Do you trust a negative PCR over a positive LFT?

[Eltonsglasses]

Person tests positive on LFT - people are notified and told to isolate then contacted again to say they don't have to isolate as PCR is negative.

I know the stance is that the PCR is the one to be believed, but from a personal POV would you trust that result? Would you be happy to crack on as normal, visiting vulnerable relatives etc? Or would you err on the side of extreme caution?

Nope. I’ve seen too many people get positives after a retest days later

my experience and general media led understanding would lead me to accept the PCR over the LFT. However keep seeing reports that LFT are in fact more accurate than we’ve been thinking (?) and reports of PCR results being overridden.

LFTs are actually very accurate!

“Research shows rapid tests are 99.9% accurate. This means the chance of getting a false-positive result (where the result shows as positive but is actually negative) is extremely low.“

www.nhs.uk/conditions/coronavirus-covid-19/testing/regular-rapid-coronavirus-tests-if-you-do-not-have-symptoms/

I just posted this on another thread but to add my two cents here…

I’ve always found the LFTs to be very reliable. I did the latest one right after eating and drinking (didn’t know you weren’t meant to). The line saying I was positive came up almost immediately - I did another one to be sure and the same thing happened. It was correct and I do have Covid.

I’ve also had a number of negative LFTs previously that have been backed up by negative PCRs.

No - I've been testing positive of LFT for four weeks. I took three PCR tests the first ten days of this, and isolated regardless of results (which were all negative). I figure I don't need to now as I did the first ten days and you can test positive for up to 90 days, but I'm fairly certain I had covid (again, also had it in Jan) because how else could I still be testing positive?! And I had very mild symptoms

We had a positive lateral flow here, negative pcr the next day. Symptoms of cough, no sense of smell or taste. Same symptoms as two other household members who tested positive. We have treated it as a positive result and ignored the negative pcr.

My OH tested positive on lft and 2 negative pcrs. He is now ill with long covid, not able to walk far, permenantly exhausted and really struggling. I'd belive the positive result.

No. False negatives are actually quite high in PCR tests whereas false positives are much rarer in either test.

I'd go with the negative if I was sure I'd done the test correctly. Not worth the risk. If work made me come in I'd want it in writing.

False negatives aren't high in PCR tests except where you have a batch of patients with extremely high positivity rates (as you do in the case of PCRs acting as confirmation for positive lateral flows).

E.g. if you have a 93% positivity rate (93 out of 100 are truly positive) in the group of tests for confirmation of lateral flows, 40% of negatives from PCRs would be false negatives.

There's some good twitter statistics on it somewhere but I read it weeks ago and not sure where...

“Research shows rapid tests are 99.9% accurate. This means the chance of getting a false-positive result (where the result shows as positive but is actually negative) is extremely low.“

So 1 in 1000 would have a false positive.

If you test 10000 people at random, at current rates (1 in 50 has Covid) , 200 would actually have Covid,( though not all would be picked up - there'd be some false negatives), and there would be 9-10 expected false positives. So still a small chance, but not as small as that 99.9% rate would lead you to believe. More like 1 in 20.

But that's still a lot lower than the 40% (8 in 20) chance that of a negative PCR that Gladioli quoted in this situation.

You've tested either because you have symptoms or have been a close contact, so there's an increased risk you have covid. I think I'd be erring on the side of caution (I certainly wouldn't be visited vulnerable people).

yes

ffs, pressed too early

yes, I'd err on a side of caution if there are symptoms

This articles explains well why the negative PCR should not trump a positive LFT: www.theguardian.com/theobserver/commentisfree/2021/oct/17/whats-the-value-of-a-confirmatory-pcr-test-covid

[quote martymcfly01]This articles explains well why the negative PCR should not trump a positive LFT: www.theguardian.com/theobserver/commentisfree/2021/oct/17/whats-the-value-of-a-confirmatory-pcr-test-covid[/quote]

Thank you. That's a really helpful article.

@ZingDramaQueenOfSheeba

ffs, pressed too early

yes, I'd err on a side of caution if there are symptoms

No symptoms the LFT was done as part of twice weekly testing.

@Lottie4

You've tested either because you have symptoms or have been a close contact, so there's an increased risk you have covid. I think I'd be erring on the side of caution (I certainly wouldn't be visited vulnerable people).

I haven't tested at all.

However, the LFT was taken as part of the usual regular testing, not because anyone had symptoms or was a close contact.

@cowburp

I'd go with the negative if I was sure I'd done the test correctly. Not worth the risk. If work made me come in I'd want it in writing.

This seems to suggest either or?

@justbananas42

I just posted this on another thread but to add my two cents here…

I’ve always found the LFTs to be very reliable. I did the latest one right after eating and drinking (didn’t know you weren’t meant to). The line saying I was positive came up almost immediately - I did another one to be sure and the same thing happened. It was correct and I do have Covid.

I’ve also had a number of negative LFTs previously that have been backed up by negative PCRs.

But what I'm asking if if the PCR didn't back up the LFT, would you trust the PCR?

@MereDintofPandiculation

“Research shows rapid tests are 99.9% accurate. This means the chance of getting a false-positive result (where the result shows as positive but is actually negative) is extremely low.“

So 1 in 1000 would have a false positive.

If you test 10000 people at random, at current rates (1 in 50 has Covid) , 200 would actually have Covid,( though not all would be picked up - there'd be some false negatives), and there would be 9-10 expected false positives. So still a small chance, but not as small as that 99.9% rate would lead you to believe. More like 1 in 20.

But that's still a lot lower than the 40% (8 in 20) chance that of a negative PCR that Gladioli quoted in this situation.

Just dug around to find the twitter stats:

twitter.com/BristOliver/status/1439966843381460992?t=f7x7ZlHRk6Vv7KiiiG0gNA&s=19

Statistics gets quite unintuitive when you have incredibly high or incredibly low positivity rates.

I received a new type of lateral flow test last week, the pink lined ones, I tested my kids and both tests were negative at 30 minutes, I left tests on the side and a few hours later went to bin them and looked again and they had faint T lines? We repeated the tests again and they were all again negative? But again an hour or so later a few of them had faint positive lines?? As we are all busy and can lose track of time, I believe this is wear all the false positives are coming from. Cos pcr’s come back negative. But even seeing a faint positive line after the time scale fills you with dread and uncertainty, not knowing what to believe?

Yes - DD had two faint positive LFTs. So we got a PCR - negative. I didn’t believe it so took her for another PCR a few days later, figuring that the PCR should be positive. It wasn’t, negative again.

She only had a sore throat for two days, then was fine. The rest of us were all fine and we had PCRs too a few days after dd and we keep getting negative LFTs. Ds had had three PCRs over the course of a fortnight because he’s been poorly (and countless LFTs as his school requires it).

So we went with the PCRs. It was over half term so dd was pretty much isolated anyway. We didn’t see anyone and stayed home.

We aren’t in the south west so fuck knows what happened there. I’m slightly worried that the PCRs aren’t picking up new variants but what else can we do?!

Yes

I have found my people….

5 faintly positive lfts over ten days and three negative pcrs following up those lfts. Am at a loss as to what to do as are the lovely people at 119 that I rang. All the lfts start off negative but come up with a very very faint line after 30 mins. Not a faulty batch as are from different boxes and testing my kids as well and theirs stay completely negative. The pcr testing place seem sick of seeing me and say that pcr is gold standard so if that’s negative I’m fine! No symptoms at all.