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.

Not a mathematician. Had to take some time over that analysis. The false positive effect you described blew my mind for a bit and I got paper and pen out ! Think I got it. Really interesting. Returned to see if I had understood. Thanks op - thought provoking.

So what do you want the government to do, test no one and let the asymptomatic pass it on unknowingly?

Aren’t false negatives a bigger issue? I thought tests were quite likely to be negative when the person has covid (cannot remember either the numbers or the terminology, sorry).

But if you have asymptomatic covid and get selected for random testing and it’s (wrongly) negative, you’re likely to be less careful about social distancing. Possibly for quite a while because many people will think “yay I don’t have it” rather than “yay I didn’t have it yesterday “. And there you are, spreading it around ...

I understand the false positives

But surely we are not behaving in an optimal way atm?

We have sectors severely restricted or closed and that means a percentage of people not doing the best thing anyway.

So weighing it up which is better?

I don’t know but would be good to assess the numbers

@TorysSuckRevokeArticle50 I think you mean to posit this opinion not posset, which is both baby sick and dessert I believe...

@chomalungma

Surely even if testing doesn’t creates some false positives it’s better than everyone locking down again

As has just been said - how many people do we lock down just to catch the ones who have it?

It is a cost-benefit analysis.

Or do we do much better contact tracing, rapid testing of contacts

We have been locking down far more people than necessary because it’s all we could do

All those 65,000 people will then be contact traced, with their school bubbles being sent home for 2 weeks and all their contacts having to isolate as well

No they wouldn’t because all of those contacts would have been tested themselves already.

[quote Malteserdiet]@chomalungma
Nothing else to add and totally agree with you. But also wanted to say..... you’re awesome

Please can you replace Chris Twitty asap?[/quote]
Why awesome?

I saw Jenny Harries on TV this morning with Naga Munchetty explaining exactly why testing many asymptomatic people when there is low disease prevalence is not a good idea, which is why there were no plans to test e.g. all university students but only to test the asymptomatic in a very targeted way.

Where in the meantime has the assertion They want to do lots of testing on asymptomatic people everyday come from during the course of the day? Who wants to? Where was this published?

@JellyBabiesSaveLives

Aren’t false negatives a bigger issue? I thought tests were quite likely to be negative when the person has covid (cannot remember either the numbers or the terminology, sorry).

But if you have asymptomatic covid and get selected for random testing and it’s (wrongly) negative, you’re likely to be less careful about social distancing. Possibly for quite a while because many people will think “yay I don’t have it” rather than “yay I didn’t have it yesterday “. And there you are, spreading it around ...

This is kinda what I meant

Can’t this issue be solved by retesting those who had positive results - this weeding out false positives and avoiding unnecessary self isolation whilst still catching those who genuinely are positive.

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

@TorysSuckRevokeArticle50

Can you please state what qualifications/experience you have that make you qualified to posset this opinion, also could you provide links to the peer reviewed research you have used to reach your conclusion?

A GcSE in maths would be more than enough to work that out, why would you need a link to anything?

Is this the same for false negatives (apologies I don't know if that's the correct term)? Is there a chance that we test everyone (or the areas proposed as project moonshot) and x number of them come back as negative but are actually positive cv carriers and therefore the virus is still out there and can infect others?

On any given day we are behaving in a way that’s best for the economy or not

With the blunt tools we have currently profit is down in many sectors and unemployment is up

Does this improve on that even with incorrectly curtailing 600000 people’s actions or not?

Why do we need to test people. So what if people are positive? Doesn’t mean the end of the world . 99% people will survive this.

Spot on OP! As you said, it isn't biology, just stats, which is my area. Plus you have the clinical experience.

I was looking this up this morning as my daughter has just tested positive which actually seems quite unlikely so I was interested in the incidence of false positives.

The maths is all sound (Maths educator at a university) and its a common mis-understood concept that has put a lot of people wrong in prison etc!

I think they were quoting between 0.5 and 5% false positives but this was before such widespread testing, as you say this will just increase.

So much interesting maths in COVID

Same happens screening everyone for cancers as well.

Why do we need to test people. So what if people are positive? Doesn’t mean the end of the world . 99% people will survive this.

Oh my god what an idiotic point. We need to test it because we need to protect the people who won't survive this (not just old people but vulnerable people who have long lives left to live). We also need to maintain a functioning healthcare system otherwise many people who would otherwise survive will die in hospital corridors.

Same happens screening everyone for cancers as well

Exactly.

Better not to miss a cancer..but you do get a lot of positives that aren't actual cancer.

It's hard getting the right balance

Hate my job, though I do have decent sick leave (orders a test...) .

[quote HandfulofDust]@chomalungma Boris Johnson is a moron but there's no way he doesn't understand this because it's very very basic. Even a very average A-level student in maths could work this out. Boris might well be less mathematically literate than an Alevel student but it will have been explained to him. You certainly haven't thought of something that wasn't very very obvious to the people working this stuff out.[/quote]
It’s unlikely he listened, if it was explained to him. He didn’t understand his own Withdrawal Agreement. And I’m not convinced that, even if he listened, he’d get it.

Handful - not really idiotic. We don’t do this for the flu.
We are testing for a disease that’s so deadly , we don’t know we have it until we test 🧐.

We have never done this for the vulnerable before.
Healthy people should not be getting tested

The government must know this but choose to ignore.

@Michaelschofield

Don't be silly. You obviously have no medical, statistical or other relevant qualifications (or common sense) so obviously your analysis isn't going to be useful. This is a novel virus which has already been shown to overwhelm even very modern healthcare systems to the extent you have people being treated in hospital corridors and doctors and nurses dying in large numbers. This doesn't happen for the flu. That's why we need to behave differently than we do for the flu.

@Michaelschofield

Plenty of healthy people ended up in ICU if they had a large dose of virus. If you're healthy and get a small dose you tend to be OK but might still need hospitilisation which we don't have the capacity for. YOu may also infect someone who is vulnerable. The vulnerable are much more at risk than they would be for previous diseases which is why we make more effort now to protect them .I understand you clearly don't have a scientific background but it's really not that hard to understand.