How can someone get both negative and positive PCR result 3 hours apart?

September 24, 2021 5:11 AM

My friend, UK-based, took a COVID-19 PCR test yesterday. At 6:30 this morning she got an email to say the test came back negative. At 9:30 she got another email to say the test came back positive. What might be going on?

She's tried calling Track and Trace (119) but they were confused and went away to investigate. They said they'd call her back but haven't done so yet. What I'd really like to know is what might be going on behind the scenes for her to receive an initial negative result and a subsequent positive result. She has only taken the one test recently.

posted by Beverley Westwood to Health & Fitness (12 answers total)

It could be that she is near the detection limit. When you're on the threshold, there's the possibility that the test can go either way. From what they told her, it's impossible to tell if they ran the test twice (unlikely) or called it negative before the PCR test was finished. It definitely is weird.
posted by kathrynm at 5:29 AM on September 24, 2021

It would seem most likely to me that there was some kind of administrative/computer glitch, but if not: the PCR process is about amplifying pieces of DNA or RNA over time, so I guess it could be possible that something got read too soon, before there was a detectable amount. But that sounds odd for a lab set up to do these tests day in and day out.
posted by needs more cowbell at 5:31 AM on September 24, 2021 [1 favorite]

I would guess one or the other calls went to the wrong person.
posted by tchemgrrl at 5:38 AM on September 24, 2021 [15 favorites]

my assumption in this case is always administrative error. either the first or the second email was sent in error.
posted by misanthropicsarah at 6:33 AM on September 24, 2021 [5 favorites]

Did the second result acknowledge the first result in any way? If not, this sounds like an administrative error in sending out the results, not anything that happened as part of the testing process.
posted by mekily at 6:33 AM on September 24, 2021 [1 favorite]

I guess it could be possible that something got read too soon, before there was a detectable amount

It's a long time since I did any PCR but it would be pretty strange to interrupt the PCR process to measure the sample. Either you ran it for enough cycles before measuring or you didn't.

I would assume the error is in the emailing rather than the testing.
posted by mskyle at 6:34 AM on September 24, 2021 [2 favorites]

Another vote for admin error. Is there a reference number quoted in the emails, and is it the same in both?
posted by ManyLeggedCreature at 6:51 AM on September 24, 2021 [1 favorite]

As others have said above, PCR works by duplicating DNA sequences and it works in a doubling sequence. 1 strand becomes 2 then 4, 8, 16, etc. There's a variable called Ct that notes how many times through the process it takes before you see the signal cross the threshold. Lower Ct means you had a lot more DNA/RNA in your sample than a higher one.

Perhaps the lab runs a quicker test with a lower Ct to look for patients that are a heavy positive but then runs a longer/overnight test on the same sample as a confirmation? Like kathrynm said, being right on the threshold might cause this situation. And the lab doesn't want a false negative to slip through.
posted by mookoz at 7:44 AM on September 24, 2021 [2 favorites]

Was this a home test that she mailed to the processing center or a test that she was given at a clinic? In either case, there should be a tracking number associated with the test - NHS home tests have a sequence number you can register on their site, and there's a reference number for clinic tests. I'm betting on admin error here too - check the emails in detail to see if the reference number is the same in both.
posted by bedhead at 8:04 AM on September 24, 2021

I would ask for another test.
posted by amtho at 8:40 AM on September 24, 2021

Response by poster: She eventually got phoned back by Test and Trace. They explained that the email that said she'd tested positive actually referred to a lateral flow test she took on Wednesday. The PCR test was negative. I've seen the "you're positive!" email and it, rather unhelpfully, doesn't say it's for an LFT.

So panic over. Thanks for your help, everyone, sorry it was nothing in the end.
posted by Beverley Westwood at 8:45 AM on September 24, 2021 [10 favorites]

That's great news! Good to hear it got resolved the best possible way.
posted by M. at 1:24 PM on September 24, 2021