Positive COVID test: how worried should I be?

August 28, 2021 7:41 AM

Someone who I have been in close contact with for the past week just got a positive COVID test result. They also, confusingly, have two negative test results, both from AFTER the positive result. How concerned should I be?

My in-laws were visiting last week to meet our newborn. Both are fully double vaccinated (as are we), and had negative PCR test results as required before arriving from Canada. They stayed in our house, but did go to the grocery store and the coffee shop once each, but were masked indoors at all times (except in our house).

In order to re-enter Canada, they needed a negative PCR test result within 72 hours of their flight. We booked them a PCR test at a local drugstore 3 days before their departure, which conceivably COULD have come back in time for their flight, but it did not. Knowing this was a possibility, we also booked them a more expensive ($200 each) test at the airport, which they took a few hours before their flight and had results within 90 minutes. The airport test came back negative, they boarded their plane and returned home, all was well….
Until we (finally) heard back from the first (drugstore) test: my mother-in-law’s test was positive (father-in-law’s was negative). My mother-in-law, upon receiving this result, had another test (not sure if PCR or antigen) in her hometown. This, too, was negative.

So, given this positive test in a sea of negatives, how concerned should I be? I am incidentally already scheduled to have a test on Tuesday for unrelated purposes, but should I rebook for a sooner test? I’m already masking indoors, should I instead be isolating? Most pressingly, how concerned do I need to be about exposure for my newborn via my mother-in-law?

Grateful for any insight you can offer an anxious new mom!!

posted by Dorinda to Health & Fitness (4 answers total)

One possibility is that it was a false positive.

But even if it wasn't, the other negative tests are telling you that the viral load they were carrying was small enough to be very much on the borderline of positive and negative test results. I've heard from relatives back home in India that this is actually common among people who are vaccinated who have come into contact with the Delta variant: that because the Delta variant is so infectious, the virus does get into you, but because you're vaccinated, you don't have too much of the virus in you and that leads to wonky almost-positive/borderline test results. As a rule, people who show this effect are asymptomatic.

That's them. What about you and your newborn? I'd suggest you test both yourself and your newborn. Can you get a drugstore test, the kind that gives you results in 15 minutes? It costs like $10 per test and comes in boxes of two tests each; I got my stash of 3 boxes last month from Walmart.
posted by MiraK at 9:27 AM on August 28, 2021 [2 favorites]

It's a rule of medicine that testing people without symptoms produces a disproportionate number of false positives. This sure seems like a classic case of that: lots of test, one single positive on just one of the tests.

(To make things even less concerning, the next most likely explanation is MiraK's point: There was a small number of viruses, rapidly being cleared by their immunized body before they could produce symptoms or spread. PCR tests are insanely sensitive.)

So I'd say not worried at all is the correct level of concern. I've had a false positive for a really serious disease, so I know it's easier said than done, but there really aren't any big warning signs in your story.
posted by mark k at 10:16 AM on August 28, 2021 [1 favorite]

Very likely to be a false positive. Talk to the PCP and see if s/he recommends one more test for peace of mind.
posted by kschang at 3:14 PM on August 28, 2021

It's not something you can use to decide that false positives are more likely than false negatives for a specific test.

Except it totally is? The observation that this is an issue for public health is made up by estimating the error in the individual cases. Broadly speaking if your chance of having Covid is less than the false positive rate then you are a prime candidate for a false positive.

The baseline rate of Covid in the US currently is around 1%. The probability of having it is obviously much lower among people who are masked, asymptomatic, or not living in a hot spot areas. The chance of having it when you've had multiple negative tests drops even more. Barring more info like "known exposure" it's just not likely this is a positive.

The false positive rate — that is, how often the test says you have the virus when you actually do not — should be close to zero. Most false-positive results are thought to be due to lab contamination or other problems with how the lab has performed the test, not limitations of the test itself.

In the real world human error is a problem. My major false positive was caused by a doctor ordering in inappropriate test. Mine was bad but I have a friend who started fucking radiotherapy for cancer because a lab mislabeled biopsy samples.

This isn't the first false positive Covid test I've heard of either. I had friends in a study in the early days and were tested frequently; one got a positive that didn't confirm on follow up.

This is exactly what happened with the OP's relatives--retested confirmed they didn't have it--just with the results coming in out of order it confused it.
posted by mark k at 10:09 PM on August 29, 2021