We’ve been slow to the discussion and use of rapid tests.
In other areas this served us well; we benefitted from others’ experience. On rapid tests, though, our tardiness has just sown confusion. The discussion only got underway when PCR tests were restricted due to Omicron’s spread. It still feels overly theoretical.
In places where they’ve long been in use, like the U.K., where, someone said, they basically throw free boxes of tests at you in pharmacies, their point has been assimilated through experience. They don’t waste time on debates.
In Thursday’s Star, David Juncker and Don Vinh from McGill make a valuable contribution on the difference between PCRs and rapid tests. The two aren’t competitive — they supplement each other. Essentially, PCRs detect whether you have COVID with high certainty. That’s because they’re “sensitive.” But they won’t tell you if you’re too infectious to go out or not, and they take a a while. Rapid tests show if you’re infectious then and there.
So if you’re about to go to work or class, you can take one first and know if you’re infectious. If it’s negative, out you go.
It’s true you could, theoretically, become infectious right after taking it, though that’s unlikely. Or you could get a false negative, which is also possible in PCR tests. That’s a good reason for taking rapid tests quite frequently; it diminishes the likelihood of possible errors. But if all goes more or less as expected, take another next time you’re ready to go out.
You can’t do any of that with a PCR test, because you need to book it in advance and wait perhaps a day for the result. Plus PCRs don’t distinguish between having some fairly harmless virus fragments and being infectious. Can rapid tests go wrong? Yes, but so can vaccines. Vaccinated people do die.
For their basic purpose, however — preventing serious illness and death — vaccines are pretty good, much of the time. Same with rapid tests. We are dealing with probabilities and general mitigation; it’s not math, though it is science.
Since we’re unclear on their basic purpose — to give a good-though-not-perfect indication of whether you’re too sick to go out — we don’t ask the right questions, and the discussion flounders. This week even infectious diseases specialist Dr. Isaac Bogoch, who’s been a brick throughout the pandemic, seemed to be reaching for something familiar when he said we don’t know how accurate they ar
That makes them sound less desirable than PCRs. But lesser accuracy may be precisely their virtue, as Zeynep Tufekci — not a doctor but damn smart — noted in the New York Times. Rapid tests, a blunter instrument, only tell if you have enough to pass it on. So you take them when what you’d like to know is whether you should go out and if you’re dangerous to others — or just want to check your status. The problem with raising the issue of accuracy is it misdirects the discussion from the real purpose of rapid tests.
There was a great example of someone trying to puzzle the issue through on CBC News. Paul Hunter, usually a model of emotionless journalism (in a good way) was interviewing Dr. Allison McGeer. Hunter said “So a positive is a positive, a negative is [LONG PAUSE] perhaps negative but you don’t know for sure … so am I right in thinking, in a sense, what the value of the rapid tests are … it kind of mitigates, in a sense, the danger you might present because you might have it, is that right? … But, I don’t know, that’s where it gets complicated … because we’re hearing about it all the time. [BIG SIGH] What does it mean when you get a negative?”
She was if anything less intelligible, since she had to present herself as an expert. He hemmed and hawed and eventually made a bit of progress. It was riveting. You hardly ever see someone actually thinking on TV.
This column was originally published in the Toronto Star.
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