Lateral flow tests - why not when symptomatic?

[NauseousNancy]

My daughter has had a temp today which nursery mentioned when I picked her up.

I have some lateral flow tests at home so used one on her. Then reading I seen that they shouldn’t be used when symptomatic, so I booked her a ‘proper’ one and waiting on the results.

Has me wondering though - why can’t they be used when you are symptomatic?

Because they aren't sensitive enough.

If they aren’t sensitive enough to pick up symptomatic cases, why do we use them?

They aren’t reliable enough but they hope they pick up some of the asymptomatic people who wouldn’t be doing a pcr test as they don’t have symtoms

Because even if they pick up 1/10 of those cases it’s better than none

I think it's just because there's a high chance of a false negative. If you've got symptoms then you're more likely to be positive than if you don't, so it's more important to get an accurate result I guess?

Seems mad we spend so much money on them and testing everyone twice a week if they are so unreliable - they are horrible as well! Seems a bit pointless if they are really a bit crap.

They are no crap. Used properly they will reduce transmission. The test is no worse than a pcr test.

They are a bit crap tbh.

We need community sniffer dogs!

I might be wrong but this is how I understand it -
Because a PCR is more accurate, but takes longer to get results, where an LFT is ready within 30 minutes.
There's more risk of someone being positive, but recieving a negative LFT result, and then not testing with a PCR and getting a positive result and then self isolating etc.
So you could have a negative LFT, because the test wasn't sensitive enough, and go on to infect others.
LFTs are for asymptomatic people who will then go on to have a PCR, and self isolate, who would otherwise be unwittingly infecting others as they have no symptoms therefore no reason to test with a PCR.
And there's always a possibility with a regular PCR test (we do them every week) that you may become infected between doing the test and getting the results, because of the time it takes, so doing LFTs regularly in that situation helps reduce that risk (I work in care)

The one I did for her today was negative, so I will wait and see what the pcr one says now!

If it’s positive I will deem them a bit crap on my hugely scientific experiment

It's not pointless OP. It seems like you don't really understand.

Lateral flow tests are useful in certain circumstances eg for those who come into contact with many people through work and they are more accessible and give a fast result which is why they are used.

They are not as sensitive as the test you might book if you are symptomatic (not sure what you call those) so it's more important to get one of those if you are symptomatic but it wouldn't be practical to do that kind of test a couple of times a week when you have no symptoms.

At our special school the staff have been doing the lateral flow tests for what seems forever. However they picked up an asymptomatic case in one staff member which then led to the 'bubble' all isolating. More and more cases did then appear but those people had all been self isolating so not spreading the virus. Whilst not perfect as long as you don't see them as a sign you are 'safe' they have their uses.

I had 2 negative lateral flow tests. 1 was on the same day at my positive PCR test. I only had nausea. They're not very good at detecting the virus but a lateral flow test did pick up my daughter's positive.

@ElderMillennial I certainly didn’t understand, which is why I asked the question!

@NauseousNancy

If they aren’t sensitive enough to pick up symptomatic cases, why do we use them?

Because one of Boris' mates was given a lot of money for something that doesn't work very well, and so a use had to be found for them.

People doing lateral flow tests are speculatively testing - ie no ‘cause’ to test such as symptoms. Therefore any positive cases they pick up are a bonus that would not otherwise have been tested at that point. They will miss some cases (report negative when they are positive) and the LFTs miss more of these than PCR tests but that is ok because you are no worse off - those cases would not have been picked up otherwise unless doing LFTs. So the benefit is finding at least some cases that would otherwise have carried in going about their business potentially spreading it.

As soon as someone is risky enough to need a proper test it is more important to be as accurate as possible to rule infection in or out. That is a PCR test.

It is not that LFTs don’t work if you have symptoms - but that in those cases you need more certainty in the result.

I just can’t get my head round how it might pick up an asymptomatic case with a fairly low viral load but not a case with symptoms and a high viral load.

Blows my mind. Which is why it’s a good job it’s not my job!!

Would you take a LFT if you were feeling ill but with none of main 3 common symptoms? I feel shocking but no cough, temperature or taste issues.

@How2Help - that makes sense. So it’s not about having symptoms or not having symptoms at all, they work both ways, it’s about having the very best one to rule in or out definitively. That’s easier to get my head round!

@NauseousNancy

The one I did for her today was negative, so I will wait and see what the pcr one says now!

If it’s positive I will deem them a bit crap on my hugely scientific experiment

That's the thing, if you are positive, but just went off the LFT results, then you'd be at risk of passing it on, because although you have symptoms, the LFT isn't sensitive enough to detect the virus, where as the PCR hopefully! is. So there's no point testing with the LFT in that case. I get 2x LFT and 1xPCR a week, I had my last PCR on Thursday, it was negative, I may have been infected since then, but not yet (or at all) develop symptoms, there's more chance that if I have, it'll be picked up by an LFT that I test with today, than when I get the results from my next PCR on Fri or Sat, than if I don't test at all.

Because they have a high false negative rate. Depending who is administering the test, it can be very high. The figure I use below is simply to illustrate

If you are screening asymtomatic people and miss 50% of those who actually have covid, it doesn't really matter, as you are still picking up the other 50% and they are all cases that would otherwise have been detected. Even though imperfect, the overall result is a gain for public health.

Now put that same 50% fail rate onto people with covid symptoms. Of those who really have covid, you are going to have 50% false negatives, and that means cases where people would have been instructed to isolate, had the more reliable test been done, are instead walking around thinking it's something else. The overall result is detriment to public health

@How2Help

People doing lateral flow tests are speculatively testing - ie no ‘cause’ to test such as symptoms. Therefore any positive cases they pick up are a bonus that would not otherwise have been tested at that point. They will miss some cases (report negative when they are positive) and the LFTs miss more of these than PCR tests but that is ok because you are no worse off - those cases would not have been picked up otherwise unless doing LFTs. So the benefit is finding at least some cases that would otherwise have carried in going about their business potentially spreading it.

As soon as someone is risky enough to need a proper test it is more important to be as accurate as possible to rule infection in or out. That is a PCR test.

It is not that LFTs don’t work if you have symptoms - but that in those cases you need more certainty in the result.

This is a perfect explanation.

So it’s not about having symptoms or not having symptoms at all, they work both ways, it’s about having the very best one to rule in or out definitively. That’s easier to get my head round!

Yup, pretty much spot on

The PCR tests are more reliable so it would be a waste of a lateral flow test to take it if you already have symptoms. They’re design to pick up potential positives of asymptomatic people and then you’d have a PCR test to double check.

My daughter got a positive LFT without symptoms and took a full test the same day which was also positive. Her actual symptoms didn't show til the next day but were so mild we'd have dismissed them - no cough, temp etc. Just slight cold symptoms. The main symptom - losing her smell and taste came five days after her positive results. So the LFT did save her passing it on for five days. Although I never caught it off her according to tests taken then and an antibody test after so I'm guessing it isn't highly transmissible. Her sister caught it but again very mild and again smell and taste loss came a few days later so the LFT are worth it for picking up early stages.