This paper used by Sage suggested 400 of 1600 positives were false. Still happy to have your school child home under house arrest for two weeks?

[Treesofwood]

Big problem with false positive us unnecessary self isolation for the person rested. Now it can lead to 300 children also being asked to self isolate for 2 weeks. This paper suggests when numbers are low, which they are at the moment, falso positives are a big problem. The more we test, the more we find them. www.gov.uk/government/publications/gos-impact-of-false-positives-and-negatives-3-june-2020
And the more children miss education and even being allowed to breathe fresh air if other threads on here are something to go by.
Obviously this is not a peer reviewed paper, but it is of enough interest and reliability to be used by Sage. Interested to hear thoughts, including if I have actually totally misinterpreted it.

Sheepandcow It is not within our control. It's a virus. That's the problem.

I have seen quite a few posters countering discussions of false positives with “but there are false negatives too”, as though that cancels out the problems caused by false positives

@Kaktus I wrote to my MP ages ago.
Presumably all the we should all ignore it and 'live' with it kill the vulnerable/develop Long Covid posters write to their MPs and not campaign/post here?
Should nobody comment here and instead only write to their MP?

Mrbucket Agree on that. Different problem. Still a problem.

I think the biggest issue with all of this is massive testing of people who are very unlikely to have Covid -19 with tests that aren't 100% specific or sensitive.

Which means that there will be lots of false positives relative to the number of true positives and also a lot of false negatives who will spread it around.

I don't know what the answer is - unless they can see if there is something that the actual positives have in common - so we can improve the criteria to get a test.

@Treesofwood

Sheepandcow It is not within our control. It's a virus. That's the problem.

Action to mitigate the risks including border restrictions and proper quarantine are within our control.

@LilyPond2

OP, have you included the wrong link? The paper you've linked to says that the UK false positive rate is unknown.

It says it's unknown then gives a very very conservative example of what sort of numbers we could be dealing with.

People are running scared, testing without symptoms, getting false positives, ramping up the rate. Forcing hundreds if not thousands of children out of school for no reason, parents out of work. Judgemental people on mumsnet berating them for even considering taking 6am walks around as they are also spreading the virus (that the original person never even had)

“ I don't know what the answer is - unless they can see if there is something that the actual positives have in common - so we can improve the criteria to get a test.”

I suppose there is re-testing (which obviously exacerbates resource problems!)

We are not just talking about 1 or 2 false positives. Hundreds of them daily.

I'm so glad this is finally being picked up on

Retesting would help. As would some guidance from WHO on PCR levels that count as a positive.

Tha paper says that the false police rate is not definitely esyabkeished but "Results of 43 EQAs were examined, giving a median false positive rate of 2.3% (interquartile range 0.8-4.0%)."

That's a bit different from the 25%!!

@Treesofwood

Retesting would help. As would some guidance from WHO on PCR levels that count as a positive.

TBH - a basic External QA programme would be the basics. As I said, this was my old life many years ago - and ensuring results were valid was so important.

I actually got a bit disillusioned by how 'within the reference range' and 'abnormal' results were reported. But that's a separate issue.

@TrojanWhore

Tha paper says that the false police rate is not definitely esyabkeished but "Results of 43 EQAs were examined, giving a median false positive rate of 2.3% (interquartile range 0.8-4.0%)."

That's a bit different from the 25%!!

It's not '25%. It could be 0.4%

Which means that 0.4% of the people who are negative would test positive.

So if you have a lot of people who are negative, relative to the true positives, then there would be a lot of false positives.

100,000 negative people - 400 would test False Positive
1200 positive people - lets say they all test positive (unlikely)

You have 1600 positive results. But 400 are false positive

This means that if they're testing 100,000 per day they can never get below at least 400 positivesb per day!
It would never look like it's been wiped out.

It would never look like it's been wiped out

Retest positives.
They take several swabs from what I understand - so retest those.

I don't know what they do to conform a positive result - if they test all the swabs, do they all need to be positive?

Trojanwhore that's not how it works.

If 100000 tests are done a day.
98.5% are negative
1.5% are positive.
So 98500 are negative and 1500 are positive.
0.4% of 100000 is 400. (operational false positive) So 400 of the 1500 positives are actually false.
It's not a percentage of the positives. But of the negatives.
Prob why MH asking people not to test with no symptoms. Not that he would be upfront about it.

Can someone please explain how you’d get a false positive? Genuinely don’t get how you would receive one. I get why you could have a false negative due to not enough/any “virus” on the swab.

The title is incorrect. The 400 and 1600 figures are presented as an example when theorising about the possibility of fake positives.

And no I wouldn’t have a problem with it if it was the case.

Trees of wood - Singapore has a higher population density than U.K. and are v dependent on imports for food and essential goods. they got their cases down to a manageable number through very strict enforcement of quarantine, masks, contact-Tracing.

Schools have some distancing measures and kids wear masks. But they can operate well because case numbers in the community are v low. It’s possible but I don’t think UK population are willing to make the trade-off

Can someone please explain how you’d get a false positive? Genuinely don’t get how you would receive one

Contamination is the main cause. Cross contamination. RNA - which is what is being looked for, can get into other tubes, into the reagents for the tests, maybe...but I am not sure about this - other viral diseases could have similar bit of RNA code that could be found by mistake.

You then amplify things up - so any small piece of RNA is suddenly amplified up and detected.

Ckbj The CT value, something to do with the number of times it is repeated amplifies dead or partial fragments of RNA. We have a positive set at 40. Other countries, such as Taiwan, have it set at 30. So less likely to get false positives.

Bluelinings I'm shocked that you don't have an issue with imposed self isolation due to false positives. Why would you think that is OK?

And no I wouldn’t have a problem with it if it was the case

Would you have a problem if it was general mass screening and you were found to be positive that way?

Where there is a much higher chance of that being a false positive?

If you test people who are ill and present with Covid like symptoms, then a positive result has value.

If you test people who are well and don't have symptoms, then I am not so sure it does have value.

And that opens up the ONS screening results.

www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/bulletins/coronaviruscovid19infectionsurveypilot/11september2020

The modelled estimates for the latest six-week period are based on 183,994 swab tests collected over this period. During these weeks, 89 people from 84 households tested positive

So out of 184,000 tests, there were 89 positive people.

(which suggests a higher specificity of course)

For information about the potential impact of false-positive and false-negative test results, see our methods article. We estimate that when different test sensitivity and specificity rates are taken into account, the number of people testing positive for COVID-19 would be similar to the main estimate presented in this section

The estimates provided in Section 2: Number of people in England who had COVID-19 are for the percentage of the private-residential population testing positive for the coronavirus (COVID-19), otherwise known as the positivity rate. We do not report the prevalence rate. To calculate the prevalence rate, we would need an accurate understanding of the swab test's sensitivity (true-positive rate) and specificity (true-negative rate

While we do not know the true sensitivity and specificity of the test, as COVID-19 is a new virus, our data and related studies provide an indication of what these are likely to be. To understand the potential impact of false-positives and false-negatives, we have estimated what the prevalence would be in two scenarios using different test sensitivity and the same specificity rates.

The results of these scenarios show that when these estimated sensitivity and specificity rates are taken into account, the prevalence rate would be fairly similar to the main estimate presented in Section 2: Number of people in England who had COVID-19.