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.

Totally agree. I'm with those who say the government knows this all along, and are choosing to ignore. Just like they knew back in January covid might be a problem, but they chose to ignore.

Since when has Saint Boris cared about facts? It's the same agenda they've been pushing all the time: they want to appear to be doing something. Mass testing makes them look good, while as others have said, those with false positives are punished by having to isolate for a fortnight.

I think the problem we face with COVID-19 is actually the other way around: false negatives will be more problematic than false positives

That's true at the moment - who knows how many people are being missed due to a test that may not be being properly carried out so a lot of positive people are being missed.

But at least it's a lab based test, working with people who think they have the disease.

As has been said, this new idea with mass testing with rapid tests that can be carried out at the point of care will cause issues as well.

Is the 99.9% either false positive or false negative though ? Why assume is false positive?

They could balance themselves out ?

And for those who love their stats, then BBC More or Less is a great programme.

@chomalungma

And for those who love their stats, then BBC More or Less is a great programme.

Yes it’s great

But I really want someone to answer my question below 😂

The really, really simple way to solve for this is just to test any positives again, Drastically reducing the number of false positives, as has already been said by others but bears repeating.

The really, really simple way to solve for this is just to test any positives again

It would depend on the cause of the false positive.

You could of course do a much better test for those false positives - such as proper lab based test. Which is what they would do in screening programme.

Follow up the positive result with a better test.

@Reallybadidea

Re false negatives - aren't they largely false negatives because the individual isn't shedding enough virus to be collected by the swab? So they may not even be infectious anyway.

That may account for some of them, but poor swab technique. problems with storing samples causing degradation of viral material and problems inherent in the test design and processing will also contribute.

*You could of course do a much better test for those false positives - such as proper lab based test. Which is what they would do in screening programme.

Follow up the positive result with a better test.*

How come you didn't mention this in your OP then?

Yes, @TheLastStarfighter - 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...

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.

Wouldn't you use the mass testing as a first hurdle with positives then being retested with a more specific test? We don't have to ditch the current test, we could continue to use it.

Not that I'm defending Boris's moonshot approach - it sounds fictional and he's hoping that each suggestion of a way out will distract us from the last failure.

You could of course do a much better test for those false positives - such as proper lab based test. Which is what they would do in screening programme. Follow up the positive result with a better test.

Why are you assuming they won't?

And do you have a better idea, being in a good position to have one?

@chomalungma

I think the problem we face with COVID-19 is actually the other way around: false negatives will be more problematic than false positives

That's true at the moment - who knows how many people are being missed due to a test that may not be being properly carried out so a lot of positive people are being missed.

But at least it's a lab based test, working with people who think they have the disease.

As has been said, this new idea with mass testing with rapid tests that can be carried out at the point of care will cause issues as well.

Yes, we may have many of the same issues with a mass test, and some different ones too.

I suspect the largest issue will be interpreting what a test result actually means without clinical oversight, particularly if the test sensitivity and specificity remain similar (or are less favourable still).

Mass testing as a multi-faceted approach to disease control is a reasonable step. It just can't be the only measure we use (that's what I meant when I said it won't work - it won't work on its own). The masks, social distancing, hand washing etc for all will need to continue alongside mass testing for quite some time.

I haven't come across Jon Deeks before

twitter.com/deeksj

Interesting person re biostats and diagnostics

And do you have a better idea, being in a good position to have one

I left lab work a long time ago. The maths is still the same though.

Effective contact tracing, local contact tracing, fast access to reliable tests, ensuring people are supported to self isolate.

Money needs to be spent effectively. If we are going to do this initial screening route, it would be much more effective to select a population to screen rather than just a 'scatter gun' approach.

You could easily screen contacts of someone quickly for example.

So operation Moonshite was proposed to be used in situations where you want an almost immediate result. With that many false positives the situations its worth using it for are really limited.

On the plus side I can see the merit in using it for international arrivals to prevent the need for everyone to quaratine for 2 weeks, where that is already the default.

But I thought the suggestion wasn't just for situations like that, but for if you wanted to go to a concert or a football match. You'd rock up, get tested and then you could return to an 'almost normal life'.

Except the reality is you could book a concert ticket, rock up with your partner and then blow positive so not only do you miss the gig (and the money you've paid for the tickets) but you AND your partner have to very needlessly isolate for two weeks, your babysitter has to go for a test - and isolate in the interim before they get a result - and your kids have to stay off school. This might endanger your jobs and in turn your house and affect your kids education. All cos you fancied a big night out.

It really doesn't sound as attractive when you put it into those terms, and sounds more like playing Russian Roulette than 'living an almost normal' life!

Won't anyone think of the children Season Ticket Holders? (Shudders and nudges the maths nerds about compound risk)

Lots of stuff on maths of Covid here plus.maths.org/content/tags/covid-19

Also see plus.maths.org/content/how-can-maths-fight-pandemic for an interview with one of the mathematical modellers working with Sage. I'm pretty sure there is an article by David S on false positives somewhere (my credentials - in case it matters - ex secondary maths teacher, now work in outreach for a university maths department)

I wonder who is tendering for the Moonshot contract ?

I haven't read it in detail yet.
This is a SAGE report on Mass testing

[[https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/914931/s0712-tfms-consensus-statement-sage.pdf
]]

Sorry

Here

assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/914931/s0712-tfms-consensus-statement-sage.pdf

@Reallybadidea

*You could of course do a much better test for those false positives - such as proper lab based test. Which is what they would do in screening programme.

Follow up the positive result with a better test.*

How come you didn't mention this in your OP then?

I'snt the problem with this that Moonshite is all about instant results for recreational activities when what we really need is availibility of access to quick testing for symptomic people to identify people who need to be contact traced as quickly as possible.

Instead of addressing the failure of the basic system which is needed for health reasons, Johnson drops an unviable, unrealistic and prohibitively expensive dead cat about recreational testing - relying on public ignorance on false positives and in the process raising false expectations of where we could be in a couple of months - in order to avoid scrunity of government failure to plan and manage track and trace.

And the media and public swallow it whole.

(and I completely agree with the OPs maths).

Retesting positive results is sensible - but would this be done? Would a school take back a child who tested positive on Monday and then negative on Tuesday - and what of the bubbles?

There are lots of questions that need to be answered before mass testing if the entire population (though I doubt that will be an issue this side of Christmas).

@frumpety

I wonder who is tendering for the Moonshot contract ?

One of Dom's mates with a company with previous experience in plumbing?

Well put OP, I've often this is something we need to be aware of.