Q and A


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.


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. 


  1. Simon Collins

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    Please note that i-Base no longer answer questions about HIV testing or HIV transmission.

  2. Lisa Thorley
  3. Richard

    I had sex with an HIV positive person unknowingly on the 9th of July, 2017 with a condom and the condom got broken. I immediately removed my penis (there was no ejaculation) and removed the torn condom and rushed into my bathroom and washed my penis immediately with a running water and soap even though I had scabies lesions on my penis. My penis was exposed to air for about 1 minutes after immediate removal. On the 25th July, 16 days after I used condom again and it got broken unknown to me until after ejaculation then I realized. I went for a rapid card test on the 28th July, I tested negative 19 days after the first exposure and 3 days after the second exposure. I was placed on PEP from that date 9pm till date, to complete PEP on the 26th August. My questions are 1.) Would I have contacted the virus in the first brief exposure of 9th July? 2.) could I believe the test I carried our prior to PEP indicative that my first exposure did not result to infection? 3.) If I go for another test on the 1st of September, 2017 could I rely on the result even though I may have just finished taking PEP? Could the result reflect my status as per my first exposure of 9th July since it may have been 54 days after(7 Weeks)? And what are my chances of infection in the overall? Thank you because I am so scare stiff now.

  4. Lisa Thorley

    Hi Avig,

    Yes this is possible. Because you’ve had both a positive and negative test, you will need this confirming. If the result comes back as positive, please get in touch.

  5. avig

    i did some test and the results was negative after 2 week i did it again and the results come positive.is that possible?