Why are Lateral flow tests useless when you have symptoms?

[Peachi82]

I have seen this statements several times (just last night in an aibu thread) and even though I'm strictly adhering to the guidance, I don't understand it.
The viral load in your mouth and throat must be quite high if you're symptomatic, so the lateral flow tests should deffo pick it up? Or is the worry that they can maybe show a false negative and people just get on with their daily life and keep on spreading?

Can someone shine light to this?

It should say nose and throat.
My mobile has a mind of its own.

It's the worry that it shows a false negative that reassures people who should be isolating.

It’s your last point. The false negative rate is quite high so if you want to check that symptoms aren’t covid you need something more reliable. Otherwise some people with covid will be falsely reassured that it’s just a cold.

If you’re simply screening people with no symptoms then every case found is a bonus. The false negative rate still matters, but the tests are worth doing because they let you pick up some asymptomatic cases rather than none of them.

Well common sense would tell you if you were experiencing symptoms to go and get a PCR test.
Lateral flow is more for asymptomatic. They're also antigen tests, they detect proteins specific to coronavirus. PCR detect viral genetic material.

The two tests are very different.

My understanding (could be wrong) is that lateral flow tests are just less accurate in general, if you have no symptoms then the lower accuracy is a more acceptable risk because you probably don't have Covid anyway.

If you have symptoms then the lower accuracy is a higher risk because if it gives a false negative and you then go round passing on Covid to everyone Willy billy that would be bad.

Ah, this is very interesting, PixieDust!
I will read up a bit about it.
I am aware that PCR is a lot more sensitive as it's run in several cycles (and is therefore a measure for the viral load). I wasn't aware that the home tests look for different things than the PCR (it does make sense though).

Has anyone got any numbers how often the home test kits show a false negative?

So which test would you take if you have wider symptoms e.g headache, blocked nose, sore throat? Presumably a lateral flow as these symptoms do not qualify you for a PCR?

It’s to map the strains of virus circulating

@Jasmine245 yes lateral flow because aa you say can't get a PCR with those symptoms.

When we had covid, lateral flows were positive for all 3 of us who had symptoms. We then got PCRs which were also positive. DP had no symptoms and a lateral flow was negative but a PCR positive, presumably because he had a low viral load. The positive lateral flows allowed our contacts to self isolate much more quuckly than they would have done if we'd had to wait for a PCR, which at the time we had covid took several days.
But as you suggested, a negative lateral flow shouldn't be used as proof that a symptomatic person doesn't have covid.

Jasmine, this is probably something I would decide depending on infection rate in the area and if it is likely that I was exposed.

I have to admit that we got PCR tests just for a sore throat, which obviously goes against guidance. But we all know that Covid can have more symptoms than the ones making you eligible for a PCR.
Waiting now for mumsnet rage descending upon me.

They're generally less accurate. So with symptoms a pcr is more reliable.

They should be seen as an ongoing 'net,' twice weekly testing. The false negatives are then more likely to be picked up with regular tests.

It usually has to be backed up by a pcr to eliminate false positive issues.

A twice weekly lateral flow test did pick up a positive in a colleague who had a bad headache and cold symptoms but no fever or cough. A child also then tested positive via pcr in the class.

Lateral flow tests are only worth using really if taking them regularly twice a week.