This year, as they prepare to let turkeys brine and pie crusts thaw, people across the country are waiting for something extra: a coronavirus test they hope can clear them to mingle with loved ones.
Because a positive test filters out people who should definitely not be out with others, many people consider a negative coronavirus test to be a ticket to freely socialize without precautions. But scientists and doctors say this is dangerously misguided.
The main reason is that a test gives information about the level of the virus at the time of the test. A person could be infected but not have enough virus for it to register yet. Or, a person may become infected in the hours or days after taking a test. Also, the tests do not have 100 percent accuracy.
“If you require all of your guests to email you a negative test result before your Thanksgiving dinner, it will definitely decrease the risk of an outbreak — but not completely,” said Dr. KJ Seung, chief of strategy and policy for the Covid response at Partners in Health. Yet this is a common misperception contact tracers hear when talking to people, he said.
Laboratory tests that rely on a technique called polymerase chain reaction, or P.C.R., can detect the virus when it’s present even at very low levels. But it might take a couple of days to return results, leaving time for someone to be exposed.
Antigen tests are faster, less expensive and more convenient — they can deliver results in a matter of minutes — but are also more prone to missing the virus when it’s scarce. And that could give someone a false sense of security en route to Thanksgiving dinner, said Paige Larkin, a clinical microbiologist at NorthShore University HealthSystem in Chicago, where she specializes in infectious disease diagnostics.
“A negative result is a snapshot in time,” Dr. Larkin said. “It’s telling you that, at that exact second you are tested, the virus was not detected. It does not mean you’re not infected.”
In summary, as Dr. Esther Choo, an emergency medicine physician and a professor at Oregon Health and Science University, put it: “Testing negative basically changes nothing about behavior. It still means wear a mask, distance, avoid indoors if you can.”