[Closed] Freedom Passes & False Positives

It does highlight an issue which is well understood by professionals in diagnostics but not necessarily by the politicians, although the issue isn't quite as dramatic as it seems. Firstly when you illustrate an example like that with real numbers its wrong to then assume those numbers apply to other examples.

So, the false positive rate on mass screening tests isn't necessarily as high as in that example - some manufacturers are claiming <1:1000 might be achievable even with simple rapid tests. There will be some trade off against false negatives here.

Secondly, the video assumes that you would test everyone at random - in the case they describe for coeliac disease people don't test the whole population, you test people who are either symptomatic or family members of people already diagnosed. That massively changes that first 1% prevalence number. It might be that this selection criteria shifts the prevalence to 20% for coeliac diseases in the population you actually test - in which case your numbers will be dramatically different. Depending how you deploy it for covid the numbers could be very small, but they could be more focussed and so less susceptible to this issue. e.g. currently we demand people who have simply been in close contact with a confirmed case to isolate for 2 weeks - how is that better "fair" if they don't actually have an infection themselves.

Thirdly, what the video only briefly touched on with the closing line was "get a second opinion". In the case of coeliac disease this is usually endoscopy which is horrible for the patient and expensive for the system. In the case of a Sars-Cov2 rapid test, a positive can be followed by a confirmation test - you can get a follow up PCR test (slower, more expensive, more specialist equipment - but exactly what we are doing right now and simple for the patient) which despite what twitter says is actually a very robust way of confirming if you have the virus. The probability of both tests giving a false positive on a healthy individual are very small. By restricting demand for PCR testing to people who have "failed" the screening test, or other specialist requirements you reduce testing bottlenecks and make it more likely people can get a result in 24 hrs.

Moreover someone who is confirmed as PCR positive, may then be entitled to compensation for self isolating, and (and this has been discussed in select committee but not in the wider media to my knowledge) may then be exempt from testing for the next X (12?) weeks after their isolation is up. That provides the incentive to get tested.

...won’t false positives mean scores of people needlessly excluded from things like going to a gig, attending the theatre, having an operation, or going on holiday etc?

Yes it will*. But currently nobody can do those things - is it reasonable to temporarily restrict 1% of people from doing something whilst their status is confirmed so that 99% of people can continue to do it? After all any individual who wishes to self impose lock down on themselves can continue to do so with no risk of mis-categorisation.

But lets be clear this is a level of testing nobody has ever done in the world for anything and designing it right is no simple feat; and getting it right in a time of 24/7 news and social media scrutiny means every screw up (and there will be plenty) is highly publicised. Its actually quite a gamble for politicians to take - because they will be blamed when Mrs Jones the school nurse hasn't followed the process right and 1/2 the school get isolated, or Shaz and Dez don't get to their dream wedding in barbados because of a false positive, or "Big Gav" misses his team win the FA cup for the first time in 120 yrs...

*at least on the literal meaning of scores.

Well it could get confusing as I already have a Freedom Pass, it's what London Councils call a bus pass.

Well it could get confusing as I already have a Freedom Pass, it’s what London Councils call a bus pass.

If it's anything like the teir system there's a good chance they'll be one and the same.

Well it could get confusing as I already have a Freedom Pass, it’s what London Councils call a bus pass.

Did boris set that up when he was Mayor and was getting IT lessons from whats-her-name? Don't be surprised if whoever gets the gig of running this has some links to the eton mafia and no actual experience of running secure IT...

I prefer this video explaining false positives.

Not sure why 😉

Secondly, the video assumes that you would test everyone at random

I understand this is what they are doing though with mass testing roll-outs, no Covid symptoms required. They are also testing ALL people being admitted to hospital so plenty of potential for false positives there.

Thirdly, what the video only briefly touched on with the closing line was “get a second opinion”. In the case of coeliac disease this is usually endoscopy which is horrible for the patient and expensive for the system. In the case of a Sars-Cov2 rapid test, a positive can be followed by a confirmation test – you can get a follow up PCR test (slower, more expensive, more specialist equipment – but exactly what we are doing right now and simple for the patient) which despite what twitter says is actually a very robust way of confirming if you have the virus. The probability of both tests giving a false positive on a healthy individual are very small.

My understanding is that very few positive tests, even 1'st PCR tests are currently backed up with a second confirmatory test. Even a 1st test is regarded as a 'Case' in the eyes of the statistics. A 'Case' of Covid even in the absence of symptoms?

I personally know 3 people who have had test kits sent out to their home but who haven't sent them back. All received a 'positive' test result despite not ever undertaking the tests. However you explain it, some big errors with the numbers are currently happening somewhere.

I understand this is what they are doing though with mass testing roll-outs, no Covid symptoms required. They are also testing ALL people being admitted to hospital so plenty of potential for false positives there.

But they haven't agreed the roll out protocols. The purpose of the Liverpool trial, is to understand these issues in real scale. Really you want to be more worried about the false negatives - those are the people the tests says don't have COVID who are then permitted to socially interact and spread the disease...

My understanding is that very few positive tests, even 1’st PCR tests are currently backed up with a second confirmatory test. Even a 1st test is regarded as a ‘Case’ in the eyes of the statistics. A ‘Case’ of Covid even in the absence of symptoms?

Correct - because PCR testing is extremely low rate of false positive - if you detect the virus, the RNA was almost certainly present. There is an argument about whether someone is infected/infectious, but that scientific debate is being twisted by the social media enthusiasts to claim that there is massive over counting in the system.

This video explain the same issues, but talks specifically about COVID, and if you stick with it all the way through gets to some interesting genuinely random sample data from the summer which shows that the FPR for PCR is better than everyone uses in their example calculations.

So, lets assume we test 10 million people - with 99.9% specificity (1:1000 false positive) and 95% specificity (5% false negatives).

Scenario 1: Only 0.5% of the population have the virus in their sample (I'll call them infected for simplicity) - current worst UK hotspots

10,000,000 => 50K infected => 47.5K detected via the test;
=> 2.5K missed (false negs).
=> 9,950K not infected => 9,940K people free to live life close to normal
=> 9.9K people asked to isolate, and possible confirmation test, despite not being infected


So is it OK to ask 10K people to self isolate (possibly only for 24-48 hrs whilst a secondary test is performed) so that 10M can live life as normal?


Scenario 2: Only 0.05% of the population have the virus in their sample (I'll call them infected for simplicity) the lowest prevalence areas we currently have.

10,000,000 => 5K infected => 4.75K detected via the test;
=> 250 missed (false negs).
=> 9,995K not infected => 9,985K people free to live life close to normal
=> 10K people asked to isolate, and possible confirmation, despite not being infected



Scenario 3: Only 2% of the population have the virus in their sample (I'll call them infected for simplicity) this might be representative of schools, employers, universities where there is a local problem.

100,000 => 2K infected => 1.9K detected via the test;
=> 100 missed (false negs).
=> 98K not infected => 97.9K people free to live life close to normal
=> 98 people asked to isolate, and possible confirmation test despite not being infected


The alternative is the status quo - none of us get to live life remotely like normal. TT&T tell people to isolate simply because of contact - which is probably something like an 80% false positive rate, and nothing to stop you being told to isolate this week and then in two weeks time and then in 5 weeks time...

I personally know 3 people who have had test kits sent out to their home but who haven’t sent them back. All received a ‘positive’ test result despite not ever undertaking the tests. However you explain it, some big errors with the numbers are currently happening somewhere.

I've no idea whats going on there. I've no idea if those are reported to the people as a positive as a fail safe, nor if such "fail safe positives" are part of the reporting nationally rather than to the individual. Certainly I don't think they should be reported even to the individual this way. Claims that "big errors" are happening sounds like social media hysteria to me unless you are on the inside and have access to the giant excel file?

The false positive rate in the pcr test has been huuugely over exaggerated by people who really should understand the science better and then amplified by clueless social media hacks

Teethgrindingly annoying to hear the same pseudoscience repeated over & over

You are being very patient Poly, but before you get too drawn in, please be aware that the OP joined the forum only yesterday, and it might be wise to be sceptical about whether their contributions are made in good faith. Keep on keeping on...

Teethgrindingly annoying to hear the same pseudoscience repeated over & over

Especially if from the same person posting under a series of different names.

Thanks for posting Richmtb

please be aware that the OP joined the forum only yesterday, and it might be wise to be sceptical about whether their contributions are made in good faith

Good spot. Busted!

My Mass-skeptometer just went past 11.

Thanks for posting Richmtb

No problem, I think she has some other videos on her channel. I'm not sure, I definitely didn't watch all of them.