HIV Prevention and Testing > Am I Infected?

4th Generation Tests

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fred:
Hi,

Can anybody explain why technically/scientifically, we cannot rely on 4th generation tests lowering the test window down to 4-6 weeks.

Forget the guidelines, I'd like the science  :)

Darkfiber:
Fred

The 4th generation assay is looking for proteins (P24) of the virus and antibodies (produced by each individuals body).

As soon as antibodies appear P24 is no longer detectable due the p24 / antibody complexing. This is true until very late stage of aids.

The twist is, that the p24 part of the assay is not very sensitive (only around 80%) which is just not high enough to exclude infection.

Eventhough the vast majority of people have detectable antibodies after 6 weeks there is a small minority who may seroconvert later (6-13 weeks)

Thatīs the main reason why you still need to restest after 13 weeks.

PS. In Switzerland we only use 4th generation assays (with exception of rapid tests) and we still have the 3 months windowperiod.

Hope this helps

Best regards

Darkfiber



fred:
But,

my understanding is that not all p24 assays are the same and that some can detect low levels of p24 (say 16 pg/ml) whereas as others have not such good sensitivity ( 50+ pg/ml )

Furthermore, the amount of p24 increases as infection progresses so, therefore, a test at 4 weeks and one at say 6 weeks would surely identify this increase ? (i.e. the 4 week might miss the p24, but the 6 would surely not ?)

I understand the resource ramifications, it's just the science I am interested in.

Fred

RapidRod:
The P24 window period is short, 1-3 weeks of first becoming infected, as antibodies are produced, the antigens decrease to where they will not be picked up by an antigen test.

Darkfiber:
Fred


--- Quote ---Furthermore, the amount of p24 increases as infection progresses so, therefore, a test at 4 weeks and one at say 6 weeks would surely identify this increase ? (i.e. the 4 week might miss the p24, but the 6 would surely not ?)
--- End quote ---

This is not true as such...

http://www.aegis.com/conferences/iac/2002/TuPeC4893.html

The P24 used on 4th generation assays is not  heat-dissociated and therefore has the lower sensitivity.

P24 was used for screening bloodsamples brfore technologies like NAT and PCR have been available. Those are much more sensitive.

Can I ask what was you exposure and why do you want to know?

Regards

Darkfiber

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