Monday, January 10, 2022

The great throat swab contoversy

I finally managed to obtain a couple of rapid at-home COVID test kits, after weeks of phone calls and failed attempts to score them at mass distribution outlets. 

We are being told not to go for PCR tests if we suspect we may have the virus, but to use rapid tests instead. But the damned things are almost impossible to find. I even ordered some online, only to be told a week later by an embarrassed vendor that "the government has taken all our capacity" and "our stock has been repurposed to the government". (I am hoping for my money back.)

Eventually, though, my persistence paid off, and a local pharmacy had some stocks of a rather dodgy looking self-administered rapid testing kit (or at least the various separate elements of a kit, with no instructions), sold to me in a brown paper bag. I jumped at it, regardless.

Now, I happened to know that there was some controversy about whether at-home rapid tests should involve a throat swab as well as nasal swabs. The only other rapid tests I had done were English ones, which specifically required a throat swab as well as nasal swabs, so it seemed like a reasonable question. I was told by the pharmacy, in no uncertain terms, that it should be nasal only, and that I should not even dream of swabbing my throat, what was I thinking?

So, what is the big controversy about rapid tests and throat swabs. Well, nasal-only rapid tests are notoriously prone to false negatives, particularly in this era of the Omicron variant, and there is lots of anecdotal evidence online about nasal swabs yielding negative results and a throat swab yielding a positive result. Plus, at least one South African study (unfortunately in pre-print only, and therefore not peer-reviewed) has concluded that throat swabs are much more accurate than nasal swabs, particularly as regards Omicron.

The official Canadian line, though, is that the rapid tests we have available have only been tested on the basis of nasal swabs. Throat swabs may indeed be better, but the test kits have not been tested for that location, and so caution dictates that we only use nasal swabs. The risk is that throat swabs may lead to false positive results (maybe). However, we can be pretty certain that the nasal swabs we are being told to use are yielding many false negatives (maybe of the order of 15%, or maybe more)

So, which is worse? I would have thought the false negatives were more dangerous, but I'm no scientist. Further study is, as they say, needed. But can it happen soon? - this is becoming reasonably critical.

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