to measure; effectively reducing the window period by average of 14 days.

A negative result at 28 days is good news but it is not conclusive. UK guidelines (BASHH) say that an early negative result at 28 days (1 month) needs to be confirmed with a second test 90 days (3 months) after the exposure.

Ask for the 4th Generation Test at one of the following services provided by AFA at $40 per test.

Click to learn more about each service.

More about p24 antigen

One distinctive HIV antigen is a viral protein called p24, a structural protein that makes up most of the HIV viral core, or ‘capsid’. High levels of p24 are present in the blood serum of newly infected individuals during the short period between infection and seroconversion, making p24 antigen assays useful in diagnosing primary HIV infection.

Antibodies to p24 are produced during seroconversion, rendering p24 antigen undetectable after seroconversion in most cases. Therefore, p24 antigen assays are not reliable for diagnosing HIV infection after its very earliest stages. However, HIV infection can be reliably diagnosed earlier with combined antibody/antigen tests than with purely antibody-detecting tests, and fourth-generation antibody/antigen tests are now the standard screening assay in the UK and some other countries.

More about HIV antibodies

Antibodies are protein molecules produced by the immune system in response to allergens, infectious organisms (including viruses, bacteria, fungi and parasites), and sometimes (in autoimmune disorders) the body’s own components.

Infectious organisms and allergens display characteristic proteins called antigens. The immune system recognises and responds to antigens by generating corresponding antibodies. An antibody is designed to ‘fit’ only one particular antigen, rather like a key in a lock. (The word ‘antigen’ in fact derives fromantibody generation.) By locking on to the antigen-bearing intruders, antibodies aim to render them harmless, to kill them outright, or to ‘tag’ them for destruction by other components of the immune system.

One or two weeks after initial exposure to HIV, antibodies to HIV antigens begin to appear in the blood, at concentrations which continue to increase for several more months. These antibodies persist for life, providing distinctive markers which can be identified by HIV screening tests

154 replies
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  1. Worriedwhy
    Worriedwhy says:

    I did my 4th gen ag/ab test today after 101 days of exposure came non reactive. Thanks to God .
    Thanks KS for the suggestions given here .

  2. Mendme
    Mendme says:


    I had an insertive anal sex encounter with another man and had many symptoms. I got a 4th generation ag/ab combo test 18 days, 49 days, and 63 days after the exposure. The results were all non-reactive.

    However, I’m worried because I keep gettin sick every other week, and I’ve read that the window period is actually 3 months and can be longer. What should I do?

    • afa_v2
      afa_v2 says:

      Hi Mendme,

      most guidelines today point towards a 62 day window period for 4th gen test. a non-reactive test at after day 62 would mean that the test is conclusive. That being said there are many other factors that may determine the final window period, such as the risk of the activity. If the insertive anal sex encounter is protected, the risk is negligible (assuming that condom was put on and taken off correctly) , however, if it is unprotected, the risk is high.

      Have you seen a doctor for these symptoms and gotten a blood test done to rule out the possibilities of other reasons?

  3. Gab
    Gab says:

    hello doctor i would like to ask i perfom oral sex with a guy msm (i sucking him)feel like my mouth got some blood and i assume its my gum are bleeding but not much no ejuculate and precum will there any high risk to hiv?im so worry bout this 🙁

  4. Must
    Must says:

    Hello Experts,

    I had unprotected vaginal sex someone which I didn’t her status for HIV.

    After that I made the couple of tests.

    14.day hiv1-2 + p24 was negative
    17.day HIV RNA negative
    33.day HIV Combi negative
    39.day vidas duo negative
    44.day vidas duo negative
    51.day hiv combi negative
    60-80-90-120 day elisa 3rd generation antibody test negative.

    Should I make more test sir?
    kind regards,

    • afa_v2
      afa_v2 says:

      Hi Must,

      there’s no need to test anymore than necessary. In the future, you might want to consider PEP instead. Post Exposure Prophylaxis (PEP) is a month of HIV medication that can stop you from becoming HIV positive and it has an efficacy rate of up to 80%. You have to start this treatment within 72 hours of exposure. For more detailed information please visit – http://afa.org.sg/pep

  5. Jack
    Jack says:

    Hey Ava,

    I had sex with someone of an unknown status, it was kind of a one night stand type of thing we had protected vaginal sex. However I think the condom might have been expired however it did not break. I got a hiv test done at 14, 19, 27, 29, 38, 41 days post exposure and all were negative. All the tests were the latest 4th generation tests. I was wondering how accurate these reuslts were? And whether I should stop worrying and move on?

    • afa_v2
      afa_v2 says:

      Hi Jack,

      I think you can stop worrying and move on. A negative test at day 42 (week 6) taken with the 4th gen test is 99% accurate. If the test was done at a reputable clinic or anonymous test site, the test result should be conclusive.

      Understandably, HIV can seem like a deadly disease, but it is not longer a terminal illness today. Nobody dies from HIV today, it is but one of the many sexually transmitted infection that cannot be cured, but can be easily managed with medication. Contracting HIV comes with a lot of stigma associated with it, and that is what affects people living with HIV, and that is what really kills people.

  6. Worriedwhy
    Worriedwhy says:

    Hi doc. Wanted to know I had anal sex with a freelance sex worker in Philippines , after we are done for 2 min ,there was a crack in the condom , not shreded ,I checked it with water . The water was not leaking from the top end.
    I did test on15th day and 33rd day again/ab combo test it came.non reactive
    Kindly suggest is it conclusive or I need to retest.?

    • afa_v2
      afa_v2 says:

      Hi Worriedwhy,

      Fourth-generation HIV tests can detect HIV infection in 50% of people by 18 days after infection; 95% of people by 34 days after infection; and 99% of people by one and a half months (6 weeks) after infection. You can make the decision if you are satisfied with this result, as there are many other factors to consider. Our counsellors will be happy to speak with you if you need further clarifications.

      • Worriedwhy
        Worriedwhy says:

        Thanks for your response .
        May I ask u few things please
        1. After going through my case how much risky was my case for you ?
        2. I have gone through Dr.Tan details as well in internet they were say 28 days is conclusive for 4th gen test , I would like to ask your thoughts on this please .

        Ki dly suggest so I stop worrying about it .

        • Worriedwhy
          Worriedwhy says:

          Thanks for your response .
          May I ask u few things please
          1. After going through my case how much risky was my case for you ?
          2. I have gone through Dr.Tan details as well in internet they were say 28 days is conclusive for 4th gen test , I would like to ask your thoughts on this please .

          Ki dly suggest so I stop worrying about it .

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