Friday - July 31, 2020

Antigen test results wobbly everywhere?

Sorry, last week’s news. And I really wish I could find something else to blog about. (hey we went bowling again last night. Our team did pretty well, I threw a 658 series (no tap league :-( ) but the other team slaughtered us anyway. So after 2 weeks we’re at the bottom of the heap. Oh well.)

Vermont Seeing Antigen Test Irregularities

65 in Manchester tested positive with Quidel’s antigen test, but then 48 of them tested negative with the standard PCR test. Which one is right? Was Quidel given an EUA too early?

65 people who tested positive last week after completing the rapid antigen test offered at a private medical clinic in the heart of Manchester, Vt., a town of fewer than 5,000. News of an outbreak set off alarms in a state that to-date had fared well in the pandemic, registering the second-fewest cases in the country. Many local restaurants and retailers quickly shut down. The Saturday farmers market was canceled. State officials with little information scrambled to answer questions from reporters and the public as to how and where the outbreak began.

But in Vermont, positive antigen tests — a relatively new way of detecting an active COVID infection — require confirmation by another test: a polymerase chain reaction, or PCR, test, the gold-standard used by the government, hospitals, and professional sports teams. Of the 65 people who’d received those positive antigen tests, 48 would ultimately test negative via PCR.

A community on edge was now a community confused. Had there been an outbreak? The state’s health department said no, while the clinic stood by its findings that showed positive test results. The Manchester town Facebook forum erupted with questions about antigen testing, authorized in May by the FDA for emergency use. What to make of this apparent testing stalemate?
Attempts at answering that question proved to be a lesson in the current fractured state of testing in the United States as well as a reminder of how little is still known six months into the pandemic about the coronavirus and the methods used to mitigate its spread.
Antigen testing, which also requires a nasal swab but can return results in just 15 minutes, has been heralded as a cheap and speedy alternative to the cumbersome and pricey PCR tests, which in the United States can take anywhere from a day to two weeks to process. Rather than lugging vials off to a lab where they’re heated, cooled, and spun, antigen test cassettes are inserted into a small on-site machine that quickly reads the results.
But although the antigen test was recently authorized by the Food and Drug Administration, it is not formally approved, meaning only a handful of studies — submitted by the very companies that markets the product — have been completed to verify its reliability.

I have read elsewhere that, while the antigen test may have an unacceptable false positive rate, it has an almost zero false negative rate. Combined with the high speed results, this may make it very useful for surgical patients, all of whom now have to be tested and cleared prior to surgery. This precaution is making for extremely difficult scheduling, and the 10-14 days results time for the PCR test is forcing most of these people to self-isolate. Now add in the requirement that all surgical patients also have to have a physical from their PCP within a month of the surgical date, which is often scheduled at least 6 or 8 weeks ahead of time.

India has been using RAT (rapid antigen testing) quite a bit, and has had problems. It’s a great idea, but it looks like it needs more development. Like anything else in life, you can’t have fast, good, and cheap, but with this, when you’re looking at hundreds of millions of daily tests, 2 out of 3 isn’t good enough.

https://www.sciencemag.org/news/2020/05/coronavirus-antigen-tests-quick-and-cheap-too-often-wrong
https://www.medicalnewstoday.com/articles/covid-19-antigen-test-could-give-quick-results-but-is-not-foolproof

Posted by Drew458     on 07/31/2020 at 03:49 PM   
Filed Under: • pandemic and epidemic diseases •  
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