Could blood on the swab cause a false positive LFT?

[Piapiano]

DD has had a headache all day and feels hot to touch but the thermometer says her temp is only 36.7.

She did LFT but it cause her nose to bleed a bit so some blood got in the sample. There was a faint positive line. She redid it after 15 mins and the second test doesn't show any line.

I've booked her a PCR for tomorrow anyway as some of her friends currently have covid. But just wondered if the blood in the sample could have affected the result?

I had a little bit of blood from my nose on an LFT the other day and it was 100% negative. Good plan to book a PCR & I hope your daughter feels better soon!

The one on the right is the one with blood in, the one on the left was done 15 minutes later.

We had a positive lft with a sample containing blood from the nose, followed by negative with no blood on the sample.
Surprisingly little information about this online though.

We had exactly the same, @Chardlettuce.

Son poked the swab so far up he had a nosebleed, and then a positive LFT.

An hour later we tried again with no blood, and a negative LFT ensued. He had no other symptoms, it was a routine test.

My DD got some nose blood on the swab but the LFT still came up negative. If in doubt, especially with the headache, I would keep her at home until she's had PCR result, and maybe retest with LFT each day whilst you wait.

Just to update this, she had a positive PCR. Still doesn't explain why the two LFTs done 15 mins apart came up with different results!

@Piapiano my completely un backed up by science view on this is that the virus is a hitchhiker rather than a part of the human body. Once you've picked up the particles with the first lft, it will take a fair while before enough little hitchhikers are present to be picked up on another swab, at least enough to test passive on a test (unless you're riddled with it, obviously). False positives are incredibly rare, maybe 1 in 10,000. False negative are rather more common so I'd trust the positive over the negative every time.

Yes I had the same thought- that I had wiped out all of the virus particles on the first test!

What I can't figure out now is that DS is getting some very faint positives one day then negative the next, then faint positive again. Negative PCR and no symptoms.

Sorry to hear she's tested positive, hope she's better soon.
Blood isn't detected by the test so no it won't cause a false positive.

The reason one was positive and one was negative is that LFTs have a high false negative rate.
The positive test only had a very faint line so the test only just picked up that there was Covid virus there. 15 mins later or was not able to detect the virus because the test is simply not sensitive enough and the levels of virus were borderline.

Hopefully that means she wasn't too infectious as she coughed in my face when I did the test 🙄

Like the pp said, false positive LFT results are very rare.

PCR false negatives are more common so your DS probably does have Covid.

If it were me I would be keeping him at home, at least until he's had another PCR test in a few days.

@Piapiano

Just to update this, she had a positive PCR. Still doesn't explain why the two LFTs done 15 mins apart came up with different results!

It's because LFTs aren't very accurate and give a lot of false negative results

@Budapestdreams where do you get that false negative pcr's are common? Do you have data please as I thought these are more accurate ( excluding recent lab issue )

Do they not mean that you're more likely to get a false negative PCR than a false positive LFT?

Would you say this is positive? DH can't see a line but I can.

I can see a very faint line

also see a faint line. soirry.

No, the opposite typically. When working as a TL for covid testing sites , we refused bloody swabs on the grounds they can provide false negatives through not giving the specimen required for testing.

To add, if there is a positive indication despite blood it shows that the swab was strong enough or prominent enough to appear.

@Piapiano

Hopefully that means she wasn't too infectious as she coughed in my face when I did the test 🙄

That doesn't matter, you should really be staying in until T and T have determined you are either negative through testing or have waited a significant length of time for it to appear.

Not sure i feel it would have contaminated the sample maybe

Well here's DS's LFT from this morning which is negative. Negative PCR, no symptoms and mainly negative LFTs for the last 7 days. A couple of faint line LFTs. It doesn't make any sense.