The predictive value of a positive lateral flow test - which is why a positive should be followed by a PCR and the PCR should overrule the LFT

[chomalungma]

Using the data supplied on the Government website to explain the tests and data

www.ox.ac.uk/news/2020-11-11-oxford-university-and-phe-confirm-lateral-flow-tests-show-high-specificity-and-are

Yes - they are highly specific and sensiitive

The specificity of the test was recorded as 99.68% -
It has an overall sensitivity of 76.8% for all PCR positive individuals but detects over 95% of individuals with high viral loads, and minimal difference between the ability of the test to pick up viral antigens in symptomatic and asymptomatic individuals.

So that's good.

But when you use them in an environment with a low prevalence of a disease, and use them on lots of people, you end up with many positive tests being false positives.

You also don't get many 'positives' at all - but on the rare occasion, you get a positive, it's more than likely to be a false positive.

So this is why a positive test needs following up with a PCR and that's why schools shouldn't insist that pupils and their contacts all self isolate for a long period.

Our local schools are allowing students back if they get a negative on a PCR after a positive LFT

@MagnoliaBeige

Our local schools are allowing students back if they get a negative on a PCR after a positive LFT

That's how it should be.

I agree OP. The LFT is a good resource to then funnel unsymptomatic people for a PCR and the PCR should dictate whether you have to isolate for 14 days.

Clearly if you get a +ve LFT you should isolate until the result of your PCR, or if you don't want to take a PCR then you should isolate for 14 days. But if you have a +ve LFT then a - ve PCR the PCR result should override the LFT.

I still appreciate the LFTs that we have at home as they do provide some additional reassurance though.

It should override and I don't understand anyone who still says you have to self isolate even if the PCR is negative.

Dds school too, positive LF test directs them to a PCR test then if that's negative they don't need to isolate

Can anyone break it down more simply for me? I don't understand how there are false positives.

Because the test may drtect something else and say that is Covid. It's very rare but given enough tests, rare events happen enough.

@chomalungma thank you this makes sense

It just really frustrates me when Government Ministers say that there is only a 1 in 1000 chance of a false positive.

That's true - but that does not answer the basic question - given that not many people have Covid-19, when you have a positive result, what are the chances if it being a True positive?

I would love to ask Boris that.