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

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 Illean/AP


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Mahmoud Illean/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 Illean/AP

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

Still, you dutifully wipe your nostrils before dinner parties, wait 15 minutes for everything to be safe, and then text the host “negative!” before leaving your KN95 mask at home.

It feels like the right thing to do, right?

The virus has mutated and then mutated again, with the tests providing at least some sense of control as the Greek letters pile up. But some experts caution against putting too much faith in a negative result.

So it’s only fitting to do a reality check of what those rapid COVID-19 tests, known as antigen tests, can do — and what they can’t.

Does the latest Omicron variant mix up the tests at home?

For the most part, the answer is no.

That’s because as the virus develops, scientists are primarily seeing changes in the spike protein, which the virus uses to attack and invade healthy cells. But the rapid antigen tests aren’t really looking for that spike protein.

“[The tests] rely on detection of the nucleocapsid protein, the protein that directly encapsulates the viral RNA,” said Dr. Robin Colgrove, a Harvard Medical School professor and chair of the Diagnostics Committee of the Infectious Diseases Society of America.

He says this internal protein hasn’t actually changed much as the virus has mutated over the years. So, at least for now, the rapid tests can detect it.

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

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

Does it take longer for antigen tests to give a positive result?

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

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

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

Many factors can make home tests seem like they take longer to register a positive result, such as the virus multiplying faster elsewhere than the nostrils in some patients, says Dr. Geoffrey Baird, chair of the Department of Laboratory Medicine and Pathology at the University of Washington School of Medicine.

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

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

And while, on average, people get a positive antigen test result around the time they become contagious, 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 sooner than most and those who test positive many times over. later test positive.

How well do these tests really work?

Antigen tests can be useful in certain situations (more on that later), but Baird emphasizes that they have their limits. That was already the case before the pandemic.

“Similar technology has been around for years for flu and the recommendation was not to use it,” he says.

Antigen tests look for specific proteins in the virus. Users usually clear their nostrils and the tests take about 15 minutes to give a positive or negative result. But these home tests take a lot more viruses to generate a positive result than a PCR test, which is done in a lab where traces of viral genetic material are “amplified” over time — usually a day or so. like this. So even if very little virus is present, there should be enough to cause a positive result (PCR tests can also remain positive long after someone has cleared the infection).

Both types of testing have their advantages and disadvantages. And there are two measures of test performance you should know: specificity and sensitivity.

Specificity is how good the test is at avoiding false positives. And sensitivity is how good the test is at finding the virus.

According to the CDC, antigen and PCR tests are both good at avoiding false positives, but PCR tests are generally more sensitive than home tests. That means antigen tests aren’t as helpful for deciding from COVID-19, but they could be valuable in really confirming that cold is COVID-19.

However, if you don’t have any symptoms, don’t count on antigen tests to give you a definitive answer on whether or not you’re clear. This is also what researchers found when they reviewed more than 100 antigen testing studies last July and published their findings in the Cochrane Database of Systematic Reviews.

“Rapid antigen tests are significantly less accurate when used in people with no signs or symptoms of infection, but perform better 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 equally accurate. Their review included 49 different types of tests.

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

So what are these tests actually good for?

While it may seem like a good idea to have everyone take a rapid COVID-19 test the day of a meeting to make sure they come back negative, experts say the tests weren’t meant to be used.

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

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

But a negative doesn’t “rule out” a COVID-19 infection, according to the Centers for Disease Control and Prevention. If someone tests negative, they should take another antigen test 48 hours later to see if it turns positive. And if that person has a known exposure or symptoms of COVID, the FDA recommends a third test 48 hours afterward.

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

“In a person who had suggestive symptoms now, in the middle of the epidemic where the prevalence of the infection is high, a single negative test is not enough to rule out infection,” says Colgrove.

If you have symptoms of COVID-19, even if your test is negative, it’s a good idea to err on the side of caution and just stay home.

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