Why is an LFT not suitable if you have symptoms?

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Is it less reliable then?

Less sensitive I believe

Also, if you get a positive LFT you will be advised to get a PCR anyway so may as well go straight for PCR if you have symptoms

It's less accurate. For the purposes of testing symptomless people it's okay as to find any is great, but to identify covid it's not.

It measures different things. LFDs test for antigens via proteins in the sample, the PCRs test for the genetic material of the virus in the sample.

I’ve never really understood it.

If it’s not accurate enough to detect Covid in a person with symptoms, how on earth can it be sensitive enough to detect Covid in someone who doesn’t have symptoms?!

Makes no logical sense to me.

It has a high false negative rate so it doesn't catch all cases. If it was positive you'd need a PCR to confirm anyway

It isn't very reliable. If you have a 100 asymptomatic people and 10 of them have covid and don't know it, then if the lateral flow test picks up 3 of them, then that's 3 more identified than would have been if you'd done nothing. So LFT better than nothing.

If you have someone with symptoms and you need to know if they have covid, then you need to use the more reliable PCR test. If you use the unreliable LFT the risk is that you would be falsely reassured by a negative result.

LFTs are designed to pick up some asymptomatic carriers. Some being the key word, a lot will be missed but it's better than nothing for testing asymptomatic people easily who would otherwise all be missed and spreading it.

Having symptoms increases your chances of actually having covid, therefore it's worth the additional time and effort to go and get a more reliable and accurate test. A PCR test is much more likely to give you the correct answer.

I know people who have tested positive via PCR but nothing showed on the LFT done the same day.

From a Guardian article:

ACochrane reviewof 64 studiesfoundthat LFTs correctly identify on average 72% of people who are infected with the virus and have symptoms and 78% within the first week of becoming ill. But in people with no symptoms, that drops to 58%.

2 main reasons are

as above, less accurate

But also more importantly, is the PCRs are lab analysed so they can track down the exact variant, find new variants and get much more useful information on transmission.

You can still do one, just can’t rely on the result.

I’d do one (as well as a pcr) as at least you can then tell people immediately if it’s positive.

Because it has a very high false negative rate.

If there are 1000 people and 10 of them have covid, even a test with a 50% false negative finds 5 cases which would otherwise have gone undiscovered (either that early, or at all). That's a gain.

But if you have the 10 people with real symptomatic covid that a PCR would detect, but the LFT only finds 50% of them, that's a detriment, because 5 people who should be in isolation are out and about.

@dementedpixie

From a Guardian article:

ACochrane reviewof 64 studiesfoundthat LFTs correctly identify on average 72% of people who are infected with the virus and have symptoms and 78% within the first week of becoming ill. But in people with no symptoms, that drops to 58%.

Aim off for who those people are - BMJ compared researchers involved in developing the test (got 80% or so accuracy), trained HCPs (down to about 70%), general public (under 50%)

I don't think it's an issue of the sensitivity changing, it's more about how likely you are to have COVID. If you take 100 people who don't have any symptoms, of which 4 have COVID, and a quarter of them get a false positive, you could end up with one person who thinks they're OK but actually isn't. Which isn't great, but if that's for asymptomatic screening of the population at large, you've taken 3 people out of circulation who wouldn't otherwise have been picked up at all.

But if you take 100 people who have symptoms of something it's more likely that they have COVID - say 40 have it, with the same false negative rate you've missed 10 people which is more of a problem. So if people with symptoms take LFTs they could end up circulating and spreading covid in a way that they wouldn't have if they'd taken a PCR.

I really think it's more about the data they would lose if everyone done lateral flows. If you have the virus in your throat and successfully swab it up, either test has the ability to detect it

@ThornAmongstRoses

I’ve never really understood it.

If it’s not accurate enough to detect Covid in a person with symptoms, how on earth can it be sensitive enough to detect Covid in someone who doesn’t have symptoms?!

Makes no logical sense to me.

Because presence of or severity of symptoms aren't indicative of infection and therefore potential to pass it on. Asymptomatic people still pass it on, people with very mild or none of the 'main' symptoms can still pass it on - both of which may not have cause to test with a PCR, so would never be identified as being infected and be passing it on to people who could become very ill through vulnerability etc. They are not as accurate as a PCR so you need to back up an LFT test with a PCR if positive, and you can do an LFT if you have symptoms, but why wouldn't you choose the most accurate test available if you have symptoms?

The best test is the PCR. Symptoms = a PCR.

LFT are there purely to mass test non-symptomatic cases in the hope of catching more positive cases and isolating as appropriate. They arent as effective as PCR but easy to mass produce/do yourself and the results come back in 30mins. So handy.

I heard a good explanation at the weekend... Lateral flows are good at picking up people who are highly infectious. Pcr will pick up anyone who is infected. So lateral flow identifies people shedding lots of virus and making others ill without realising. Pcr will detect if the illness you are experiencing is covid and as its lab processed will help scientists learn more, detect new variants for future vaccine tweaks etc. You can take a lateral flow if you have symptoms but it won't tell you that you don't have the virus, just that you are not highly infectious right now, but by tomorrow or the next day you could be. Pcr rules this out so that's why you need one for symptoms.

When it says LFT tests are for asymptomatic testing only, does that include having none of the official symptoms (so not eligible to book a pcr test) yet having sore throat, runny nose, fatigue, grogginess etc. I’ve now done 6 LFT tests (2 a day since symptoms started), all negative… if I was COVID positive with these symptoms would at least one of the LFTs have been positive by now?

LFT are unreliable and bring up many false negatives.

Ideally we'd be using PCR tests on everyone with and without symptoms. We do not have the resources necessary to do that. So we're reserving the reliable (PCR) tests to people who have symptoms.

The idea is it's better to catch some asymptomatic cases with LFTs than catch no asymptomatic cases by not testing at all. Simple as that.

My friends son who is 16 felt a bit rotten over the last week, just general cold symptoms, not the Covid symptoms. Although he felt a bit off it was only mild, nothing that stopped him carrying out his daily activities....just a bit of a runny nose, feeling a bit more tired than normal and the occasional headache that didn’t even require pain relief. He was doing LFTs during that time, all of which came back negative.

As a result of them coming back negative he was carrying on with life as normal, going to school, going to his hobbies etc, but when after a week he still wasn’t feeling completely himself, despite three negative LFTs he went for a PCR which came back positive.

Who knows how many people he may have potentially infected during the week he was getting negative LFTs.

Maybe they do more harm than good if their false negatives mean that infected people are carrying on with their daily lives.

Maybe as a previous poster said, PCR should be the go to as the first line of action, as opposed to only be used if a person has the standard Covid symptoms.