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!

Very unlikely to get a false positive LFT as they are very specific to the covid virus. You are more likely to get a false negative PCR

If you put orange juice or fruit shoot on the test it comes back with a false positive. This is why with the throat swab ones you weren’t allowed to eat or drink before doing it.

@dementedpixie

Very unlikely to get a false positive LFT as they are very specific to the covid virus. You are more likely to get a false negative PCR

PCR are meant to be more reliable and accurate I thought? My dc lft was negative this morning, it was just one yesterday morning. Really strange

I’m getting very wary of either PCR or LFT now tbh. There’s been a number of children test positive on PCR but not LFT at out school recently (both with and without symptoms).

I’m taking the view that a positive on either overrules a negative on the other if you’re in a situation where both have been done 🤷‍♀️

@Fallagain

If you put orange juice or fruit shoot on the test it comes back with a false positive. This is why with the throat swab ones you weren’t allowed to eat or drink before doing it.

I've heard that. I witnessed the whole test so definitely no OJ involved plus it was just the nose one.

@Lilah2435 had she eaten or drunk anything before hand or maybe sprayed something? It may be there’s another factor that affected the LFT? Wasn’t it oj that interferes with them previously?

LFT have a higher false negative rate but have a very low false positive rate.

Despite PCR being the 'gold standard' they have a higher false negative rate than the false positive rate of LFT

The rate of false negatives on PCRs is higher than the rate of false positives on LFT. So I'd assume it's the PCR that is wrong.

[quote Cornettoninja]@Lilah2435 had she eaten or drunk anything before hand or maybe sprayed something? It may be there’s another factor that affected the LFT? Wasn’t it oj that interferes with them previously?[/quote]
It was a nose only test. They were having OJ for breakfast but wouldn't affect it if just up the nose I was there the whole time too, they swabbed and then I was holding the liquid, they put the stick in and I did everything else.

I was advised not to drink or eat anything (or chew gum) at least 30 minutes before LFT as this could interfere with the validity of the result. Could that be it?

This was a nose test in connection with travelling.

@grapewine

I was advised not to drink or eat anything (or chew gum) at least 30 minutes before LFT as this could interfere with the validity of the result. Could that be it?

Even with the nose only ones?

We've had this palaver twice now in our household, resulting in so much lost time isolating waiting for PCR results. No one of a scientific/medical background has been able to explain it, but anecdotally, it's way more common than that make out as I know at least three other families it's happened in too. And one woman I spoke to at test & trace said they see it frequently too. "Trust the PCR" is the advice. As long as you aren't exhibiting any symptoms?

@grapewine

This was a nose test in connection with travelling.

Nothing to do with travelling. The only ones my dc school has now are the nose only ones

Lilah2435 yes. It was in the email from the testing provider.

@Meecrowavay

We've had this palaver twice now in our household, resulting in so much lost time isolating waiting for PCR results. No one of a scientific/medical background has been able to explain it, but anecdotally, it's way more common than that make out as I know at least three other families it's happened in too. And one woman I spoke to at test & trace said they see it frequently too. "Trust the PCR" is the advice. As long as you aren't exhibiting any symptoms?

No symptoms at all! Dc back to school this afternoon because the PCR is negative

@grapewine

Lilah2435 yes. It was in the email from the testing provider.

Oh that's interesting. I didn't think food or drink would affect unless it was a throat and nose one X

@Fallagain

If you put orange juice or fruit shoot on the test it comes back with a false positive. This is why with the throat swab ones you weren’t allowed to eat or drink before doing it.

This only works if juice is put directly on the test - as long as the swab has been mixed in the correct solution first that won't happen, even if the person had snorted orange juice immediately beforehand 🤪.

Sure, I understand. But I gather the conditions would be the same for the same kind of LFT.

This has happened to us twice now in the space of a few weeks.

Positive LFTs, multiple negative PCRs. While I accept that one PCR may be wrong, I don’t accept that several are wrong.

So something is up. It may be that there is latent covid virus hanging around DD’s system (it’s only happened to her), so when she’s run down it flares up again? That’s my pseudo science theory 😂

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.

Sorry, crosspost.

DH has tested faintly positive on LFTs twice in the last month with the new flow flex tests. Both times he's done more LFTs that have all been negative and followed up with a PCRs that are negative. Either he's got/had a very mild case of covid or something is cross reacting with the test. He's been totally asymptomatic both times so he has to follow the negative PCR test and go to work! If he'd had big glaring lines I'd say clearly he's positive and the PCR is wrong but when it's so faint and followed by more negative tests it's very difficult to know what to do!

All the science theories in world doesn't get away from the fact that most schools and workplaces want you back ASAP if you've got a negative PCR 🤷‍♀️

There needs to be some new messaging / rules if that's not the case.

@Niki14

DH has tested faintly positive on LFTs twice in the last month with the new flow flex tests. Both times he's done more LFTs that have all been negative and followed up with a PCRs that are negative. Either he's got/had a very mild case of covid or something is cross reacting with the test. He's been totally asymptomatic both times so he has to follow the negative PCR test and go to work! If he'd had big glaring lines I'd say clearly he's positive and the PCR is wrong but when it's so faint and followed by more negative tests it's very difficult to know what to do!

Similar but in DD’s cases she did have mild symptoms of something. First time, a mild sore throat. Second time, her sense of taste changed and she had headaches (but that happens anyway when she’s unwell). Both times we had two PCRs spread out across a few days and sometimes postal, sometimes drive through.