How are rapid COVID tests holding up four years into the pandemic? : Blows

Rapid antigen tests are ubiquitous, but some Americans have learned the hard way that a negative test result isn’t necessarily the last word.

Mahmoud Illéan/AP


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Mahmoud Illéan/AP

Rapid antigen tests are ubiquitous, but some Americans have learned the hard way that a negative test result isn’t necessarily the last word.

Mahmoud Illéan/AP

As the COVID-19 pandemic enters its fourth year, a negative result on a small home plastic test is a little less comforting than before.

Yet you dutifully swab your nostrils before dinners, wait 15 minutes for the green light and then text the host “negative!” before leaving your KN95 mask at home.

Sounds like the right thing to do, doesn’t it?

The virus mutated and then mutated again, with the tests offering at least some sense of control as the Greek letters piled up. But some experts warn against relying too much on a negative result.

So it’s worth doing a reality check on what these rapid COVID-19 tests, also called antigen tests, can do — and what they can’t.

Does the latest omicron variant trigger home testing?

For most, the answer is no.

Indeed, as the virus evolves, scientists primarily see changes in its spike protein, which is what the virus uses to attack and enter healthy cells. But rapid antigen tests don’t actually look for this spike protein.

“[The tests] rely on the detection of the nucleocapsid protein, which is the protein that directly encapsulates viral RNA,” says Dr. Robin Colgrove, professor at Harvard Medical School and chair of the diagnostic committee of the Infectious Diseases Society of America .

He says this interior protein really hasn’t changed much because the virus has mutated over the years. So, at least for now, quick tests can pick it up.

Federal health agencies are monitoring the situation in case this changes. The Food and Drug Administration is working with the National Institutes of Health to study how well home testing is working as the virus continues to evolve.

So far, agencies have identified only one test — the Luminostics Inc. Clip COVID Rapid Antigen Test — that has been made less reliable in the face of newer variants. And even then, the FDA says “the impact does not appear to be significant.”

Do antigen tests take longer to show a positive result?

Some people report having negative antigen test results for days, despite known exposure to COVID-19 and the telltale symptoms. Eventually they test positive, but sometimes it can take up to a week.

The phenomenon is somewhat mysterious, says Colgrove. He acknowledges that doctors see it, but so far it’s only anecdotal.

“What kind of experiment would you have to do to answer that question?” he said, explaining that it would be difficult to study.

Many factors could make home tests appear to take longer to register a positive result, such as the virus multiplying faster elsewhere than in the nostrils in some patients, says Dr. Geoffrey Baird, director of the Department of Medicine laboratory and pathology at the University of Washington School of Medicine.

But Baird says perhaps the biggest factor is human error. After all, the people who do these tests at home make mistakes and are not trained like those who do the COVID-19 tests in the laboratory.

“There will be people putting it in their mouths,” he says, explaining that not everyone follows the test instructions as they are written. Some people even get mucus on the swab, mistakenly thinking the mucus will contain a lot of virus. “Actually, you don’t want snot on the thing.”

And while on average people test positive for antigens the moment they become infectious, Baird says it’s important to remember that there will always be plenty of people on either side of that average. : those who test positive much earlier than most and those who test positive much later.

How well do these tests really work?

Antigen tests can be useful in some situations (more on that in a minute), but Baird points out they have their limitations. This was true even before the pandemic.

“Similar technologies have existed for influenza for years and the recommendation was not to use them,” he says.

Antigen tests look for specific proteins inside the virus. Users usually swab their nostrils and the tests take around 15 minutes to give a positive or negative result. But these home tests require a lot more virus to generate a positive result than a PCR test, which is done in a lab and involves leaving traces of viral genetic material to “grow” over time – usually a day. or two. So even if very little virus is present, there should be enough to trigger a positive result (PCR tests can also remain positive long after someone has cleared the infection).

Both types of tests have their pros and cons. And there are two measures of test performance to be aware of: specificity and sensitivity.

Specificity is the ability of the test to avoid false positives. And sensitivity is the ability of the test to find the virus.

Both antigen and PCR tests are good at avoiding false positives, according to the CDC, but PCR tests are generally more sensitive than home tests. This means that antigen tests are not very helpful in deciding outside COVID-19, but they can be valuable in confirming that the cold is really is COVID-19[FEMALE[FEMININE

If you don’t have any symptoms, don’t rely on antigen testing to give you a definitive answer as to whether you’re clear or not. That’s also what the researchers found when they reviewed more than 100 antigen testing studies and published their results in the Cochrane Database of Systematic Reviews last July.

“Rapid antigen tests are considerably less accurate when used in people with no signs or symptoms of infection, but they work best in people who have been in contact with someone who has confirmed COVID‐19” , they wrote.

The same researchers also found that not all home tests were as accurate. Their exam included 49 different types of tests.

“We found a lot of variation in the sensitivity of different brands of tests and our overall results combine results from different studies that evaluated the same tests,” lead author Jacqueline Dinnes from the University of Birmingham said in a podcast. on the report.

So what are these tests really for?

While it seems like a good idea to ask everyone to take a rapid COVID-19 test on the day of a rally to make sure they’re negative, experts say that’s not how it is. the tests were supposed to be used.

“A positive test is almost always true,” says Colgrove. “So in an exposed person or someone with suggestive symptoms, if they do a test and it’s positive, you’re done. You have your diagnosis.”

It’s a slightly different story if you get over COVID-19 and test to see if you’re still positive.

But a negative “does not rule out” a COVID-19 infection, according to the Centers for Disease Control and Prevention. If someone tests negative, they are supposed to take another antigen test 48 hours later to see if they come back positive. And if that person has known COVID exposure or symptoms, the FDA recommends a third test 48 hours after that.

The best way to use the tests is to know their limitations and follow the instructions for retesting when you get a negative result.

“In someone who had suggestive symptoms now, in the midst of the outbreak where the prevalence of infection is high, a single negative test is not enough to rule out infection,” Colgrove said.

If you have symptoms of COVID-19, even if you test negative, it’s a good idea to be careful and stay home.

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