As the COVID-19 pandemic enters its fourth year, a negative result on a small plastic home test seems a little less comforting than it used to be.
Yet, you blow your nostrils before dinner parties, wait 15 minutes for everything to clear up, and then shout “Negative!” to the host. Before leaving your KN95 mask at home.
Seems like the right thing to do, doesn’t it?
the virus has mutated and mutated again, with tests providing at least a sense of control in the form of a stack of Greek letters. But some experts caution against placing too much faith in a negative result.
So it’s worth a reality check on what those rapid COVID-19 tests, also called antigen tests, can do — and what they can’t do.
Is the latest Omicron variant tripping in the at-home tests?
For the most part, the answer is no.
This is because as the virus evolves, scientists are mainly looking for changes in its spike protein, which the virus uses to attack and enter healthy cells. But rapid antigen tests aren’t really looking for that spike protein.
,[The tests] rely on the detection of the nucleocapsid protein, which is the protein directly encapsulating the viral RNA,” says Dr Robin Colgroveis a professor at Harvard Medical School and chairman of the Diagnostics Committee of the Infectious Diseases Society of America.
He says this internal protein hasn’t really changed much as the virus has mutated over the years. So, at least for now, rapid tests can detect it.
Federal health agencies are monitoring the situation in case it changes. The Food and Drug Administration is working with the National Institutes of Health to study how well the at-home tests work 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 forms. And yet, the FDA says “the effect does not appear to be significant.”
Are antigen tests taking longer to show positive?
Some people have reported having been exposed to COVID-19 and showing symptoms, but antigen test results remained negative for several days. Eventually, they test positive, but sometimes it can take up to a week.
This phenomenon is somewhat mysterious, says Colgrove. He acknowledges that doctors are looking into it, but so far, it’s only anecdotal.
“What kind of experiment would you have to do to answer that question?” He says no, explaining that studying will be difficult.
Several factors may make it seem like home tests are taking longer to register a positive result, such as the virus multiplying faster in some patients elsewhere than in the nostrils, say Dr Geoffrey BairdChair 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 doing these tests at home are prone to mistakes and are not as trained as those who are doing COVID-19 tests in a lab.
“There will be some people who will stick it in their mouth,” he says, explaining that not everyone follows test instructions as written. Some people even get mucus on the swab, mistakenly thinking that the mucus will contain a lot of virus. “Really you don’t want to pay attention to this.”
And while, on average, people will get a positive antigen test result around the time they are contagious, Baird says it’s important to remember that there will always be a lot of people on either side of that average: those who test positive first 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 stresses that they have limitations. This was true even before the pandemic.
“Similar techniques 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 typically sniff their nostrils, and the test takes about 15 minutes to produce a positive or negative result. But these home tests require much more virus to produce a positive result than the PCR test, which is performed in a laboratory and involves “amplifying” trace amounts of viral genetic material over time. Usually a day or two. So even if very little virus is present, it should be enough to trigger a positive result (the PCR test can be positive even after someone has cleared the infection).
Both types of tests have their advantages and disadvantages. And there are two measures of test performance to know about: specificity and sensitivity,
Specificity is how well the test is able to avoid false positives. And sensitivity is how good the test is at detecting the virus.
According to the CDCAntigen and PCR tests are both good at avoiding false positives, but PCR tests are generally more sensitive than home tests. This means that antigen tests are not useful to rule out out COVID-19, but they can be valuable for confirming you actually have a cold Is COVID-19.
If you don’t have any symptoms, don’t rely on antigen tests to give you a definitive answer on whether or not you’re in the clear. That’s what came up when researchers took a look at more than 100 studies of antigen tests and published their findings last July in the Cochrane Database of Systematic Reviews.
“Rapid antigen tests are significantly less accurate when they are used in people who have no signs or symptoms of infection, but perform better in people who have been in contact with someone who has had COVID-19 Confirmed,” he wrote.
Those same researchers also found that not all home tests were equally accurate. Their review included 49 different types of tests.
“We found a great deal of variation in the sensitivity of different brands of tests and our overall results combine findings from different studies that evaluated similar tests,” said lead author Jacqueline Dines, from the University of Birmingham. said in a podcast About the report.
So what exactly are these tests good for?
Even though it seems like a good idea to have everyone do a rapid COVID-19 test on the day of a gathering to make sure they’re negative, experts say that’s how the tests should have been used.
“A positive test is almost always true,” Colgrove says. “So in an at-risk person or a person with suggestive symptoms, if they do a test and it’s positive, then you’re done. You have your diagnosis.
this is a slightly different story If you are recovering from COVID-19 and getting tested to see if you are 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, he has to undergo another antigen test after 48 hours to see if he is positive or not. and if that person has Known COVID exposure or symptomsFDA Recommends third test 48 hours after that.
The best way to use the tests is to know their limitations and follow the instructions for re-testing if you get a negative result.
“In the midst of an epidemic where the prevalence of infection is high, in a person who still had suggestive symptoms, a single negative test is not enough to rule out infection,” says Colgrove.
If you have COVID-19 symptoms, even if you test negative, it’s a good idea to be cautious and just stay home.
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