Why can’t you use an LFT when you have symptoms?

[Magistera]

I know you’re not supposed to, but why? Currently having an argument with a symptomatic coworker who insists they’re fine because they did a negative LFT this morning. I said they need a PCR. They said why isn’t a negative LFT acceptable? And I don’t know the answer, other than to say it’s government policy.

Because you may get a false negative and be reassured that it isn’t Covid and then go spreading it about?

I believe LFT’s have a higher false negative rate than PCR’s. They’re not as sensitive.

That said, people are getting some conflicting results from both and I have heard that a negative LFT is a good indicator of how infectious you might be at the point of testing.

With symptoms a PCR is needed to be as sure as you can be whether it is or isn’t covid.

@MeetMeAtOurSpot
So can’t you just do two to make sure? Why would a lateral give a false negative and not a pcr?

I know 2 people in RL (both in education sector) that had cold symptoms so were told not to have PCR, LFT came back negative 2 days running and then positive on 3rd day. I think it is something to do with antigens but I am a bit like you in that, it is policy and so just do it.

Not very helpful - sorry

A positive on either a PCR or a LFT are both highly likely to be correct.
A negative on a LFT could be correct, or could miss an infection. So, if you are asymptomatic, a LFT may pick up an infection you would otherwise have missed - great. If you are symptomatic, you are much more likely to have covid than someone asymptomatic, and so missing a positive can have much larger consequences.

Like every else said, it is because they have a high false negative rate. Significantly higher than PCRs. A PCR is always a better, more accurate test, but they are more expensive and time consuming than lateral flows.

Whether it is "worth" doing a PCR depends on the prior probabilities. If you are unlikely to have covid (no symptoms, no contacts with cases) then if you have a negative lateral flow you are very unlikely to have covid.

If you are likely to have covid (symptoms, close contact with cases) then even with a negative lateral flow you are still quite likely to have covid. Much less likely after a PCR. That's why you are also advised to have a PCR test after a known close contact with covid.

In addition to the sensitivity of the tests themselves, I imagine it's also that if everyone used LFTs, Test & Trace would never be informed or people could just lie about having a positive or a negative test (based on their circumstances) - when it's sent off to a lab, it should in theory be more trustworthy and official... (yes I'm aware of all the lab problems).

Four of us were at a client meeting together. We’ve been informed the client has tested positive. Me and another person have symptoms so we need a PCR, the other two don’t have symptoms so can just use a LFT.

I’ve had a PCR this morning. But the other person who has symptoms is saying “I have a negative LFT so that should be acceptable, and how come the other two colleagues can get away with a just negative LFT but I’m being told to get a PCR as well?” I said because government policy is if you have symptoms you need a PCR. And the response is “that’s ridiculous, a test is a test, if I had Covid it would show up on the LFT, and if your LFT is positive then you get a PCR to confirm, it’s discrimination to let those two off with only an LFT but insist I need a PCR”.

LFT have a higher false negative rate so a negative test on LFT isn't as reliable as a negative test on a PCR. LFT and PCR tests look for different things so although the method of taking the test is the same they are not the same type of test.

Lateral flow tests are not as sensitive as PCRs, so they miss a significant number of positives. If you have symptoms you need to know that you don't have covid. If you have no symptoms then a LFT is like an extra level of reassurance that you probably don't have it.
My DH had a negative LFT and a positive PCR taken within an hour of each other (he was testing because DS tested positive). He got symptoms the following day.

When my (mild) Covid symptoms started, the LFT still said negative but my PCR was positive.

In my experience this autumn a pcr (from close contacts at schools in our numerous cases) seems to be able to pick it up approx 48 hours before LFT turns positive. LFT does go positive eventually after positive pcr in most cases.

Not sure. Probably if your viral load is low the LFTs are less likely to pick it compare to PCR?
However, it was LFT that triggered me getting a pcr after I started showing symptoms. Sore throat isn’t on the list of symptoms to get PCR but this is why I did LFT in the first place. Was super positive, never seen the line so bright . Husband was the same, symptomatic but LFT positive.
But if in doubt get a PCR. I m still surprised the list ofPCR eligible symptoms is so short.

LFTs don’t work any less well if you have symptoms. They’re just not good enough to say your symptoms aren’t due to covid. You need a pcr for that.

They’re good for alerting you to the fact you likely have covid if you test positive on one.

Four of us were at a client meeting together. We’ve been informed the client has tested positive. Me and another person have symptoms so we need a PCR, the other two don’t have symptoms so can just use a LFT.

You are encouraged to have a PCR following close contact whether or not you have symptoms. LFTs are really for people with no symptoms or contacts.

You are required to isolate if not vaccinated regardless of test results (only legally required if contacted by track and trace)

The presence of symptoms affects how the outcome of positive/ negative is calculated. PCRs take this into account and LFTs don't.
I agree the list of 3 official symptoms seems wildly out of date.

The reluctance to do a PCR test despite having symptoms absolutely winds me up - I've known several people display the same ignorance. People are still dying because of this wretched virus.

Because LFT's aren't accurate enough. I had a negative LFT on the same day I had a positive PCR.

Because Boris knows what he's doing!

The whole point of the LFTs was to check the population for asymptomatic cases - yes they aren't as accurate as PCR but if used regularly are likely to pick up a fair number of people who would otherwise be wandering aroung spreading the virus. They still work if you are symptomiatic- they don't suddenly stop detecting virus once you get symptoms - how would they know?
It now appears that they are more accurate than at first thought, particularly if you are doing them regularly - the chances of them missing a positive when repeated are less of a risk. Mind you, I susect there may be problems with technique - for example it took me a while to realise that the FlexFlow ones tell you to twiddle the swab in the buffer for a while - I don't think my original ones did. Another one I used at a client's had slightly different intructions again.

There is a very small window in which a lft will be positive when you have symptoms

@RuthW

There is a very small window in which a lft will be positive when you have symptoms

What do you mean? Where is this information from?

@RuthW

There is a very small window in which a lft will be positive when you have symptoms

Mine was positive just as my (non classic) symptoms started and stayed positive until at least two weeks later when I got fed up of waiting to see when it went negative.

The system is set up to track pcr tests, so I suppose it makes sense to stick with those, even though lateral flow tests are more reliable than originally thought.

My DD did 6 LFTs feom monday to friday last week. PCR was positive on wedsnesday. Every single LFT was negative. She only tested positive on the LFT 4 days after the PCR result.

LFTs are a useful tool for asymptomatic people. But for people with symptoms - PLEASE get a PCR because LFTs do not alwats pick up covid

@RuthW

There is a very small window in which a lft will be positive when you have symptoms

This isn't my experience at all and I haven't read this anywhere either. Do you have a source?