Q and A

Question

Why do I need to repeat a 28-day negative HIV test at three months?

I don’t understand how if a 28 day Duo test when negative is conclusive, why does the information on i-Base say …

“As with antibody tests, a small percentage of people may have a delayed response to HIV so people using this test four weeks after any potential exposure are routinely recommended to confirm a negative result three months later.”

I was negative at 35 days and was moving on with my life until I read this,
please help.

Answer

Note: Since 2021, UK guidelines recommend that a negative result at six weeks using a 4th generation HIV Ag/Ab test does NOT need to be confirmed with a second test. Testing earlier than 6 weeks still needs a second test to confirm a negative result. The earlier answer is kept online to explain the complications of HIV teting. See https://i-base.info/qa/11844

……………

UK recommendations have changed several times on this question. In 2010 when this question was first posted, UK guidelines routinely recommended retesting so the i-Base information referenced this.

Since 2014, the guidelines change to only recommend retesting if this was linked to a high risk of HIV. (See statement below).

Updated guidelines also recommend not having to wait for 28 days before testing. Instead, it is better to test as soon as possible and then retest 4 weeks later. This change was important. It stops people worrying and might pick up early infections. Unfortunately, we still hear of clinics ask people to wait 4 weeks (or longer) before testing.

Even with tests that are incredibly accurate, health care workers have a duty to explain the best chance for a confirmed negative status. The reason for recommending the confirmatory test is two-fold:

  1. A small number of people have individual responses to HIV that may not be picked up by the test at 28 days. When the tests are approved, this is based on a panel of responses from an extensive store of timed blood samples. These show that some samples pick up a positive test result after only a week and some after more than a month, but that most are positive for the p-24 antigen at around 15 days. Some people may not produce enough p-24 antigen for the test to pick this up though, so together with the antibody response, the combined test are still more than 95% accurate at four weeks.
  2. The second reason relates to the possibility that someone who is not picked up, might put another person at risk of HIV. This duty of care should be explained when you take the test the test and are given the results. So although you have almost certainly not caught HIV, you should use condoms with sexual partners, until the confirmatory result.

I wish there was an accurate test at two weeks or one day, but there isn’t. The technology is more complicated than most people realise, and the concern for future transmission of HIV, however slight the possibility, is a professional health care concern.

Please don’t let this cause you stress, you are almost certainly HIV-negative, but this is the detailed reason.

BASHH/EAGA Statement on HIV window period

November 2014

HIV testing using the latest (fourth generation) tests is recommended in the BHIVA / BASHH / BIS UK guidelines for HIV testing (2008). These test for HIV antibodies and p24 antigen simultaneously. A fourth generation HIV test on a venous blood sample performed in a laboratory will detect the great majority of individuals who have been infected with HIV at 4 weeks after specific exposure.

Patients attending for HIV testing who identify a specific risk occurring less than 4 weeks previously should not be made to wait before HIV testing as doing so may miss an opportunity to diagnose HIV infection (and in particular acute HIV infection during which a person is highly infectious). They should be offered a fourth generation laboratory HIV test and be advised to repeat it when 4 weeks have elapsed from the time of the last exposure.

A negative result on a fourth generation test performed at 4 weeks post-exposure is highly likely to exclude HIV infection. A further test at 8 weeks post-exposure need only be considered following an event assessed as carrying a high risk of infection.

Patients at ongoing risk of HIV infection should be advised to retest at regular intervals.

Patients should be advised to have tests for other sexually transmitted infections in line with advice on window periods for those infections (see BASHH guidelines at: www.bashh.org ).

Note: This answer was updated in January 2018 from an original post from October 2010. The additional note was added in September 2023. 

115 comments

  1. Lisa Thorley

    Hi Mercy,

    The only thing you can do is test. Please see quesiton 1 here:

    https://i-base.info/qa/what-are-the-most-asked-questions

  2. mercy

    I slept with this guy unprotected. After that day I been been having what I think are HIV symptoms, should I worry?

  3. Lisa Thorley
  4. Kim J

    I had unprotected sex 11-04-16. And i’ve taken 4th generation combinationHIV test 1-15-16 and it came out negative. What are my chances for not being HIV positive???

  5. Lisa Thorley

    Hi Bebero,

    Please see question one here:

    https://i-base.info/qa/what-are-the-most-asked-questions

  6. Bebero

    I had unprotected sec with a woman on August 15th . I went to see a doctor within 72 hours and he recommended me ” Prep” (isentress & truvada). I get tested for hiv 4th generation on Sept 12 (28 days) after exposure and it was negatif. Now it almost 3 months since exposure.andToday 11/10, I get tested again but ora quick and the result still negative. Should I trust those results or Get tested again ?

  7. Simon Collins

    Hi Muhammed

    I am sorry that we can’t interpret test results but please look at question 1 at this link – which has links to more information online:
    https://i-base.info/qa/what-are-the-most-asked-questions

  8. Muhammed

    Since 4 september 2016, i hve slept with a lady who is not my wife for 4 tmes with protected sex in the first round but i remove the condom in the second round, am scared coz when ever i go with her, i become sickly, body becomes weak the following day, its now almost 40 days since the first exposure but upto now i have never been normal lyk before i feel joint pain, tencilz, musle pain and night sweats, i went to see the doctot and tested negtve hiv, but postve with typhoid and UTI, am i safe doctor bcoz am worried

    e

  9. Simon Collins

    Hi Distressed, please see Q1 at this link:
    https://i-base.info/qa/what-are-the-most-asked-questions

  10. distressed

    my has been having unprotected sex with a woman. and he was intimate with me too without any protection. my last sex encounter (unprotected) with my husband is on 5th Oct 2016., after which i discovered about his activities. how many days should i wait to get tested? pl help me out. I have already lost my appetite and sleep over this from that day.

    Pl pl help me out.