Rapid Tests and PCR Tests

It’s odd that, since the pandemic began, I haven’t really had an accurate distinction in my mind regarding the differences between a Rapid vs PCR test. It’s been over two years now, and you’d think that I’d know more about the differences, or why one is preferred over the other.

It seems that the number of people contracting Covid is growing – more specifically, it seems that the number of people we all personally know contracting Covid is growing. And testing is more of an issue, for us to be able to interact with one another.

Previously, the only times Liz and I were testing involved our trip to Scotland. We needed to test before boarding a plane to get there, as well as a test to return home to the States.

But those tests were all Rapid tests. And in my mind, Rapid and PCR tests were identical. If a rapid test was good enough for the airlines, that’s the one I want to be taking.

But it turns out, there are subtle differences between the two tests. As I learned in this article by Rebecca Heilweil.

Well, rapid antigen tests, which look for a specific protein on the Covid-19 virus, remain extremely effective at confirming positive cases. Put simply, if you test positive on a rapid test, you almost certainly have Covid-19. If you test negative, in some cases, you might still test positive on a PCR test, which is much more sensitive because it tests for genetic evidence of the virus.

This was the situation we found ourselves in, when visiting family in Indianapolis: a negative Rapid test but a positive PCR test.

Perhaps more importantly, rapid tests can indicate whether someone is contagious enough to spread the virus to others, which is what many people are most worried about.
Both PCR and rapid tests have high specificity, which means that their positive results are very trustworthy. But while PCR tests tend to have near-perfect sensitivity, rapid antigen tests tend to have a sensitivity around 80 to 90 percent. This means that rapid tests tend to produce more false negatives than PCR tests do.

So the takeaway for me here seems to be:

Rapid Test, positive: Trust this result.
Rapid Test, negative: Be slightly skeptical of this result.
PCR Test, positive: Trust this result.
PCR Test, negative: Trust this result.

The other thing I’ve read is that repeated Rapid tests will help reduce the problems of false negatives.

Contracting Covid now feels like it’s an inevitability, as more and more people I know have gotten it. As the Coronavirus becomes more endemic, I imagine we’ll be employing testing more frequently, as a means to ensure that we can safely interact with friends and family.

It’s an interesting shift for me, mentally, thinking about testing in this way. Mostly because Liz and I still have limited out errands, kept to ourselves. We’ve visited family over the holidays, but also took some tests ourselves between visits.

But I still think of testing as a requirement of travel (and airline travel, at that). It’s not yet sunk in for me that testing will/is becoming a step we undertake, prior to visiting others.

[photo via Mika Baumeister]

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