Lateral Flow Tests when symptomatic

[swill453]

The official guidance is that you shouldn't use lateral flow tests if you have Covid symptoms:

Do not use a rapid lateral flow test if you have COVID-19 symptoms. Get a PCR test as soon as possible and self-isolate, even if symptoms are mild.

https://www.gov.uk/order-coronavirus-ra ... flow-tests

Is this because a lateral flow test genuinely won't give an accurate result if you have symptoms?

Or is it because they want to encourage symptomatic people to get a PCR test for other reasons (e.g. so they're in "the system", so they can do the genomic testing etc.)?

Scott.

[servodude]

Is this because a lateral flow test genuinely won't give an accurate result if you have symptoms?

Gavi who know this stuff say they're about 72% sensitive if you have symptoms https://www.gavi.org/vaccineswork/how-likely-positive-lateral-flow-test-covid-19-be-wrong

So around 30% of those who would be using it would get the wrong advice
- that could be a good leg up for Omicron!?

-sd

[swill453]

Is this because a lateral flow test genuinely won't give an accurate result if you have symptoms?

Gavi who know this stuff say they're about 72% sensitive if you have symptoms https://www.gavi.org/vaccineswork/how-likely-positive-lateral-flow-test-covid-19-be-wrong

Vs. 58% if you don't have symptoms.

The implication being given by the official advice is that lateral flow tests are less accurate with symptoms than without*, whereas the opposite is the case.

* I know this because that is how my wife interpreted the advice.

EDIT: Note I'm simply discussing technicalities here, not implying people shouldn't go for a PCR test if they have symptoms.

Scott.

[servodude]

Is this because a lateral flow test genuinely won't give an accurate result if you have symptoms?

Gavi who know this stuff say they're about 72% sensitive if you have symptoms https://www.gavi.org/vaccineswork/how-likely-positive-lateral-flow-test-covid-19-be-wrong

Vs. 58% if you don't have symptoms.

The implication being given by the official advice is that lateral flow tests are less accurate with symptoms than without*, whereas the opposite is the case.

* I know this because that is how my wife interpreted the advice.

EDIT: Note I'm simply discussing technicalities here, not implying people shouldn't go for a PCR test if they have symptoms.

Scott.

I think it's more of a "knife to a gunfight fight" thing
- in a population where there's good coverage of a vaccine that protects well against infection (mostly from people with symptoms) that 30% error won't be a huge problem
- that has probably changed?
- sd

[vagrantbrain]

I suspect it's more to do with being able to enforce isolation requirements and allow track and trace to get involved than any scientific reasons.

Take a LFT and if it's positive you can just bin the test and no-one would ever know. PCR tests need your contact details to book and an ID check at the test site (in theory) so it's much harder to hide a positive result. I also think there would be legal challenges if people were required to isolate by law following the result of something that's relatively inaccurate.

[Hallucigenia]

- in a population where there's good coverage of a vaccine that protects well against infection (mostly from people with symptoms) that 30% error won't be a huge problem
- that has probably changed?

The argument for LFTs against "classic" Covid is that their lower sensitivity vs PCR actually helps, as transmissibility falls off with viral load, so the cases you miss are probably going to fall into the 75% of classic Covid cases that don't transmit to anyone else and so you can concentrate resources on high-risk patients.

I'm guessing (but don't know) that delta and omicron change the game by giving symptoms and transmission at below the viral load that LFTs can detect, and we have the PCR capacity to PCR all symptomed patients.

[Gersemi]

I assume the rules are because most people are asymptomatic (mainly because they don't have Covid). It is expensive to administer a PCR test, so they don't want to waste them on people who probably aren't infected, but they are willing to expend the cost of a LFT to catch some cases that would otherwise go undected.

If you have symptoms you are more likely to be infected (although many people don't actually have Covid even then) and the authorites are willing to expend the cost of a PCR test to identify you so you can isolate.

If you aren't prepared to isolate then just don't take any test. I'm not recommending that course of action.

[9873210]

I think it's more of a "knife to a gunfight fight" thing

- sd

You can look at this from a Bayesian perspective. The chances of somebody who is symptomatic having COVID is almost certainly higher than that of somebody who is asymptomatic having COVID.

To put it yet a third way: You want to isolate everyone who is symptomatic OR has a positive LFT, not everybody who is symptomatic AND has a positive LFT. So once you have symptoms the LFT should not change the isolation status, so there is no point doing the test.

[Hallucigenia]

It is expensive to administer a PCR test, so they don't want to waste them on people who probably aren't infected, but they are willing to expend the cost of a LFT to catch some cases that would otherwise go undected.

It's not so much a question of cost. If you're trying to prevent infection chains, then an imperfect test that gives a result "immediately" is more effective than a "perfect" test that gives a result in 3 days (or more in some cases) when most of the transmission has already happened.