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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 result is not necessarily the final word.
Mahmoud Illean/AP
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As the COVID-19 pandemic enters its fourth year, a negative result on a small plastic home test seems a little less reassuring than before.
Still, you dutifully clear your nostrils before dinners, wait 15 minutes for everything to clear, then text the host "negative!" before leaving your KN95 mask at home.
Sounds like the right thing to do, right?
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 caution against putting too much faith in a negative result.
So it's appropriate to do a reality check on what these rapid COVID-19 tests, also called antigen tests, can do — and what they can't.
Is the latest omicron variant getting in the way of home testing?
For the most part, the answer is no.
That's because as the virus evolves, scientists are mostly looking at changes in its spike protein, which is what 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 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 Disease Society of America.
He says that this interior protein really hasn't 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 that changes. The Food and Drug Administration is working with the National Institutes of Health to study how well home tests work as the virus continues to evolve.
So far, agencies have identified only one test – Luminostics Inc. Clip COVID Rapid Antigen Test – which has become less reliable in the face of new variants. And yet, the FDA says "the impact does not appear to be significant."
Are antigen tests taking longer to show a positive?
Some people report having negative antigen test results for days despite having 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 of a mystery, says Colgrove. He acknowledges that doctors are seeing this, but so far, it's only been 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 appear to be taking longer to register a positive result, such as the virus multiplying faster somewhere beyond the nostrils in some patients, says Dr. Geoffrey Baird, chairman 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, those who do these tests at home make mistakes and are not trained like those who do COVID-19 tests in the laboratory.
"Some people will stick it in their mouth," he says, explaining that not everyone follows the test instructions as written. Some people even get mucus on the cotton swab, mistakenly thinking that the mucus contains a lot of viruses. "Actually, you don't want snot on the thing."
And while, on average, people get a positive antigen test result around the time they become infectious, 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 much sooner than than most and those who test positive much later.
How well do these tests actually work?
Antigen tests can be useful in certain situations (more on that in a minute), but Baird emphasizes that they have their limits. This was true even before the pandemic.
"There have been similar technologies for influenza for years and the recommendation was not to use them," he says.
Antigen tests look for specific proteins within the virus. Users typically swab their nostrils and the tests take about 15 minutes to give a positive or negative result. But these home tests need a lot more virus to generate a positive result than a PCR test, which is done in a lab and involves letting trace amounts of viral genetic material "amplify" over time — usually a day or so. So even if there is very little virus 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 advantages and disadvantages. And there are two measures of test performance to be aware of: specificity and sensitivity.
Specificity is how good the test is at avoiding false positives. And sensitivity is the quality of the test in finding the virus.
According to the CDC, antigen and PCR tests are good at preventing false positives, but PCR tests are generally more sensitive than home tests. This means that antigen tests are not as useful in deciding Outside COVID-19, but they can be valuable in confirming that the cold really It is COVID-19.
If you don't have any symptoms, don't count on antigen tests to give you a definitive answer as to whether or not you are free. That's also what the researchers found when they analyzed more than 100 antigen testing studies and published their findings in the Cochrane Database of Systematic Reviews last July.
"Rapid antigen tests are considerably less accurate when used in people without signs or symptoms of infection, but they perform better in people who have been in contact with someone with confirmed COVID-19," they wrote.
The same researchers also found that not all home tests were equally accurate. His 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 different studies that evaluated the same tests," said lead author Jacqueline Dinnes of the University of Birmingham in a podcast about the report.
So what are these tests for?
While it seems like a good idea to have everyone take a rapid COVID-19 test the day of a meeting to make sure they're negative, experts say that's not how the tests should 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 get tested and it's positive, you're lost. You've got your diagnosis."
It's a bit of a different story if you're getting over COVID-19 and you're 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 have another antigen test 48 hours later to see if it comes 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 your limits and follow the instructions to retake the test when you get a negative result.
"In a person who has had suggestive symptoms now, in the midst of the epidemic, where the prevalence of 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 you test negative, it's a good idea to be cautious and stay home.
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