Archive for January 4th, 2022

So, do rapid tests work … or not?

January 4, 2022

With covid prevalence spiking, as usual, CDC guidance muddies the water.
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A couple of weeks ago, with omicron spreading like wildfire and workforces getting depleted by quarantined workers, Biden finally left his bunker and promised January home-delivery of 500 million hard-to-find antigen rapid tests.

Current reporting is that orders are “in the process” (i..e. they have not yet been officially placed) for 250 million in the last half of January and 250 million in February and March.

That works out to about 1 test per month for every adult.

Biden assured that the testing surge would be another game-changer (akin to getting LA ports to stay open nights & weekends to unclog supply chains) … and that he was, of course, “following the science” …

So, it seems reasonable to conclude that his homeboy scientists advised him that the antigen rapid tests worked.

That is, except for CDC Director Wolensky who told CNN that the CDC doesn’t “actually know how well rapid tests perform and how well they predict transmissible presence of the virus”.

So, do the rapid tests work … or not?

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On that question, Epidemiologist Michael Mina published the most informative piece that I’ve found…

Dr. Mina’s overall conclusion:

Antigen tests are extremely sensitive for very contagious people.

When taken at peak viral load, these tests approach 100% sensitivity.

Here’s his visual recap … click it to enlarge it.

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My takeaways:

> In the first couple of days after getting infected, neither the PCR nor the antigen tests are sensitive enough to detect the infection.

> Around the 5th day after getting infected, the tests are able to detect the virus.

> PCR tests may be able to detect the virus a day or two sooner than the antigen test … but, since the PCR tests require a day or two for processing & reporting, antigen and PCR tests are practically equivalent for early detection.

> Infectees have the highest viral load (and are most contagious) from day 6 to day 12 after getting infected.

> During days 6 to 12, when infectees are most contagious, both PCR and antigen tests are reliably able to detect the virus.

> After day 12, as the level of viral load quickly diminishes, PCR tests are able to detect the residual, non-contagious levels of the virus … but, the less sensitive antigen tests do not.

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Again, according to Dr. Mina’s research and analysis:

When taken at peak viral load (when infectees are most contagious), antigen tests approach 100% sensitivity.

Accordingly, Dr. Ashish Jha, the dean of Brown’s school of public health, calls antigen tests “contagiousness tests” … and says that they are very good at detecting people who are still infectious to others.

They won’t detect every speck of virus that their PCR counterparts are attuned to do, but they can detect the important part — if someone is producing enough of the virus that they’re likely to spread it.

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Bottom line

If I could get my hands on in-home rapid tests, I would confidently use them…

(1) When I’m exhibiting any symptoms (e.g. fever or sniffles)

(2) After the rare occasions when I’ve attended an indoor gathering with non-family members.

(3) Before visiting my grandkids … for re-assurance that  I’m not contagious.

But, first step is getting my hands on some of the in-home tests!

And, as I like remind readers…

DISCLAIMER: I’m not a medical professional or scientist — just a curious, self-interested guy.  So, don’t take anything that I say or write as medical advice. Get that from your doctor!


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