This explains false positives https://www.google.com/amp/s/www.bbc...s/amp/54270373
This explains false positives https://www.google.com/amp/s/www.bbc...s/amp/54270373
Hi Mark - we have covered this extensively on the forum so at risk of repeating myself to some of the avid followers of the thread here goes.
If I get a bit of terminology incorrect I apologise, but I firmly believe the thrust of what I am posting here is correct.
All tests have a False Negative and False Positive rate and they are usually declared. They refer to the clinical, laboratory errors that are associated with all tests.
Normally these FNs and FPs are declared so that analyse can take the range of error in to account.
With the PCR test, these figures have not been declared.
In addition to the technical shortcomings of tests, there can be additional shortcomings and this is particularly relevant where Guinea Pigs are mostly using home testing kits, or turning up at outdoor testing stations where mostly non-clinically trained staff are handling the testing process and batches of samples are being picked up at times by self-employed couriers and taxis, more often handling Just Eat deliveries or Hermes orders from Next.
As Governments haven't declared the FNs and FPs for the PCT tests, there has been analysis of various findings that have led to a consensus on FNs in the range of 20-30% and of FPs in the range of 0.8% to 4%.
I'm going to set out some illustrations now using 20% and 0.8% to give you an idea of outcome.
Scenario 1.
In hospital testing where likelyhood of CV infection is high and assuming a 50% rate of infection with CV.
Test 1000 people. Expect 500 positives.
400 Positives will test positive.
4 Negatives will test positive.
596 test negative.
The consequences are that 400 will be treated correctly, 4 people might be treated incorrectly as having CV but they clearly have a severe respiratory illness as they are in hospital.
96 people will not be treated as having covid who should, but repeat testing will end up catching most.
Most importantly, the testing results whether right or wrong are backed up by a clinical diagnosis.
Scenario 2.
General Public Testing where most are not showing any symptoms and community infection is thought to be 1%
Test 1000 people. Expect 10 positives.
8 positives will test positive.
8 negatives will test positive.
984 will test negative.
The consequences are that 2 of the infected cases will go about their business as usual. 8 will be quarantined, kept off work without genuine cause.
The figures are inflated by 60%. Policy decisions are based figures that are out by a significant amount.
Scenario 3.
Covid has disappeared from the general population.
Test 1000 people. Expect 0 positives.
There are no positives to test negative as a result of the FN error.
8 negatives will test positive.
The consequences are that Covid can not be eliminated. That this rate of FPs gives us a community rate of 800 per 100,000 population.
Here is the table for the Top 30 Middle Layer Super Output areas (statistical areas of around 7500 population).
https://twitter.com/RP131/status/133...407747/photo/1
Our Scenario 3 would fall in to 4th place at the moment nationally, be placed in Tier 3 and yet the reality is that there is no community infection.
I hope that sets out in a straightforward way the problems that have been highlighted around use of the PCR test for mass public surveys. It's an issue that I first came across in July, but experts at Porton Down flagged it up to SAGE in May, asking for SAGE to deal with this.
Richard Taylor
"William Tell could take an apple off your head. Taylor could take out a processed pea."
Sid Waddell
The problem with this explanation is that it asserts that the people attending the test centres were symptomatic.
1. Symptomatic of what? September is rhino virus season. Back from holidays, back to school, back to University and so it cannot be assumed that even someone with symptoms had Covid.
2. Mass testing was taking place in schools and Universities - no symptoms required.
3. Local authorities often had a "Come of Down" policy for their mobile testing centres, such as here in Blackburn where as late as 9th September the Council announced on Facebook that anyone can be tested.
They even had teams going around knocking on doors asking people to sign up for a test.
4. People were booking tests and ticking the appropriate symptoms box to get a test. Sometimes prior to, or after travelling overseas.
So whilst it sets out the issue of false positives, it seeks to discredit the validity of the concerns raised.
BBC speaks with forked tongue.
Richard Taylor
"William Tell could take an apple off your head. Taylor could take out a processed pea."
Sid Waddell
The test’s instructions for use state that it should not be used on asymptomatic people.
https://www.bmj.com/content/371/bmj.m4436
Richard Taylor
"William Tell could take an apple off your head. Taylor could take out a processed pea."
Sid Waddell
WP, do you believe epidemiologists are not taking false positives into account when analysing results from mass testing?
As WP points out, the nuances of false negative and false positive categories can take a while to get your head around.
Add to that the fact that the principles of epidemiology and disease control can also be nuanced and poorly understood by the general population and then stir in a healthy dose of fake news/conspiracy theory and you can appreciate that this whole sh!t-show is almost impossible to navigate through. Especially for a politician who is unlikely to be trained in any of the above and also subjected to all the other distractions such as the economy, keeping voters happy etc etc.
The main news channels like to give nice, easily digestible packages to the great unwashed but the daily stats they show are pretty meaningless. Would be much better to give daily capacity figures for NHS regions. If you knew that your local hospital was full (as in Stoke on Trent) then you might think twice about doing anything risky or ignoring covid advice.
Simon Blease
Monmouth
As a number of people on this thread like statistics here are a few figures to consider:-
Member and number of posts on this thread.
Witton Park 548
Dave_Mole 326
Oracle 262
Mike T 251
Fellbeast 238
Stagger 231
Muddy Retriever 229
Graham Breeze 226
Travs 144
Llani Boy 117
Flem 108
Mossdog 87
molehill 83
Wheeze 81
anthonykay 63
noel 61
JohnK 61
Derby Tup 51
Wetherby whaler 40
CL 33
Mark G 24
bigfella 22
Daletownrunner 21
MattPo 19
Steph 18
jackd 17
RichA 16
Marco 15
TheGrump 15
PeteS 14
mr brightside 12
creaky 11
brett 10
There may be a few false positives or non-negatives in the above figures
Fox Avatar "Protected" by Hester Cox - Printmaker
I can't answer that. Epidemiologists are not a group that has one opinion on this.
But consider the makeup of SAGE.
No Clinical Immunologists.
No Biology Degree.
There are a few medics, some sociologists, psychologists.
They do have 7 mathematicians - the discipline most represented.
People think SAGE is a co-opted group of people that are experts in virology/immunology. I thought as much. BUt they aren't and that was clear when the membership was published.
https://www.bmj.com/content/bmj/369/bmj.m1808.full.pdf
This was May.
It is highlighting the lack of the "Gold Standard" for the Covid test. At that time they are looking at it from the perspective of a False Negative because the prevalence in the particular setting is high.
More recently the concerns have moved to the False Positive, because the test has been used among the public en mass, and the prevalence there is low.
Being generous, I think the issue is one of mindset. Ferguson et al have over egged the potential of outbreaks in the past.
What are the consequences of being wrong on the side of caution?
Imagine if Ferguson had modelled 25k would have died of Covid without a lockdown? Better to be on the high side.
Being ungenerous, I sense a political motive behind it.
Academia and the Public Sector are steeped in the left.
In the UK they abhor Brexit and the Tory Government, as exemplified by the Dr Deepti Gurdasani quoted by Mike.
Across Europe they have Le Penn, AFD, Salvini....
In the USA Trump.
The western world was heading the wrong way. What better way to enact a handbrake turn?
Richard Taylor
"William Tell could take an apple off your head. Taylor could take out a processed pea."
Sid Waddell
already out of date but I have to say Oracle is impressive in 3rd place as he was only contributing for around 1/3 of the time the thread has been running.
In mitigation for myself and like-minded people, we have been a little out-numbered (2 have been banned) so tend to post more than others to answer questions or deal with points raised, a roleI currently seem to share with Muddy.
MR - you need to pull your weight a little more and lighten my load
Richard Taylor
"William Tell could take an apple off your head. Taylor could take out a processed pea."
Sid Waddell