PCR test after a positive infection - what to expect?

December 28, 2021 9:33 AM

This is a question about getting a PCR test for possible Covid re-infection after a recent (November) positive test. Are false positives something to worry about?

First of all - I'm sorry if this has been asked before. I have had a cursory check but am getting pretty worried about this.

More info:
- I'm in the UK
- toddler and me both got Covid at the end of November (no idea what strain if it matters; I'm assuming not Omicron as it was before Omicron was really making a splash here, who really knows though)
- toddler has cold symptoms, we also have a close contact who has tested positive
- I went to ask for a PCR test, but was told that we couldn't get a test within 90 days of a confirmed infection. The staff member checked with their supervisor and then said we could go ahead with the test as toddler has symptoms, but that it might just flag up as positive because of the recent infection.

My question is, is this true? Is there a way of distinguishing between past/present infection? I don't actually know a lot about PCR tests, and I'm struggling to find up to date information about getting re-tested within that 90 day window, bearing in mind what is seemingly a high risk of reinfection with Omicron. Any information about how PCR tests work would be helpful, but I will also accept anecdotes.

posted by the cat's pyjamas to Health & Fitness (6 answers total)

So, PCR is very very sensitive, and you can have a tiny amount of viral particles floating around inside you for a long time post-infection, long after you're over the infection, clinically speaking. That's why they don't like to test you 90 days after recovery, because such a test isn't particularly conclusive - if you test positive it could mean you have a new infection, or it could just be that you still have residual gunk from your earlier infection.

Still, in this case you could potentially get a useful result - if your kiddo has a *negative* PCR test result that's going to be a pretty solid indicator that they do not currently have a COVID infection. If they have a positive PCR result you won't really know for sure what they have.
posted by mskyle at 9:45 AM on December 28, 2021 [1 favorite]

I'm not an expert, but my understanding is that the rapid tests are better at distinguishing a current infection, since PCR tests are so sensitive that they do often detect DNA from old infections. So it might be useful to start with a rapid test.
posted by pinochiette at 9:46 AM on December 28, 2021 [1 favorite]

I posted a similar question, but regarding rapid tests.

From the CDC which doesn't specifically mentions PCR testing but is relevant:
Patients who have recovered from COVID-19 can continue to have detectable SARS-CoV-2 RNA in upper respiratory specimens for up to 3 months after illness onset. However, replication-competent virus has not been reliably recovered and infectiousness is unlikely.

From Cleveland Clinic
How long do you test positive after having had COVID-19?
Because the PCR test is so sensitive, it can detect very small amounts of virus material. This means that the test can continue to detect fragments of SARS-CoV-2 virus even after you’ve recovered from COVID-19 and are no longer contagious. So you may continue to test positive if you've had COVID-19 in the distant past, even though you can’t spread the SARS-CoV-2 virus to others.

Prolonged infection in immunocompromised individuals can occur where they shed infectious virus for months. Also, healthy people can become reinfected. If you test positive for SARS-CoV-2 but you think you might have already recovered from COVID-19, please discuss with a healthcare provider.
posted by joeyjoejoejr at 10:02 AM on December 28, 2021

You may have already seen them, but the NHS guidelines state:

You should not do another PCR test within 90 days of your positive result, unless you:

- have any new symptoms of COVID-19
- get a positive rapid lateral flow test result
- need to get a PCR test when you arrive in the UK from abroad

Since your toddler has cold symptoms, it may be worth getting another PCR. But either way, if you self-isolate from now for 10 days, you should be clear of potential contagiousness either way. If your need to see a positive result is for work reasons, I would contact your work and ask them what they think. Otherwise, if all you need is reassurance that you'd be doing the right thing by isolating with your toddler and assuming you have it: yes, this would be a good idea.

I believe it is possible to be infected by Delta and then get Omicron within a short period, so it is possible that you might have picked up another strain. However it might also be a cold (plenty of those around right now as well!). But whatever happens, being cautious and acting as if you're positive (if you're able to) is unlikely to do any harm in the long run.
posted by fight or flight at 11:23 AM on December 28, 2021 [1 favorite]

The antigen test is probably more reliable in the case of recent and resolved infection. As others have noted, the pcr tests are so sensitive that they can detect lingering, inactive, non-contagious viral remnants.
posted by Doc_Sock at 12:33 PM on December 28, 2021

Response by poster: Thank you everyone for your responses. I had seen the NHS guidelines posted by fight or flight and was going by the book, so got the PCR test. I was just really down at the thought of having to self isolate for (possibly) no reason.

I did speak to a member of the test and trace team who reassured me the system does flag up previous infection and somehow interprets the results in light of this (I would love to know in detail how this works, just out of curiosity!). Anyway I got the results back today - negative, a relief all round.
posted by the cat's pyjamas at 7:35 AM on December 29, 2021 [1 favorite]