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

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  1. Kent chako
    Kent chako says:

    I had unprotected sex with a prostitute, I tested @ 4 weeks, 5,6 and 7 weeks respectively. All results came out negative using 3rd generation rapid test kit.

    Can the results turn positive @ 12 weeks?please help.

    From Zambia Africa.

    • afa_v2
      afa_v2 says:

      Hi Kent,

      thank you for writing to us, however, it seems like you are based in Zambia, and we are not familiar with the test kits that are being used, hence we are not able to give you a concrete answer, and can only recommend that you take a 4th gen test or a 3rd gen test 3 months (12 weeks) after your exposure.

      At the same time, it sounds like your experience with a sex worker was a stressful one and you may want to consider speaking to a counsellor regarding that too. In some culture, there is a great deal of shame with regards to having sex with sex workers, when there is nothing to be ashamed about. Sex work is work and many people around the world rely on it to make a living.

    • afa_v2
      afa_v2 says:

      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 after infection.

  2. Shawn
    Shawn says:

    Hi sir,I had checked for the HIV p24 + antibody check at a healthcare centre.Checked at 18th and 20th day of exposure.. Both negative,but still scared as within window period.Can you please advise on my condition?

    • afa_v2
      afa_v2 says:

      Hi Shawn,

      If you tested at our centre, we will be better equipped to give you an advice. In most cases, testing at day 18 with a gen 4 test kit gives you a 50% accuracy rate, and is increased to 99% at day 28. If you are unsure, we will strongly recommend that you consider another 4th gen test at 6 week mark.
      our counsellors will be more than happy to explain these in detail

  3. Rick
    Rick says:


    I had unprotected sex for 3 days with a women who I had been seeing off and on for 5 years. After we left two days later I developed a pain in my testicles and tip of penis. I freaked out went to the doctor and told her of what happened. She went and got tested for everything and those rest all came back negative. I took a 4th gen test at 21 days and days and those came back negative I guess my question is how realizable is that negative at 21 days. I also tested negative for everything else and was diagnosed with prostatitis. I’ve been on Cipro but I’m having random itching and small little itchy bumps all over my body at random times. They disappear rather quickly, I also lost 10 pounds of weight either from stress or the Doxcy I was on originallly. Should I still be worried?

  4. Anxious boy
    Anxious boy says:

    Hello doctor,

    I had sex with an indonesian sex work. We had a very long makeout session which involves alot of french kissing. We then proceed to unprotected oral sex and protected sex. Towards the end of service, while removing the condom with tissue, i notice that the condom tore towards the entry of the condom. I’m not sure if it tore because of the way i pull out the condom or during sex. If it’s during sex, am i at risked for HIV? The sex lasted for quite some time, roughly 15-20mins, so Im assuming it broke during sex but i hope not! This happened in April and only recently i got worried. But no symptoms of fever or anything.

    Need your professional advice. Thank you doctor!!!

    • afa_v2
      afa_v2 says:

      Hi Anxious Boy,

      we are not doctors but do consult our doctors to make sure the information is accurate.

      It is very hard for us to make a judgement call, and the only way is to take a test. The whole process is fairly simple and safe. The symptoms associated with HIV are sometimes hard to identify, and only 60% of people recalled that they experience any form of symptoms. Taking a test will give you the answer right away.

      Regardless of the result, we are here for you at every step of the way.
      You can find out more about the process and charges for taking at test – afa.org.sg/ats

  5. Joshua
    Joshua says:

    Sir, thank you for all the hard work. I have a question to ask, i had a risky hiv exposure with a prostitute and i got sick after 10 day, severe fever that only in midnight with backache, dry throat and mouth and white tongue. I went to hiv test 3, 4 and 5.5 weeks, all the tests show negative. Then i went to a private clinic and i was diagnosed with hep b, the doctor put me on lamivudine which i stopped taken after 2 weeks because of allergic reaction. I tested again after a fear of thinking its hiv that caused my allergy at 7 week and the result was negative. I tested negative again at 11 weeks (4weeks post lamivudine) after learning that lamivudine might affect the test result. Another test at 14 weeks (6weeks post lamivudine) and the test was negative. Sir, please is my tests, conclusive or i should test again after 12 weeks post lamivudine (20 weeks post exposure) to be definitive

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