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So here's my limited understanding of how viral infections work.

Some virus enters the body and causes an infection. The virus then begins to replicate itself. It takes some time for there to be enough replicated virus to be detectable.

The body reacts to this infection by producing antigens and antibodies. Likewise, it takes some time for there to be enough antigens and antibodies to be detectable.

In the current state of HIV testing, the 4th generation test looks for antigens and antibodies while the PCR/NAAT test (often referred to as the RNA test) looks for the viral material itself. Naturally the RNA test is able to detect an infection sooner than a 4th generation test can. I learned this as we went through COVID and its testing phases. But it seems like COVID goes through infection - detectable - cured/not-detectable rather quickly. Especially now where people can go through that whole phase in a week.

Most of the information out there suggest that the window period of the 4th generation HIV test to be in the 20-40 day range with a general suggestion for test timing at 30 days. For the RNA test it's in the 7-30 day range however a lot of sources suggest that it is accurate at 10 days.

Various HIV testing window as per CDC

So my question is, why are the far ends of the window periods not too dissimilar between the RNA test and the 4th generation test? Does it actually take that long (30 days) for there to be enough viral material to be detected? With HIV RNA tests the detectable range is like 20-50 copies, and in the initial stages of HIV infection the viral load is spiked to 3-5 degrees of magnitudes above that. Given this context, and especially with people who have a higher risk of infection, shouldn't there be enough confidence to say that the window period is more in the 15 day range? It just seemed to me (who is not very familiar or educated in this field) like the PCR test doesn't provide tons more value than the antibody test.

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  • Do you not think that during the validation of these tests, that someone would have tested them on real patient samples and worked out the kinetics for the sensitivity?
    – bob1
    Sep 4, 2022 at 21:59

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