Why A (Single) Negative Rapid Antigen Test Result Is Not Necessarily A Sure Thing.

TwoLogsWhenTen

AreALotVille


There is a whole lot of testing going on these days.

And the gold standard for SARS-CoV-2 sensitivity is still the polymerase chain reaction (PCR) test.

An alternative is the rapid antigen test (RAT) which can also detect viral load, but only at the top half of the curve.

The graph below, from the Ontario Science Table, a group that has been doing a great job of disseminating critical information and making important calls throughout the the entire pandemic, shows the difference in sensitivity between the PCR and RAT tests:

Please note that the Y-axis here is a log10 scale which means that there is a 100 fold difference between the PCR sensitvity threshold of 3 (1,000 viral copies per millilitre) and the Rapid Antigen Test sensitivity threshold of 5 (100,000 viral copies per millilitre).

So.

What does this mean, practically?

Well, if you are worried about exposure before you go to visit an at-risk loved one this holiday season and you test negative just once with the RAT it is possible that you could be infected but under the threshold of detection for that test (and before you become infectious and/or symptomatic).

Thus, if you are using the RAT test alone as a means of knowing if you are good to go it should be used just before you visit that loved one so that know, for sure, over a short period (i.e. a few hours), that you are below the level where you are infectious. 

And if you want to be really sure about your status you should use the RAT sequentially over 2 to 3 days which will ensure that, if you are infected, you will get over the sensitivity bar and the test will find it.

Of course, the thing about the latter regimen is that, here in Lotusland at least, the only way to do that is to order a batch of them yourself online (and then wait by the front door for them to arrive which is what we're doing in our house right now).

The following video and Twittmachine thread by science communicator and neuroscientist Samantha Yammine, with an assist from Saskatchewan-based virologist (and Seahawks fan) Angie Rasmussen, is really good on all this stuff... 

 

I was very lucky to get to chat with virologist @angie_rasmussen about rapid antigen tests.

This short video (also on IG here: https://t.co/0I222KjHHR) shares 2 key things we want you to know before using them over the holidays ⤵️
[& 🧵with more info] pic.twitter.com/UEwTsfJH05

— 🔬Samantha Yammine, PhD (@heysciencesam) December 17, 2021

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We want to go see Grandpops, who has had his booster, over on the Island this holiday season...As you might imagine, I''m treating it like an 'overseas' trip, where I am the pain-in-the-behind immigration officer....Thus, we have also shelled out for travel clinic PCR tests...

Just a note about the graph.... It only shows the OG wild type and Delta strains...Now, it is entirely possible, given it's ability to rapidly infect  cells lining the upper portion of the respiratory tract, that the Omicron strain may show a more vertical ramp-up slope...If that does turn out to be the case it could bring the difference in the time to sensitivity between the PCR to RAT closer together.

Of course, an important thing to remember here is that vaccination that, regardless infection status, is still doing a really great job of keeping a large number of folks out of our hospitals.

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