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Mainly the thing I'm trying to understand is "how do repeated LFT testing and PCR testing interact with false negatives".

But I think the answer to that is a fairly direct function of what causes false -ves on LFTs.

  • Is it simply that whatever the testing "pallet" does has a random noise component to it, and sometime you get unlucky?
  • Is it that the sampling process is noisy, and thus sometimes you get unlucky with the swab and don't catch any viral particles?
  • Is it that sometimes the body is compromised but immune system controls the viruses presence in such a way that there isn't anything in the upper-respiratory tract?
  • Is it that some mutations of covid don't trigger the tests?
  • Something else?

Respectively for the above possibilities, they would mean that:

  • If you get a false -ve, you could just put new drops form the same sample in another pallet and immediately get a true +ve.
  • If you get a false -ve, you could take another swab and immediately get a true +ve.
  • Either:
    • If you get a false -ve, then that might indicate that LFTs are mostly worthless for you personally. (If your personal immune system fundamentally prevents them doing their job)
    • If you get a false -ve, you would consistently get false -ves for at least another <x days/hours>
  • If you get a false -ve, you will continue to get false -ves for the entire course of this infection ... but LFTs might work again on a different infection.
  • ???

And likewise for the interaction with PCR testing:

  • LFT result has no interaction whatsoever with PCR.
  • If you had used that particular sample for the PCR test then you would have gotten another False -ve.
  • If you take 2 samples for LFT and PCR at the same time then you'll get False -ves on both.
  • LFT result has no interaction whatsoever with PCR.
  • ???

Obviously, plenty of those guesses may not be biologically or statistically possible ... but hopefully they're useful as an indication of the sorts of distinctions I'm asking about and the effects that the mechanics of a False -ve could conceivably have.

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  • $\begingroup$ Another way to phrase this could be ... "If I take lots of LFTs one-after-another, after a high-risk exposure, and they're all -ve ... does that change the odds of whether I'm actually covid-free?". "What if I take them over the course of a day/week?" $\endgroup$
    – Brondahl
    Mar 21, 2022 at 10:02
  • $\begingroup$ Or "If I tested continuously LFT -ve for 2 weeks, but consistently PCR +ve in the middle ... does that tell me anything interesting/useful about future LFTs" $\endgroup$
    – Brondahl
    Mar 21, 2022 at 10:03

1 Answer 1

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  • Is it simply that whatever the testing "pallet" does has a random noise component to it, and sometime you get unlucky?
  • Is it that the sampling process is noisy, and thus sometimes you get unlucky with the swab and don't catch any viral particles?
  • Is it that sometimes the body is compromised but immune system controls the viruses presence in such a way that there isn't anything in the upper-respiratory tract?
  • Is it that some mutations of covid don't trigger the tests?
  • Something else?

Yes -all of the above, apart from the "immune system controls viral presence to not upper respiratory tract" point. That bit is almost entirely down to viral factors and attachment factors such as the presence of the receptor proteins/sialic acids etc.

Basically the way that lateral flow tests work is that they solubilize the sample, which is then dropped into the well. From there the liquid moves along into the flow part of the test, filtering out large junk and binding to labeled antibodies. When the sample reaches the test line, a second antibody binds to the label on the first antibody, which results in a coloured reaction between two chemical components. The control line does similar, but is designed to detect any antibody or perhaps something that is spiked (added) into the kit as a control. There's a description and video of how these work here.

If any one of these steps goes wrong, from getting a sample without enough virus, to no reaction at the control line (or at the test line), then you could have a false negative. A common problem is not sampling enough virus to work. I don't know how much virus is needed to be added, but in my tests on early versions of these with purified virus (not nasal swabs), I could get down to about 1000 viral particles per test based on qRT-PCR assays of viral concentration. The sensitivity of these tests will depend on the brand and exactly what they are testing for, and should come on an information sheet which you can either get with the test or look up on your health authorities' authorization of the tests.

However, the most common false negative I suspect will be that the test subject has been tested outside the test window, and there is not enough virus present to trigger the test as positive. Generally you need to be between about 3-days and 2-weeks post-infection for these tests to detect SARS-CoV-2 in patient samples.

On the other hand, PCR tests are much more sensitive, and can detect the virus before you are infectious, and long after the infectious window has passed.

There's a good graph of this from a paper by Dankova et al at the International Journal of Environmental Research and Public Health1:

LFA PCR sensitivity

In this case the lateral flow test is marked as "Ag" (for antigen; these tests are often called rapid antigen tests (RATs) too). As you can see the Ag section is quite short and starts well after the infectious period starts.

Now for your next bits:

Respectively for the above possibilities, they would mean that:

  • If you get a false -ve, you could just put new drops form the same sample in another pallet and immediately get a true +ve.

Possibly, but this would be a failure of the first test rather than a testing/sampling problem

  • If you get a false -ve, you could take another swab and immediately get a true +ve.

Maybe, this could be a sampling error, nothing to do with the test itself, or a test failure.

Either: * If you get a false -ve, then that might indicate that LFTs are mostly worthless for you personally. (If your personal immune system fundamentally prevents them doing their job)

No, they shouldn't be personally worthless to you, they don't have anything to do with your immune system or any part of your body. When there is enough virus there, they should detect it.

  * If you get a false -ve, you would consistently get false -ves for at least another <x days/hours>

Maybe - this could be that you've sampled outside the window for the test to work, as you sort of hint at, or that you didn't sample properly

If you get a false -ve, you will continue to get false -ves for the entire course of this infection ... but LFTs might work again on a different infection.

No, this is very unlikely unless you have a dramatic genetic change in the virus such that the antigen is no longer picked up by the test.

And likewise for the interaction with PCR testing:

*LFT result has no interaction whatsoever with PCR.

They don't interact - they don't detect the same thing. The PCR detects the viral RNA, the LFA/RAT detects viral proteins.

  • If you had used that particular sample for the PCR test then you would have gotten another False -ve.

Maybe, PCR is much much more sensitive and capable of detecting the virus for longer, so a negative on a LFA/RAT doesn't mean a PCR would be negative. False negatives by the PCR tests are very rare as the tests include internal controls to ensure a correct sample has been taken.

If you take 2 samples for LFT and PCR at the same time then you'll get False -ves on both.

Unlikely. Same as immediately above. Repeat sampling of the same sites could remove the necessary material for the test to work. However, generally the PCR swab is taken in the deep nasal tissues, while the RAT/LFA is from shallow and/or throat. The PCR should also work on the RAT/LFA samples, but I believe is less reliable due to the presence of nasal mucus which interferes with the reaction. I'd have to really dig through the literature to find a reference for that, but I think I read one a couple of years ago.

  • LFT result has no interaction whatsoever with PCR.

Same as 3 points above.

1: Dankova, Z.; Novakova, E.; Skerenova, M.; Holubekova, V.; Lucansky, V.; Dvorska, D.; Brany, D.; Kolkova, Z.; Strnadel, J.; Mersakova, S.; Janikova, K.; Samec, M.; Pokusa, M.; Petras, M.; Sarlinova, M.; Kasubova, I.; Loderer, D.; Sadlonova, V.; Kompanikova, J.; Kotlebova, N.; Kompanikova, A.; Hrnciarova, M.; Stanclova, A.; Antosova, M.; Dzian, A.; Nosal, V.; Kocan, I.; Murgas, D.; Krkoska, D.; Calkovska, A.; Halasova, E. Comparison of SARS-CoV-2 Detection by Rapid Antigen and by Three Commercial RT-qPCR Tests: A Study from Martin University Hospital in Slovakia. Int. J. Environ. Res. Public Health 2021, 18, 7037. https://doi.org/10.3390/ijerph18137037

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