What's the point in lateral flow tests?

[IAmConfusion]

Bare with me on this one!

I work in a school, so will be regularly taking lateral flow tests to make sure I'm ok to keep working and they're fine to take when I have no symptoms, but why if I then go on to develop symptoms a lateral flow test wouldn't suffice? If they can pick up if you have covid asymptomatically, why do they suddenly not work if you have symptoms?

I've seen people post that they've had a positive pcr test who then use lateral flows daily to check if the virus is still in their system so I'm just confused!

Can anyone make it make it sense?

They do work with symptoms. I did one it came up big fat positive in seconds, I then went on to have it confirmed with a PCR. The only reasthey say get PCR with symptoms is you might get a false negative with an LFT.

We all had positive lateral flows here with symptoms. Then confirmed with PCR

I'm.not sure why people are using daily lateral flows following a positive pcr. It makes no sense. The virus can be there for up to 90 days so you arent going to stick a swab down your throat every day for that long surely? And you still have to isolate anyway.
The pcrs I think are more sensitive but more importantly can see which variant you have so that public health england can track new variants which may have different symptoms or have an effect on the vaccine. It also allows tracing of contacts and to get the number of people positive each day/week. I imagine this will change in time like the isolation has done

Its similar to sending off a stool sample to see if you have food poisoning etc. You can probably estimate yourself but it's the information that public health/goes etc needs that is useful

They are far from perfect but they will pick up infections in some cases so I guess that's why we continue to do them. Are we an outlier compared to the rest of Europe an other western countries? I don't think they do these in Ireland but maybe this has changed. My sister who works as a nurse there did her first test of any kind when she visited the UK this year and used one we had! I think some very lucrative contracts were dished out to supply these tests.

If they can pick up if you have covid asymptomatically, why do they suddenly not work if you have symptoms?

This is a misunderstanding about how they work. They can pick up covid with symptoms. They’re not good enough to rule it out if someone has symptoms but isn’t sure whether it’s covid or a cold. That’s why a pcr is needed to see if someone had covid.

@PurpleDaisies

If they can pick up if you have covid asymptomatically, why do they suddenly not work if you have symptoms?

This is a misunderstanding about how they work. They can pick up covid with symptoms. They’re not good enough to rule it out if someone has symptoms but isn’t sure whether it’s covid or a cold. That’s why a pcr is needed to see if someone had covid.

Basically this. They do work if you have symptoms. They are not accurate enough to catch all cases though

They are not as accurate as PCRs but do pick up a number of cases especially without symptoms.

Also PCRs put you in the system so to speak as the results are recorded automatically

LFTs and PCRs are designed for different purposes because of their differing accuracies.

PCRs are a diagnostic test because they are reliable in detecting positive cases. It’s possible (although not common) that a LFT gives a false positive which is superseded by a negative PCR.

LFTs are much more likely to give false negative results but are useful for mass screening because they will at least pick up some cases which would otherwise have gone undetected (although they will miss many others).

They're not hugely accurate, so if you don't have any symptoms and feel fine they are better than just guessing you are covid negative (eg some people will have covid with no symptoms and it will pick up some cases that otherwise wouldn't have been diagnosed).

However, if you do have symptoms so therefore have a greater likelihood of actually having covid, you want to do a more accurate test.

they definitely work with symptoms - but are good for screening huge numbers quickly of asymptomatic people (that otherwise wouldn't get a test)
PCR is the better test for those with symptoms or to confirm an LFT

It's not that a lateral flow doesn't work on cases that have symptoms, they're not as sensitive as a PCR.
For example, I do a PCR weekly because of my job, but 2x LFT as well, because I work with very vulnerable people. I tested negative on Monday on my LFT, which was the day I also took routine PCR, the results for the PCR came back on Wednesday as being positive, but my LFT came back as positive on Tuesday. The LFT took a further 24 hours to detect the virus, but due to the PCR taking longer to deliver a result, I had a positive LFT result first, so I was prevented from spreading the virus for those 24 hours due to the LFT. I didn't have symptoms until the day the PCR came back, so didn't know I was positive and had no reason, except the LFT, to know I had covid.
When you consider that some people are asymptomatic right through, and unless you're in a job like mine with regular PCR testing, LFTs are detecting cases that may well go undetected and therefore spreading the virus unknowingly, because that person would have no reason to test with a PCR as they don't have symptoms, or don't have symptoms yet, but have the virus and are at risk of passing it on.

The point is that you shouldn't do them INSTEAD of a PCR if you have symptoms, not that they don't work. They aren't sensitive enough to catch all positives so if you've got symptoms you need the more reliable PCR to reduce the risk of spreading it after a false negative LFT.

My Friend is a teacher. Regularly takes LFTs and gets negatives. She took a PCR the same day she got a negative LFT and the PCR came back positive. Despite mild symptoms, no matter how many LFTs she takes now, they're still negative. Same goes for her colleague who's positive via PCR and negative with LFTs.
So now I don't trust them at all. What's the point?

@OhRene

My Friend is a teacher. Regularly takes LFTs and gets negatives. She took a PCR the same day she got a negative LFT and the PCR came back positive. Despite mild symptoms, no matter how many LFTs she takes now, they're still negative. Same goes for her colleague who's positive via PCR and negative with LFTs. So now I don't trust them at all. What's the point?

The point is people do test positive on LFTs. Not everyone. They’re not supposed to be “trusted” to clear you from having covid.

@HipTightOnions

LFTs and PCRs are designed for different purposes because of their differing accuracies.

PCRs are a diagnostic test because they are reliable in detecting positive cases. It’s possible (although not common) that a LFT gives a false positive which is superseded by a negative PCR.

LFTs are much more likely to give false negative results but are useful for mass screening because they will at least pick up some cases which would otherwise have gone undetected (although they will miss many others).

This

In summary, LFTs aren’t reliable but they will pick up some cases so are better than nothing. And they’re much cheaper and quicker than PCRs.

Also LFT will only pick up when you have very high infectious levels of the virus so people who get a negative LFT and then a positive PCR probably just tested at a point when levels weren't that high.

Can't believe how many people have no understanding that a test cannot tell whether you have a cough or not. It's literally a chemical reaction to the presence of the virus. If there is enough of the virus it will react. If there isn't it won't. It makes no difference whether you have symptoms or not.

LFTs are sometimes described as “red light” tests: positive => stop/isolate.

PCRs are “green light” tests: negative => go.

We've never had a positive lft on any staff where we work...but we also get pcr every week and we have on them. I don't think a lot of people are doing them right tbh.

@OhRene

My Friend is a teacher. Regularly takes LFTs and gets negatives. She took a PCR the same day she got a negative LFT and the PCR came back positive. Despite mild symptoms, no matter how many LFTs she takes now, they're still negative. Same goes for her colleague who's positive via PCR and negative with LFTs. So now I don't trust them at all. What's the point?

Whereas I know loads of people who have taken PCR’s because their LFT is positive, and their PCR has been positive too. A negative LFT doesn’t necessarily mean you don’t have Covid, that’s why they’re not used if you have symptoms. However they will pick up some (not all) asymptomatic cases that wouldn’t be picked up otherwise.

of course they also work if you have symptoms but anyone symptomatic should be relying on PCR result not LFT.

From my friends in schools I gather LFTs are picking up a number (def not all) of asymptomatic positive cases which otherwise might not have been picked up, either for several days or in some cases at all. Not perfect at all, but an extra tool in the box to fight the pandemic.

They definitely do work. Might not be 100% accurate but two friends of mine have tested positive for covid this week and both of their LFTs have tested positive (it has been confirmed by PCR test now).

It’s definitely a good tool to weed out asymptomatic cases and sometimes unusual symptoms which aren’t on the government’s official list of symptoms (one friend only had a sore throat and felt rough and would have carried on for two more days before her symptoms worsened if I didn’t encourage her to take a LFT just in case). I did a few LFT tests and a PCR test which both came back as negative.

I wouldn’t completely trash them as useless

The point in lateral flows is to detect asymptomatic cases. But you are totally right. If they can't detect symptomatic cases how can they detect asymptomatic?? It doesn't make sense. I guess if they pick up just a few cases that wouldn't have been detected otherwise that could be a win. But I guess the point is don't rely on them if you have symptoms!