Accuracy of tests
- Tessa K
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Accuracy of tests
Sorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
Re: Accuracy of tests
The short answer is no.Tessa K wrote: ↑Wed Sep 30, 2020 11:36 amSorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
I've been trying to find a definitive answer for the correct number but I don't think there is one. Most reliable sources say, however, that the PCR test has very high specificity and the only numbers I could find say <1%
https://fullfact.org/health/coronavirus ... -accuracy/ is worth a read
I suspect the 40% comes from the rate of false negatives where people are tested very soon after exposure and the infection isn't detectable yet.
Re: Accuracy of tests
It's possible that if the incidence is sufficiently low and there is random testing, then 40% of the positive results could be false, but that would rely on a very low incidence rate.Tessa K wrote: ↑Wed Sep 30, 2020 11:36 amSorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
As it is, the tests are being conducted on people who have some level of symptoms, so false positives will be a much lower percentage.
I think More or Less covered this recently
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ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn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!
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn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!
Re: Accuracy of tests
This might be of interest: https://www.bmj.com/content/bmj/369/bmj.m1808.full.pdf
Re: Accuracy of tests
There's also the point that false positives should occur with a roughly constant probability, with an upper ceiling for this set by the observed rate for random asymptomatic testing during a period of low prevalence (ie not through contact tracing for example). Whatever the lowest % of positive tests has been over the whole pandemic, the false positive rate must be equal or less. Most importantly, false positives cannot explain an increase in the proportion of positive tests.Gfamily wrote: ↑Wed Sep 30, 2020 12:35 pmIt's possible that if the incidence is sufficiently low and there is random testing, then 40% of the positive results could be false, but that would rely on a very low incidence rate.Tessa K wrote: ↑Wed Sep 30, 2020 11:36 amSorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
As it is, the tests are being conducted on people who have some level of symptoms, so false positives will be a much lower percentage.
I think More or Less covered this recently
Re: Accuracy of tests
Exactly - the dashboard that @TravellingTabby on Twitter produces atAMS wrote: ↑Wed Sep 30, 2020 8:13 pmThere's also the point that false positives should occur with a roughly constant probability, with an upper ceiling for this set by the observed rate for random asymptomatic testing during a period of low prevalence (ie not through contact tracing for example). Whatever the lowest % of positive tests has been over the whole pandemic, the false positive rate must be equal or less. Most importantly, false positives cannot explain an increase in the proportion of positive tests.Gfamily wrote: ↑Wed Sep 30, 2020 12:35 pmIt's possible that if the incidence is sufficiently low and there is random testing, then 40% of the positive results could be false, but that would rely on a very low incidence rate.Tessa K wrote: ↑Wed Sep 30, 2020 11:36 amSorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
As it is, the tests are being conducted on people who have some level of symptoms, so false positives will be a much lower percentage.
I think More or Less covered this recently
https://www.travellingtabby.com/uk-coronavirus-tracker/
Shows this positivity rate rising to about 4% overall and ICU occupancy highest for 3 months
Have you considered stupidity as an explanation
- Tessa K
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Re: Accuracy of tests
I did ask him if that was false positives or negatives and he said definitely positives. It did seem an unlikely high percentage which is why I thought I'd check here before repeating it.bagpuss wrote: ↑Wed Sep 30, 2020 12:13 pmThe short answer is no.Tessa K wrote: ↑Wed Sep 30, 2020 11:36 amSorry if this has been covered but there are many threads I could have posted in and couldn't choose between them.
I had a flu jab today at Boots and the pharmacist told me the Covid test is giving around 40% false positives, the quick test even worse. Is this right?
I've been trying to find a definitive answer for the correct number but I don't think there is one. Most reliable sources say, however, that the PCR test has very high specificity and the only numbers I could find say <1%
https://fullfact.org/health/coronavirus ... -accuracy/ is worth a read
I suspect the 40% comes from the rate of false negatives where people are tested very soon after exposure and the infection isn't detectable yet.
Re: Accuracy of tests
Yeah, my understanding was that the false negative rate is quite high, but the false positive rate is low enough to be negligible for most purposes. For a layman like me, this makes sense if you consider that there are a bunch of reasons for a false negative (low viral load, poor swab, too early in infection so virus hasn't reached swabbed areas, degraded sample etc) but few reasons for a false positive (result assigned to wrong person, that's about all I can think of).
Re: Accuracy of tests
Yes, this is something that could be a possible issue, much loved by the covid sceptics, but isn't
https://www.bbc.co.uk/sounds/play/m000mr42 9 minutes - best explanation
https://www.bbc.co.uk/sounds/play/m000mr42 9 minutes - best explanation
Re: Accuracy of tests
I couldn't think of any but I googled to see if someone else could:Sciolus wrote: ↑Thu Oct 01, 2020 11:05 amYeah, my understanding was that the false negative rate is quite high, but the false positive rate is low enough to be negligible for most purposes. For a layman like me, this makes sense if you consider that there are a bunch of reasons for a false negative (low viral load, poor swab, too early in infection so virus hasn't reached swabbed areas, degraded sample etc) but few reasons for a false positive (result assigned to wrong person, that's about all I can think of).
They go on to say:Technical problems including contamination during sampling (eg, a swab accidentally touches a contaminated glove or surface), contamination by PCR amplicons, contamination of reagents, sample cross-contamination, and cross-reactions with other viruses or genetic material could also be responsible for false-positive results.2 These problems are not only theoretical; the US Center for Disease Control and Prevention had to withdraw testing kits in March, 2020, when they were shown to have a high rate of false-positives due to reagent contamination.5
https://www.thelancet.com/journals/lanr ... 7/fulltextThe current rate of operational false-positive swab tests in the UK is unknown; preliminary estimates show it could be somewhere between 0·8% and 4·0%.
Re: Accuracy of tests
We know from New Zealand that the false positive rate has to be very low - otherwise they'd have been chasing imaginary outbreaks.jdc wrote: ↑Thu Oct 01, 2020 5:30 pmI couldn't think of any but I googled to see if someone else could:Sciolus wrote: ↑Thu Oct 01, 2020 11:05 amYeah, my understanding was that the false negative rate is quite high, but the false positive rate is low enough to be negligible for most purposes. For a layman like me, this makes sense if you consider that there are a bunch of reasons for a false negative (low viral load, poor swab, too early in infection so virus hasn't reached swabbed areas, degraded sample etc) but few reasons for a false positive (result assigned to wrong person, that's about all I can think of).
They go on to say:Technical problems including contamination during sampling (eg, a swab accidentally touches a contaminated glove or surface), contamination by PCR amplicons, contamination of reagents, sample cross-contamination, and cross-reactions with other viruses or genetic material could also be responsible for false-positive results.2 These problems are not only theoretical; the US Center for Disease Control and Prevention had to withdraw testing kits in March, 2020, when they were shown to have a high rate of false-positives due to reagent contamination.5https://www.thelancet.com/journals/lanr ... 7/fulltextThe current rate of operational false-positive swab tests in the UK is unknown; preliminary estimates show it could be somewhere between 0·8% and 4·0%.
Have you considered stupidity as an explanation
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Re: Accuracy of tests
Good point. At the time of writing, overall totals are
Total people tested 746,478; Test rate per 1000 head of population 151; Tests returning positive result 0.2% (1,492 out of the 746,478)
The current figures from the NZ Ministry of Health can be found here:
https://www.health.govt.nz/our-work/dis ... ty-and-dhb
Money is just a substitute for luck anyway. - Tom Siddell