to explain why testing lots of asymptomatic people everyday is a bad idea

[chomalungma]

They want to do lots of testing on asymptomatic people everyday.
Aim is to see if people are positive, and if so, then those people have to self isolate.

Issue is: There will be many people who will have to self isolate who don't need to because of false positives

All to do with how specific a test is, how sensitive it is, and the prevalence of disease in the population you are screening.

Let's say the levels of disease are 1 in 10,000

So in 1 million people, 100 people will have the disease

And 999,900 people won't have the disease.

They get a really good test; It detects disease in 99% of the people who have it.

So in the 'ill' population, we have 99 positives and 1 negative

The test is also very specific. Say 99.5% specific. That means in the 999,900 people who don't have the disease, we will get 994,900 people who test negative and 5000 who test positive (False positives)

So we have 5099 positive tests.
And 99 of them are actual positives

So 98% of the positive results are false positves

(this is based on a level of 1 in 10,000 and those figures for specificity and sensitivity) Obviously a test needs to have really good specificity and sensitivity and it needs to be done on a population with a relatively high chance of having the disease.

From that:

Testing should be driven by public health considerations and priorities. Priority groups for mass testing to reduce transmission should be identified according to their likely contribution to reducing R and outbreak risks, and improving health, social and economic outcomes including reducing inequalities.

Mass testing is most likely to be beneficial and feasible in cluster outbreak scenarios and well-defined higher risk settings (e.g. health and social care settings, higher risk occupations such as food production facilities, and universities), where it can help detect and prevent large outbreaks early, and compliance can be measured and moderated

From highly accessible fast turn-around testing to structured financial and other support (particularly for most disadvantaged groups), a system-wide capability must be built

In populations with low prevalence of infection, mass testing that lacks extremely high specificity would result in many individuals receiving false positive results. In such circumstances, rapid follow-up confirmatory testing will be needed to determine whether individuals should continue to self-isolate – it is important to rapidly isolate infectious individuals, but efforts will be needed to quickly release false positives

(Which is what @TheLastStarfighter said)

@RedToothBrush

I think you have hit the nail on the head here! Possibly also trying to deflect from the government planning to break international laws...

@chomalungma

although the OP doesn't seem keen to engage with this point, simply retesting all those who test positive would avoid almost all false positives, at the cost of only 1-2 days of isolation for most

The OP has been busy for the last hour.

Sorry, OP, but this also made me laugh a lot. How dare you be busy, when there are people on Mumsnet waiting to be updated.

This paragraph is interesting

The population prevalence of infection and relevant test performance have critical implications for effectiveness and risks of mass testing.

Mass testing could enable earlier detection of infection clusters, but it is essential to consider the implications of both false positives and false negatives.

In a population with very low prevalence twice weekly tests with 99% specificity – and 10 day isolation if positive - would lead to ~3% in isolation at any given time (as 1% isolated every 3.5 days) and 41% of the population receiving a false positive over 6 months (i.e. the probability of getting at least one false positive in 52 tests). This example highlights the importance of follow-up confirmatory testing in low prevalence settings, addressed in Annex B. Finding the right combination of tests is not trivial and will require careful pilot studies

I'd recommend people who are interested to take a read of that report.

There's a bit about people trying to game the system if a negative result gave them access to say a football match or the theatre.

Thanks for responding. I don't think it's ever going to happen anyway.

SAGE warned against apparently because over the course of a year 41% of the population would have to self isolate unnecessarily.

However, if you retest the positives, you could whittle away almost all the false ones, If, of course, money were no object for all of this.

The biggest problem with this moonshot testing (though I can think of ruder words for it) is the proposed cost. 100 billion is £1500 per man, woman and child in the UK. It's £3000 per economically active person. It's actually insane. The idea that the government is going to hand over this much cash to Serco and Deloittes.

For a fraction of it they could pay local councils, labs and GPs to run a decent track and trace process.

Matt Hancock in Parliament today

""At the same time, we are developing new types of test that are simple, quick and scalable. They use swabs or saliva and can be turned round in 90 minutes or even 20 minutes. So-called Operation Moonshot, to deploy mass testing, will allow people to lead more normal lives and reduce the need for social distancing.

For instance, it could mean that theatres and sports venues could test audience members on the day and let in those with a negative result, workplaces could be opened up to all those who test negative that morning, and anyone isolating because they are a contact or quarantining after travelling abroad could be tested and released. We are piloting that approach right now and verifying the new technology, and then it can be rolled out nationwide. [Laughter.]"

It's ok.

Matt's on it

Clive Efford (Eltham) (Lab)
I am sympathetic to the idea of mass testing and have been for some time, but what does the Secretary of State say to someone like Professor David Spiegelhalter, who said that mass testing could lead to hundreds of thousands of false positives, with the knock-on effect of over a million people who have been in contact with those individuals being told to self-isolate? What assessment has the Government made of that issue?

Matt Hancock
I welcome the hon. Gentleman’s enthusiasm for mass testing. He might want to have a word with his Front-Bench colleague, the hon. Member for Leicester South (Jonathan Ashworth). Of course the specificity of the test is incredibly important, so that we do not get false positives. There are ways to deal with it, and those are taken into account in the moonshot programme.

In a population with very low prevalence twice weekly tests with 99% specificity – and 10 day isolation if positive - would lead to ~3% in isolation at any given time (as 1% isolated every 3.5 days) and 41% of the population receiving a false positive over 6 months (i.e. the probability of getting at least one false positive in 52 tests).

I still think this has the potential to be a “least bad” solution depending on how the pandemic progresses.

Given the potential cost of it I tend to agree with the op. Let’s be honest out testing system is a shambles, before the government spends any more on it they need to evaluate the cost benefit ratio.

Hopefully the moonshot program will go the way of the bridge between Scotland and Ireland (rather than the Garden bridge).

(I'm a loyal listener of More or Less and got B at GCSE maths)

I'm a loyal listener of More or Less

Is there any other kind

@Taswama

Hopefully the moonshot program will go the way of the bridge between Scotland and Ireland (rather than the Garden bridge).

(I'm a loyal listener of More or Less and got B at GCSE maths)

You mean be announced for a second and then a third time as a completely new idea that one one has suggested before in order to distract from other news that day?

You mean its not just a dead cat story, but a dead cat story that can be recycled?

"Does this mean that there possibly are no asymptomatic people? That they could all be false positives so far?"

Interesting - I think it has been suggested by some that some may be, and that you ideally need a clinical analysis to be sure of what a positive result means. (Leaving aside a recent debate about whether labs are undertaking so many 'cycles' when testing a sample that the test is picking up very old fragments which aren't infectious. As I understand it that is slightly different from the argument about false positives)

To the pp who said it's an easily understood concept, I'm not so sure - I think it's counter-intuitive to many people! If the false positive rate is 0.5% you might assume that each positive test has a 99.5% chance of being 'true'. It's quite a conceptual leap to understand that if prevalence is low the false positive percentage of the total number of positives will be higher than that. But seeing the actual calculation does explain it very clearly! (Relevant qualifications - none really!)

A leaked (? deliberately) report of the Moonshot programme

www.theguardian.com/world/2020/sep/10/moonshot-five-key-points-from-no-10s-leaked-coronavirus-testing-plan

Part of it

As well as catching asymptomatic carriers, the new system would allow people to test negative and so attend mass-gathering events, in theory at least. The draft “UK government mass-testing narrative” suggests that testing can also be used to give people assurance that, at least for a limited time, they are unlikely to have the virus and are at low risk of transmitting it to others.
“This could be tied to a specific event or activity – taking a test directly before attending a football stadium or theatre or visiting elderly relatives,” it suggests. “Or it could potentially be used to confer ‘safe status’ on an individual – testing to confirm they are negative now and so at low risk of transmitting for several days, or to confirm a level of immunity that will prevent them from becoming infected again for a defined, longer period of time via confirming a previous positive [virus] or antibody test, or, in time, vaccination.

So they had better get a really good test that is sensitive enough - or they are going to have lots of people who are positive who are falsely reassured they are negative and go about their life - for a while at least - thinking they are negative.

I am still trying to envisage the practicalities of testing say 20,000 people for a football match - and what steps people will take to ensure they can get in.

That is a LOT of tests to administer for an event.

@SherryPalmer

Do we want a lot of people self isolating who are actually disease free?

Again, isn’t this exactly what we have with a general lockdown. Surely even if testing doesn’t creates some false positives it’s better than everyone locking down again?

But it also means that next time those people with a false positive get ill, they will assume it can't be coronavirus because they've already had it, and go out spreading it.

So paradoxically, mass random testing could actually increase infection rates.

(not an expert)

But as well as the statistical implications of this, what about the economics of it ... are there better ways of spending the money for tackling the virus?

You could get round that by explaining the false positive issue to people, I suppose. Though it is not that easy to explain (see previous post!). And then if people know there is a reasonably significant chance that a positive test is wrong, they may be less inclined to self isolate.

Interesting point though that compared with a second lockdown, a mass test results in fewer uninfected people isolating. There's discussion about the 'second test' possibility in the report in the Times this morning - someone suggested that you would use a different, more specific test for the second test.

Is any other country looking into this, I wonder?

You could get round that by explaining the false positive issue to people, I suppose

Well - if I wanted to go to a gig, and I was one of the 10,000 people they tested before entry using the rapid test lateral flow kits - and it said I was negative, I'd believe it and they would allow me in.

But if it said I was positive - well, I wouldn't believe that I was actually positive, but they wouldn't let me in. Would I then be expected to self isolate for a few days - whilst I tried to get a second test - which would then confirm the diagnosis? Probably - but it's a big ask of a lot of people, especially given the overwhelming chance of that initial positive being negative.

Yes Choma, I should have made clear I was replying to Sherry's comment - "But it also means that next time those people with a false positive get ill, they will assume it can't be coronavirus because they've already had it, and go out spreading it. So paradoxically, mass random testing could actually increase infection rates."

But as you say, if people know the positive test may well be false, it may influence compliance with self-isolation.

There is much more discussion about false positives now in the media over the past couple of days - so Bayesian analysis may be become more generally known about!

Where did the £100bn estimate come from by the way - is that the government's estimate? I think Matt Hancock announced £500 mn had been allocated so far.

Thie pandemic has certainly been a good lesson in biostatistics and misuse of statistics.

@UntamedWisteria

(not an expert)

But as well as the statistical implications of this, what about the economics of it ... are there better ways of spending the money for tackling the virus?

The economics is interesting.

Hopefully More or Less will do a programme and what would cost more, doing it or not

Looks like they haven't discussed this with the UK Health Screening Committee

www.theguardian.com/world/2020/sep/11/uk-health-screening-advisers-not-involved-in-moonshot-covid-plan-mass-testing-coronavirus