Dec 16, 2022 - Health

Changing the way we think about rapid tests

Illustration of a covid rapid test with a change symbol in the results panel

Illustration: Sarah Grillo/Axios

​​The COVID-19 rapid tests that public health officials are counting on to help fight a wintertime surge may not give an on-the-spot picture of whether a person is infected. And that could influence the way people approach the holiday season.

Driving the news: The FDA last month urged repeat testing after a negative result to reduce the risk of missing an infection amid concerns rapid tests aren't sensitive enough for early detection.

  • But that guidance may not resonate with people eager to be done with COVID, or those consumed with holiday obligations and navigating a "tripledemic" of respiratory diseases.
  • Even the willing can get confused over how much testing is enough, Michael Mina, an epidemiologist and chief scientific officer for at-home testing company eMed, told Axios. "How does the average person know? That’s why the best advice at this point in time, and really for any viruses, is if you don’t feel well, probably don’t go out.”
  • Questions about some of the antigen tests surfaced this week after the FDA announced the voluntary recall of more than 11,000 tests from Detect Inc. amid concerns they may give false negative results.

By the numbers: Early in the pandemic, before COVID vaccines and multiple variants, rapid antigen tests had a much higher sensitivity, where 95% of results were true positives, Marc Sala, co-director of the Northwestern Medicine Comprehensive COVID Center, told Axios.

  • With omicron in an asymptomatic and vaccinated person, that sensitivity might be closer to 25%.
  • A PCR test, in contrast, can detect the most minute amounts of SARS-COV-2 in a person for an average of 18 days, Bob Wachter, chair of the Department of Medicine at the University of California in San Francisco, told Axios.
  • But PCR testing takes longer to process and can be less accessible, making rapid tests the practical choice when people need to know fast if they are infectious and need to isolate, Mina said.
  • Beyond that, a PCR has never been the best indicator for whether someone is currently contagious, Mina said.

Between the lines: Because rapid tests can't pick up low amounts of the virus, a negative result may be common for people with immunity from prior infections or those who are up-to-date on booster shots.

  • Repeat testing can establish whether the person's immune system is gradually being overwhelmed.
  • The tests remain highly effective at establishing if a person is a superspreader, Mina said. Errors are more often due to people inadvertently testing too early than a faulty test.

Be smart: Understanding the limitations could lead to better decision-making ahead of holiday gatherings, especially with the flu and RSV also looming.

  • The FDA recommends re-testing 48 hours after the first negative antigen test for people who have symptoms.
  • If someone doesn’t have symptoms but has been exposed to the virus, they should test every 48 hours for a total of at least three rapid tests, per the federal agency.
  • Because a fever is usually the most immediate sign a person is sick with COVID, Mina said that can serve as an immediate sign to take precautions.

But, but, but: The "stay home if sick" advice can be difficult to follow when a person is required to work in-person or lacks reliable childcare or paid sick leave.

Our thought bubble: The course of the pandemic may be influenced more by our perceptions of risk and desire to get back to pre-COVID life than by test results.

  • And it's possible the tests will be tweaked as new variants emerge, to ensure the antigen can be detected.
  • "There will always be a role for these tests, but they might have to be updated with the viral proteins and mutations," Sala said.

Bottom line: You're not free and clear just because one test comes back negative.

  • "If you don’t feel well, don’t go hang out with an immunocompromised friend," Mina said. "Don’t go to dinner at a tightly packed bar. That’s a pretty good rule of thumb.

Editor’s note: This piece was updated with detail on Michael Mina’s role at eMed.

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