Effectiveness of Covid test without symptoms

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discovolante
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Effectiveness of Covid test without symptoms

Post by discovolante » Sat Sep 26, 2020 8:16 pm

Something I havent been sure about but now suddenly need to know fairly quickly...

Is it just a bit more likely to produce a false negative if you don't have symptoms or what?

This is a genuine clinical question not an urgh Tories one, at least for now.
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jdc
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Re: Effectiveness of Covid test without symptoms

Post by jdc » Sat Sep 26, 2020 10:39 pm

I found this: https://www.bmj.com/content/bmj/369/bmj.m1808.full.pdf which might be helpful?
While positive tests for covid-19 are clinically useful, negative
tests need to be interpreted with caution, taking into account
the pre-test probability of disease.
As I understand it (and it's possible I don't), your negative test is less likely to be false if you don't have symptoms. Anything that reduces your prior probability of having the virus will reduce the chance of you falsely testing negative:
Pre-test probability is high in someone with typical symptoms
of covid-19, an occupational risk of exposure, and working in
a high prevalence region, and negative test results can therefore
be misleading.
A systematic review of the accuracy of covid-19 tests reported
false negative rates of between 2% and 29% (equating to
sensitivity of 71-98%)

They've a table of estimates ("Pre- and post- test probabilities for covid-19 RT-PCR tests") based on 70% sensitivity and 95% specificity on p5

A pre-test probability of 15% drops to one third of that figure on testing negative, while a pre-test probability of 90% only falls to five sixths of that figure on testing negative.

Good luck finding the sensitivity and specificity of whatever tests we're using in the UK. I gave up looking.

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discovolante
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Re: Effectiveness of Covid test without symptoms

Post by discovolante » Sun Sep 27, 2020 8:52 am

Thanks, jdc. I'll try and wrap my head round that later. I'm technically on holiday!
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Sciolus
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Re: Effectiveness of Covid test without symptoms

Post by Sciolus » Sun Sep 27, 2020 11:15 am

There are two distinct questions here, I'm not sure which one you're after. One is, what are the chances that my test result is wrong, given that I do/do not have symptoms and the test came back negative/positive. The other is, are the error rates for infected people different depending whether they are/are not exhibiting the classic symptoms.

The first one is fairly simple: if you show the classic symptoms and haven't been self-isolating, but the test comes back negative, there's a decent chance (at least one in 3) the test is wrong. There's standard maths around this.

The second one, which I think is what you actually asked, is harder. It may be that asymptomatic cases have lower viral loads which are more likely to be missed in swabs, but I dunno. I haven't seen any sensible reports on how infectiousness varies with symptomaticness, which considering it's critical to reopening schools among other things is pretty shocking, so I'm not sure anyone knows.

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Brightonian
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Re: Effectiveness of Covid test without symptoms

Post by Brightonian » Tue Jan 25, 2022 11:36 am

Technical question about self-testing antigen kits. Yesterday I tested myself, and 5-10 seconds after putting the drops onto the place where you're meant to put the drops onto, the result area (the bit with the C and T) got flooded pink - maybe two thirds, probably down past the T. After 10-12 minutes, the C had a pink line, and the T had a very faint pink line (so faint I almost didn't see it and assumed it was negative). What do you think? Assume faulty test* and/or kit and ignore, or assume positive? This morning I did another self test with a different brand and it came up negative.

*Conditions were not ideal: was on a train, hot and sweaty having had to rush with heavy bags, and did the test while train was bouncing along so maybe does't qualify as a flat surface (and one which I'd not been able to clean effectively beforehand).

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discovolante
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Re: Effectiveness of Covid test without symptoms

Post by discovolante » Tue Jan 25, 2022 11:43 am

If you can maybe try and do one with a throat swab as well, seems to be more accurate that way. But I think the flowflex ones do often have just a very very almost invisible faint line on them even when negative (either that or I've had covid multiple times). Who knows though.
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Gfamily
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Re: Effectiveness of Covid test without symptoms

Post by Gfamily » Tue Jan 25, 2022 11:49 am

discovolante wrote:
Tue Jan 25, 2022 11:43 am
If you can maybe try and do one with a throat swab as well, seems to be more accurate that way. But I think the flowflex ones do often have just a very very almost invisible faint line on them even when negative (either that or I've had covid multiple times). Who knows though.
+1
I've had very faint lines - that more or less say "this is where a positive test would show if the test was positive" and taken that as a negative.
I am confident that it is a negative result, because I got pinged by T&T a couple of weeks after one of those tests, and the required PCR test came back negative as well. Given that PCRs can show results for up to 90 days after infection, I take that as indicative that the -ve LFT was genuine.
My avatar was a scientific result that was later found to be 'mistaken' - I rarely claim to be 100% correct
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
Meta? I'd say so!

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