Why 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,
Answer: Simon Collins
The context of HIV testing is quite complicated.
Even with tests that are incredibly accurate, health care workers have a duty to explain the best chance for a confirmed negative status.
In the UK, even with dual antibody/antigen test, guidelines for many years stuck to the three-month window from exposure before testing. This was historical and in my personal opinion, out of date given the advances in the testing technology.
In March 2010, one of the professional associations involved in HIV testing – the British Association for Sexual Health and HIV (BASHH) – released updated guidelines, recognising that the vast majority of infections (over 95%) were likely to be detected after four weeks (ie by four weeks after the exposure). They also recommend a confirmation test three months later, and as i-Base are based in the UK, we want to provide consistent information, as long as this is supported by evidence. See below for the statement’s exact wording.
This change is welcomed. The current technology does not support any need for most people to have the stress of waiting an additional two months because of outdated guidelines. Unfortunately, we still hear of clinics that have still not changed their practice.
I understand that the reason for recommending the confirmatory test is two-fold:
i) 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.
ii) 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.
15 March 2010
HIV testing using the latest (4th generation) tests are recommended in the BHIVA/BASHH/BIS UK guidelines for HIV testing (2008). These assays [tests] test for HIV antibodies and p24 antigen simultaneously. They will detect the great majority of individuals who have been infected with HIV at one month (4 weeks) after specific exposure.
Patients attending for HIV testing who identify a specific risk occurring more that 4 weeks previously, should not be made to wait 3 months (12 weeks) before HIV testing. They should be offered a 4th generation laboratory HIV test and advised that a negative result at 4 weeks post exposure is very reassuring/highly likely to exclude HIV infection. An additional HIV test should be offered to all persons at 3 months (12 weeks) to definitively exclude HIV infection. Patients at lower risk may opt to wait until 3 months to avoid the need for HIV testing twice.