What is a 4th generation HIV test?
I have just looked through the Q & A pages but I am a little bit unsure about one thing.
I live in the midlands and have asked a few clinics if they do they p24 tests I’m guessing this is the 4th generation test? However they are all giving me different information. Some websites say wait 28 days exactly. Others say repeat after 3 months and I have seen on your website that 3-4 weeks after exposure is good enough?
I am feeling a little bit confused. Also if going to a private clinic are there any questions I can ask to make sure it’s legitimate establishment?
Thank you for your question.
Fourth-generation tests look for both antibodies AND antigens. 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.
The p24 tests are fourth generation tests as are Tri-Dot and Duo tests.
3rd generation tests take 6-8 weeks (usually clinics say 3 months) and is only test for antibodies. 2nd generation tests take a few weeks longer after infection before they are able to recognise infection.
4th generation tests are accurate 3-4 weeks after exposure, because this is when the p24 antigen becomes high enough to measure. Everyone makes antibodies at slightly different rates so to make sure the tests are accurate they air on the side of caution and say you should get the tests done at 28 days.
A negative result at 28 days is good news but it is not conclusive. However, UK guidelines (BASHH, 2014) say that an early negative result at 28 days it good enough for most people (and situations),
However, if this was a high risk situation, getting a second test 8 weeks later is recommended to confirm the result.
Most private clinics will use the same tests as free clinics. To check the standard of care of the clinic you are interested in using please follow this link.
This answer was updated in December 2017 from a Q&A first posted in July 2010.