Lateral Flow Tests and false positives

[whataboutbob]

My son has already been sent home for 12 days after re starting school, because someone from another class, who shared a coach trip to the sports fields, has tested positive on a LFT.
I have read contradictory accounts of the specificity of this test, with the government assuring us it’s very high, to articles in the BMJ saying it’s poor. Does anyone know how common false positives are? Maybe I’m clutching at straws but I’m worried we’ll have a succession of the positives and school will be interrupted repeatedly.

I read in the media that false positives are rare (0.1%). At the two secondary schools my children go to, last week they administered over 3000 tests and they were all negative.

The drawback is I believe more that they will give false negatives in a lot of cases where someone is infected (but asymptomatic).

Much more likely to get a false negative than a false positive

@dementedpixie

Much more likely to get a false negative than a false positive

Yes, this.

Ok then, definitely clutching at straws . I think another issue is that a further PCR test on the positive kid at home is likely to be less reliable than a PCR performed under supervision at home. Either way, back to lockdown learning. So fed up of this.

That should be “ than a lateral flow test performed at home”

At school even

In trials, estimates ranged from 99.7 to 99.9%.

The recent program in schools suggests it's even higher as the number of +ve results is I think below that expected from 0.1% false positives although I haven't seen full data here.

So at worst it looks like 1 in 1000 tests falsely give a +ve.

LOTS of false -ves though!

Isn't self isolation 10 days max from contact now, not 14 days? Are schools still doing 14 days?

Yes it does work out at 10 days, then there’s the weekend so he finished on Tuesday yesterday and doesn’t go back until Monday the 29th ( for four days then it’s Easter).

Much more likely to get a false negative than a false positive

No, you cannot say they is, it's deeply misleading.

Given you know that a result of a test is WRONG, then it is much more likely that it was a false negative, but that does not mean that you are more likely to get a false negative. Because the ratio of positive to negative depends on the prevalence.

The most optimistic validation has a result of 1 in 1000 false positive, the original validation in non lab conditions has a result of 1 in 300 false positive. The current prevalence on average across the country is around 1 in 300 and the chance of a false negative is maybe 50% so that is 1 in 600 people will have a false negative result, with the range of false positive positives being 1 in 300 to 1 in 1000.

So it actually means around half the wrong results are false positives and half are false negatives, with the prevalence of positive cases continuing to drop, and in areas with lower case numbers the chance of a wrong result is more likely to be a false positive.

So no, you can't say it's much more likely, at the moment the best you can say is it's a little bit more likely, but in general the evidence of the performance of the tests is terrible in all ways.

I understand that false positives are unlikely but my friend’s ds had to isolate because someone he sat next to tested positive on lft. They tested negative on the pcr but he still had to isolate . He was really annoyed.

One of the nursery workers in DD's bubble had a false positive yesterday

My son went in last Thursday then yesterday a case was found in his year group but not in his class. He said that he had been with a group of lads yesterday morning then they had the test before break and the lads he was with got sent home but not him.
I had to ring the school and ask if he's been with the positive case should he stay home and they have said yes.
So now he's off till the 29th which is Easter holidays anyway so a long time off now.
If he was with this boy not social distancing or wearing a mask what are the chances of him getting it now.
.

Sal

False positives on lateral flow around about 1/1000. False negatives on a PCR are about 1/10. That’s why a negative PCR shouldn’t be override a positive lateral flow. It’s very simple maths!

@Sadless it depends on a few factors such as how infectious was the contact. If he had lots of virus on board ie he was half way through his infection rather than right at the start or the end, he’d be more likely to infect someone else . Were they outside ( less risk) or inside ( more risk of passing on). Was the infected contact shouting/ sneezing/ coughing, etc.

Just asked him he said about 25 minutes outside but I think because the year group only went back on Thursday and this boy would have tested negative then so it started before the test on Tuesday. He says the boy wasn't coughing or anything.

Sal

All other things being equal probably a lo wish chance, but hard to say for sure.

At my school we had one class where 2 boys sitting next to each other just tested positive on a LFT. We gave them each a second test to make sure. Same result. None of the other pupils in the class tested positive but they have all been sent home to isolate regardless. I'll be interested to know if any end up testing positive while isolating.

We had another positive in another year group too. I must admit, it took me aback a bit after days of negatives.