How can false LFT happen?

[Lilah2435]

I'm just curious. I've attached a picture of my dc positive LFT from yest, no symptoms at all. Kept them off school yest, preparing ourselves for the pcr results. They've come through all negative today. But how can an lft show positive when it's not, what triggers the second line to come up if there's no virus. I'm just curious!

@Knownbyanothername

It’s about sensitivity and specificity and false negatives and false positives If you’re positive on a LFT there’s about a 99% chance you have covid so very low chance of false positive. If you redo a PCR in about 3 days it will most likely be positive. The line on your test is weak so probably in very early days of infection. LFTs can miss up to 25 percent of positive cases apparently so a higher chance of false negatives.

This wasn't the case with us and we've been through it on two occasions now. All follow up PCRs for all involved were negative too and no symptoms developed.

My theory (non scientific!) is that there is coronavirus present in the body at the time of the faint lft but a very early infection that you go on to fight off (because of vaccine or whatever else). In our last household experience of this, we did actually have COVID in the house - but not from me who had the initial faint positive lfts. I went on to have three negative PCRs in the days that followed as other members of my family fell ill. And the lfts were negative in the days after too. All very odd.

All the people I know this has happened to have only ever had faint lines. I've never heard of anyone having a strong line and then testing negative on a PCR.

It's probably worth doing another lft in a few days to make sure the line isn't more obvious, but no guarantee it'll lead to a negative.

No guarantee it'll lead to a positive I mean!

The liquid in the tube is a buffer so the pH is neutralised. It's only if something acidic is out on the test on its own that you get the ok/fruit shoot positive. That's the point of the liquid in that little tube. Eating/drinking irrelevant anyway for nasal tests.

@Meecrowavay that makes sense. The LFTS aren’t actually wrong, just picking up minor infection. So the vaccine is doing it’s job and there’s enough immunity to stop it becoming full blown, or there’s a milder strain going about. Either way it’s good news 🙂

I also don't understand what can make tests show a false positive if their specificity is so high. My DD produced this test a few weeks ago but two PCRs (one done the same day and one a few days later) came back negative. She didn't eat or drink before the test and no chance of cross contamination as no other cases in the house. We can't have just caught the tail end of the virus as she'd been doing regular LFTs for a few weeks and they were all negative. 119 were not very helpful, just said to go with the PCR result. She had been sick a few hours before the positive LFT and I did wonder whether that could have contaminated the test, but probably not if the buffer fluid neutralises acid.

My theory (non scientific!) is that there is coronavirus present in the body at the time of the faint lft but a very early infection that you go on to fight off (because of vaccine or whatever else). In our last household experience of this, we did actually have COVID in the house - but not from me who had the initial faint positive lfts. I went on to have three negative PCRs in the days that followed as other members of my family fell ill. And the lfts were negative in the days after too. All very odd

I like this theory

A positive result on LFT is about 7-8 times more reliable than a negative on a PCR. The official advice does not acknowledge this.

Additionally, there is now another private lab (same owners!) which may have been issuing incorrect results:

www.independent.co.uk/news/science/covid-testing-immensa-lab-nhs-latest-b1960760.html

[quote JanglyBeads]A positive result on LFT is about 7-8 times more reliable than a negative on a PCR. The official advice does not acknowledge this.

Additionally, there is now another private lab (same owners!) which may have been issuing incorrect results:

www.independent.co.uk/news/science/covid-testing-immensa-lab-nhs-latest-b1960760.html[/quote]
Jesus.
Well it’s behind a paywall and I can’t see which lab it is. If it’s south east then could affect us. I have to say I was stunned when DD’s first lft positive happened (three of them) then negative PCRs all around. We all got ill to some extent but we all tested negative many times.

The line on your test is weak so probably in very early days of infection

Or in the very late days of infection. Maybe some are positive on the LFT but then negative on the PCR because by then the virus is going and so just isn't dectable anymore?

*detectable

I've spoken to test and trace about this and they said PCR trumps LFT every time.I've had false positives from the nose only ones but -ve for the other LFTs and PCRs. Not had symptoms at all.

We had covid in the house (confirmed by PCR test) and my other DC had two positive LTFs followed by two negative PCRs.

@Flaxmeadow we can rule that out I think in our situation as I had done LTFs several times before including only 3 days or so before the positive ones began and they were always negative.

We’ve also had this- faintly positive LFT followed by negative PCRs 72 hours apart. Baffling.

In the last three weeks I’ve had 7 faintly positive lfts and 4 negative pcrs. Interestingly all of them have been nose only lfts. When I did a throat and nose last week at a friend’s house that was negative. 119 have not been able to help and the pcr testing place are sick of seeing me! (I feel like I have a frequent fliers card right now!!)

Try this link, no paywall?
www.google.co.uk/amp/s/www.independent.co.uk/news/science/covid-testing-immensa-lab-nhs-latest-b1960760.html%3famp

No indication of where the tests came from and no specifics about definite incorrect results but it certainly sounds like there could have been with those poor practices.

We had the same positive LFT and negative PCR. Useless things

The instruction leaflet that comes with the nose-only LFTs says not to eat or drink before taking the test @Lilah2435

@DavidDevantsSpiritWife it must depend on the brand of test. My Flowflex ones say not to eat drink or smoke for 30 minutes prior to the test but my Orient Gene ones don't mention it at all.

I had a false positive ltf. It was a thin faint grey line very different to my kids positive ltfs which were positive pcrs.
I went for a pcr which was negative
I had felt a bit unwell the day I did the test but kept testing and only had the one positive on a lateral flow. From threads on here I was expecting to start to get strong positives and I would have kept isolating and pcring if that had happened but it didn't and I felt fine from the day of negative test result . My nhs boss said to go with the pcr as there have been a number of seemingly false positive ltfs in our trust

I also think there may be something about you've been exposed to covid snd are fighting it off which caused the faint positive
When I had mine I had a bad cold and read an article at the time which said if you're exposed to a cold and covid at the same time the cold will 'win' out so I wonder if that's what happened with mine
Plus I was three weeks post booster so probably super immune . Just to add with mine I test x 2 a week for work so I know mine wasn't the end of something as all tests week before were also negative
Mine was flow flex as well