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.

[quote chomalungma]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[/quote]
Is anyone surprised at this revelation?

I’m in the UAE where mass testing is being used to great success and our infection rate is very low at the moment. We are getting a lot of false positives, but if you test positive you have to have another test the next day before your case is confirmed. You are then sent to either a hotel or hospital for two weeks’ isolation (depending on your symptoms). We have done an absolutely brilliant job at managing this virus and I would rather be here than back home in the U.K.

The false positives aren’t actually that disruptive and it means our lives are going on as reasonably normal. We are all being tested all the time for various reasons (going to the office, going inside a school, having a public facing job, travelling between two cities etc etc, plus they go around the city at night testing people in their homes).