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.

You can, but you've a little higher chance of getting a false negative. It's all about probabilities. I'll do my best to explain, but be aware I don't know what the probabilities of incorrect test results actually are, so the numbers in this are just for illustration.

A test can give you the wrong result in two different ways: a false negative means the test says you don't have it but you actually do, while a false positive means the test says you have it but you actually don't.

My understanding is that the LFTs are slightly more likely to give false negatives and PCRs slightly more likely to give false positives. So, plugging in some numbers to illustrate this, let's say if you're actually infected and you take an LFT, you've got a 1% chance of the test being negative. If you're infected and you take a PCR, you've got a 0.5% chance of the test being negative.

Now, if you're a member of the population with no symptoms, your chance of having Covid at any given moment is (about) the Covid rate, i.e. the proportion of people in the population who do have Covid. Not sure what that is right now, let's say 1%.

If you do have symptoms, you have a higher chance of having Covid, let's say 50%. Again, this is made up to illustrate!

Here's the thing. If we test everyone without symptoms with a LFT then we'd expect a false negative in 1% of 1% of the total people we test (as about 1% are positive and 1% of them will get a false negative). That's 2 in 20,000, which isn't too bad. If we test people with symptoms with a LFT, 1% of 50% will get a false negative. That's 100 in 20,000, quite a bit more common.

If we test people without symptoms with a PCR, 0.5% of 1% will have a false negative, so 1 in 20,000. Test those with symptoms and it's 0.5% of 50%, so 50 in 20,000.

So, using my figures, which I can't emphasise enough are just made up ones to illustrate how it works, out of every 20,000 people without symptoms, we'd have 1 less false negative if we used PCR compared to LFT. Out of every 20,000 people with symptoms, we'd have 50 less false negatives if we test with PCR compared to LFT.

So, given that PCR tests are more expensive, and more of a faff to do than LFTs, it makes sense to use them primarily with the group that's more likely to be infected, as we'll miss fewer cases that way.

@Magistera

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”.

I think your colleague raises a good point but I disagree with their points about LFT’s. I think all of you should have PCR’s as close contacts and use LFT’s as an extra precaution if negative symptoms or not.

If you have symptoms, are you not meant to isolate until you have a negative PCR? Or has that changed now?

Yeah you're right. If you have symptoms you isolate and go for a PCR

To be fair he colleague can pint blank refuse any test as contact tests are voluntary so be glad shes taken a lft!

Lat flows aren’t accurate. There’s more room for user error too not enough/flooding tests with the buffers and ignoring faint lines mostly.

Lat flow never turned positive until 6 days after a positive pcr for me and even then it’d have been debated if it was a pregnancy test. And it never got darker (full box and curious about it as I was asymptotic initially)

LFTs are 70% accurate at detecting covid. They are useful because they are cheap and easy to use, and will pick up 7/10 cases.
However if you have symptoms then this is not a high enough ratio to guarantee that you do not have covid, so you need a PCR which will say with a greater rate of certainty whether the symptoms are caused by covid.

@LarkspurLane

If you have symptoms, are you not meant to isolate until you have a negative PCR? Or has that changed now?

If you have classic covid symptoms (one of the big three) you have to isolate until you get a negative pcr. I think people are talking about the other symptoms which pretty much everyone knows are potential covid symptoms but aren’t on that list.

@Watapalava

To be fair he colleague can pint blank refuse any test as contact tests are voluntary so be glad shes taken a lft!

But she has symptoms so should do a PCR

LFTs allow regular, population wide, testing of non-symptomatic people which gives us a chance of identifying asymptomatic cases and getting them to isolate & therefore not spreading the virus.

If you have symptoms there's a stronger chance that you have COVID. LFTs show up loads of false negatives (test says you don't have covid when you do) so they're not good enough to be confident a symptomatic person is covid-free. A PCR test is more accurate so a negative result is good enough to test and release.

Case in point: my colleague had symptoms, did 3x LFTs while waiting for PCR. All LFTs were (false) negatives, PCR was positive. She had covid, LFTs didn't show it.

The LFT is less reliable than a PCR. I think it's something like 80% reliability. This means that it won't pick up every infection. It may be in cases where: there is a smaller viral load; infection in it's very early stages (so it hasn't had time to replicate); or very late stage.

The above will lead to a false negative result.

Your colleague sounds a bit thick and a right pain in the bum tbh.

2 years in and people still don't understand how to not kill people

I tested negative on 3 separate LFTs then positive on 2 separate PCRs before my symptoms started. They're unreliable. Tell her that.

As soon as you get symptoms, you need the higher rate of accuracy of a PCR to rule out covid.

If you’ve got covid, then the LFT is likely to be right. However if it’s a cold with covid like symptoms, the LFT isn’t reliable. You won’t know you’ve got covid without the PCR

My sister and brother in law are medical and I have asked them this before.

As I understand it (from them) LTF are simply less sensitive than PCR which are tested in a lab etc. So to dumb it down a bit and use a simple example, say you have 1 speck of Covid and test on a LTR, the LTR might only magnify the sample by 10x and therefore the test might miss it. The PCR might magnify the sample by 100x and therefore more likely to pick it up than 1 speck of the virus.

Also add in the human error for doing the test at home with LTR and swirling the sample in the liquid and then dropping 4 drops onto the test etc and reading it yourself. All of that seems just obviously less sensitive than a trained person/lab dealing with it and interpreting the results.

Also, to echo others' comments, it is nothing about actually having symptoms that affects either test. We are told to get a PCR test if you have symptons purely because if you have symptoms you are more likely to have Covid than someone testing with a LTF who has no symptoms. So they want to ensure they don't miss a positive case with the LTF and that they can do track and trace etc etc..

I did 8 lfts in about 4 days all were negative but I got a positive pcr when tested in the middle of those lfts.
I was pretty sure I had it which is why I did so many lfts, I've since thrown my big box of lfts out as there is obviously not point me using them anymore.

If you've got covid will an LFT show when you're no longer infectious?

Don't know but my daughter

Has she been living under a fucking rock!!

Almost two years in. Just tell her to follow the rules FFS!!

(BTW, to answer your question it’s to do with the viral load. By the time you’re showing symptoms it’s likely to have dropped and the LFTs aren’t as sensitive as the PCRs)

Oops. Posted to early. My daughter's LFT negative an hour before she took PCR that was positive.

Friend's ds woke up with a sniffly nose Friday, so she did lft, which was positive. Booked pcr later that day, also positive. She did an lft on her ds the following day out of interest and negative, as had been the one last Wednesday. Lfts done every day since, all negative.

So in this case, there was indeed a very small window. If she'd just stuck to the Wednesday and Sunday lfts for school, they would have missed it.

@hopeishere

If you've got covid will an LFT show when you're no longer infectious?

No.

My husband tested negative on a LFT the day he got a positive PCR. The lateral flows became positive as he became symptomatic.