Lateral flow test turning positive after many hours?!

[pleasenomoreisolation]

DS did a LFT yesterday. We are doing them daily and are in isolation as we are close contacts of someone who tested positive earlier this week.

Yesterday's LFTs were negative but I've come down for my morning coffee and DS's test from yesterday now looks positive to me!

Is this a false reading? Or is this a real positive? The line is a squinter but definitely there.

I'm sure an expert will come along but as I understand it you take the result at after the 30 mins and ignore what happens after that. But sensible to keep testing and isolating

Do another test

You read after 30 minutes and then discard.

You cannot rely on a reading that's that old.

Re test if you're concerned

They will nearly always turn positive eventually. It’s the technology. Take the result at 30 mins then bin.

@Greenandcabbagelooking

They will nearly always turn positive eventually. It’s the technology. Take the result at 30 mins then bin.

All our other LFTs are crystal clear. Me and DP's tests from yesterday still have no hint of a line.

Here's mine from yesterday vs DS's.

I think I'll do another LFT and then perhaps book a PCR.

dd2 got a line after the time and then tested positive the next day. But she also got symptoms.

You aren't supposed to read them after the time but since you are close contact then theres really no harm in getting a pcr.

OK I'm a scientist (a biochemist) and have spent several years of my life developing these kind of tests so an expert. As PPs have said you do sometimes get a shadow if the test is left to develop for longer than the recommended incubation time. This can be due to non-specific binding and will vary from test to test. Of course there is the possibility that your DS is positive but has such low levels of virus that it is below the limit of detection of the assay but the longer incubation gave a positive result.

So the question is what to do next? Well you are taking daily LFTs so keep doing that, his test might be positive today. If it's not throw away the used tests after the 30 mins and don't look at them again and worry yourself. If it's positive then get a PCR to confirm. Your behaviour isn't going to change, you are isolating already so try not to worry, for most people it's a mild illness.

@JaninaDuszejko

OK I'm a scientist (a biochemist) and have spent several years of my life developing these kind of tests so an expert. As PPs have said you do sometimes get a shadow if the test is left to develop for longer than the recommended incubation time. This can be due to non-specific binding and will vary from test to test. Of course there is the possibility that your DS is positive but has such low levels of virus that it is below the limit of detection of the assay but the longer incubation gave a positive result.

So the question is what to do next? Well you are taking daily LFTs so keep doing that, his test might be positive today. If it's not throw away the used tests after the 30 mins and don't look at them again and worry yourself. If it's positive then get a PCR to confirm. Your behaviour isn't going to change, you are isolating already so try not to worry, for most people it's a mild illness.

Thanks, this is a really interesting reply. We've taken another LFT and all seem fine. They are all now going in the bin after 30 minutes! PCR results pending.

I posted an identical thread a few days ago. Are they the new LFTs with the nasal swabs only?

@pinkynonk

I posted an identical thread a few days ago. Are they the new LFTs with the nasal swabs only?

Yes, it was!