Widespread covid LFT testing Options

[notmeatloaf]

I can't help but think the government has lost it's mind.

They plan to send out lateral flow tests, free, by post, to anyone who wants them.

These packs cost a minimum of £20 each, probably more. And because they are "free" lots of people will order them with little appreciation of their value.

They are recommending a technique already ruled out for nurses because it would increase non-compliance - swabbing the back of the throat, versus swabbing both nostrils.

And there is no proper record. When we received these through work you had to record 20 results before you could get a new box. Now you can order as many as you like.. If you record test results they disappear off into the ether with seemingly no record.

The whole idea is idiotic. We know that a significant proportion of PCR tests produce false negatives because a trained healthcare professional does not jam it far enough down your throat or nose. The idea people will be able to do the test properly from a paper booklet is insane.

Yet despite the cost of these largely useless tests being projected at £1bn a month, we can expect the inevitable purse tightening before long.

If they have tons of money spend it on a British holiday subsidy scheme. Restrict foreign travel as much as possible. That is the top risk at the moment - a variant coming in which wipes out the benefits of the vaccine overnight as the Kent variant proved when we were Plague Island.

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[The Slithy Tove]

Dido Harding, she of massive data leaks, has already spaffed away billions on a world-beatingly crap test & trace scheme that still isn't fit for purpose. What's a few billion more?

[The Rookie]

My wife gets home tests as she works at the local school.

She has already been given enough tests to last until November (2 tests a week when at school and two in the preceding holiday week).

[nigelbb]

The real scandal is not the cost but the fact that LFT should not be used for mass testing because of inaccuracy. This is especially so when disease incidence is low so that the majority of positive tests will be false positives. The false positive rate is around 1:500 with anything up to 1:4 subjects who test positive with the gold standard PCR test falsely testing negative with LFT.

[Steve_999]

The real scandal is not the cost but the fact that LFT should not be used for mass testing because of inaccuracy. This is especially so when disease incidence is low so that the majority of positive tests will be false positives. The false positive rate is around 1:500 with anything up to 1:4 subjects who test positive with the gold standard PCR test falsely testing negative with LFT.

A false positive rate of 0.02% would not make me worry about being tested as positive when I am not! The false negative rate is far higher of course, but at least it will capture the vast majority of positives.

What I find amazing is that the government seem to think that people will take these tests and, if positive, will self isolate (along with their entire household) pending a PCR test. Without any support when they cannot get sick-pay or any form of grant. Most people will not self-isolate when symptomatic, so why would they do so when they have no symptoms?

[nigelbb]

The real scandal is not the cost but the fact that LFT should not be used for mass testing because of inaccuracy. This is especially so when disease incidence is low so that the majority of positive tests will be false positives. The false positive rate is around 1:500 with anything up to 1:4 subjects who test positive with the gold standard PCR test falsely testing negative with LFT.

A false positive rate of 0.02% would not make me worry about being tested as positive when I am not! The false negative rate is far higher of course, but at least it will capture the vast majority of positives.

What I find amazing is that the government seem to think that people will take these tests and, if positive, will self isolate (along with their entire household) pending a PCR test. Without any support when they cannot get sick-pay or any form of grant. Most people will not self-isolate when symptomatic, so why would they do so when they have no symptoms?

Testing twice a week with a false positive rate of 1:500 means that 1 in 5 will falsely test positive each year. If the false positive rate is 1:200 then it would mean that 50% would falsely test positive each year.

This post has been edited by nigelbb: Wed, 7 Apr 2021 - 08:55

[notmeatloaf]

It is worse than that. Bayes Theorem means that as prevalence decreased, the number of false positives increases.

No-one knows the exact figure for false positives but Professor Jon Deeks (not one of the nutjob Daily Mail professors) has estimated that if covid prevalence is 0.5% then the rate of false positives will be about half. E.g. if you have a positive test it is 50/50 whether it is correct.

However, this is skewed further by the fact that positive rates from secondary school pupils are incredibly low compared to known infection rates. So either they aren't being done properly, or positive tests aren't being reported. So that reduces the notional detection rate still further.

In the end mass testing makes sense when prevalence is high. If the government actually expects a third spike it would make sense to hold back the tests until then.

I say that as someone who has been doing these tests since they first came out, but because I work on covid wards and am trained to use them properly. They are useful when rates are high OR when they are highly targeted.

[andy_foster]

It is worse than that. Bayes Theorem means that as prevalence decreased, the number of false positives increases.

No it doesn't. It has no effect whatsoever on the number of false positives (unless it affects the number of tests). What it does is increase the probability that any given positive test is false.

[notmeatloaf]

Relative number, if you want to be pedantic.

[The Rookie]

Well there is a huge difference between relative and absolute numbers increasing, that's not pedantic - it's the difference between being right and outright wrong!

Saying the numbers of false positives would go up is also a massive failure of common sense, you don't need a theorem to see that surely?

[Ahelpinggand]

More to the point PCR tests are to be used to confirm a positive anyway.

I have to take these regularly due to the sites I cover (NHS, HMPS,) sometimes multiple times per day.

Partner is also Early Years so she also uses twice a week.

I fail to see an issue. Yes the issue of people not complying is there. But if people aren't going to isolate then they won't anyway. BUT a positive LFT result MIGHT make them change their behaviour, even sub consciously.

Also recognise the financial implication for self isolation, it annoys me that it's assumed someone earning £35k for example cannot claim a support payment even if they live hand to mouth. Yet someone who doesn't work, on benefits who doesn't lose any money by isolating (and has a minimal cost increase due to it if any) is able to claim £500.

The issue is if you brought the payment in as universal the amount of fraud it would be left open too is unreal. Everyone would be testing positive left right and centre. We all know it shouldn't happen. We all know however it would.

More to the point data shows the chain is beginning to break due to the vaccine program, and also the high take up of said vaccines.

Once this chain is broken fully the case numbers will not be much of a concern being realistic.

What we should be doing is banning any and all travel from South America regardless of nationality etc. The current danger is Brazil pure and simple.

On topic the kits cost nowhere near £20 for a single LFT


This post has been edited by Ahelpinggand: Fri, 9 Apr 2021 - 08:40

[notmeatloaf]

On topic the kits cost nowhere near £20 for a single LFT

They are being sent out in packs of seven. I've seen quotes it's £5 per test, my understanding is more like £3 per test but still by the time postage and admin is paid it's >£20 a box.

The better thing to do would be to heavily subsidise them. If they charged £3 a box people would be more likely to value them. And if you can't afford £3 a box, then you probably won't self isolate if you do test positive.

[Ahelpinggand]

Sorry NML thought you meant cost per test

Your figure match what I've been told as well.

Issue is they're being used to encourage people to test themselves. Charge people to use them and they are less likely to do so.

As my previous if people are unlikely to isolate regardless it won't change that BUT it might affect their actions even sub consciously

The best way to encourage isolation is simple. Bring in legislation that forces employers to pay full pay for 10 days if isolation is required. This however can only be used once in a 12 month period. And only from a positive PCR test.

This could be paid for by government. End of day the cost of test and trace etc is nothing compared to Furlough!

This post has been edited by Ahelpinggand: Sat, 10 Apr 2021 - 08:11