How Confident Can You Be in a Coronavirus Test?

Many coronavirus test results already say “detected” or “not detected” as their default readout, a distinction that several experts call more useful.

“‘Not detected’ really points to the moment,” said Dr. Valerie Fitzhugh, a pathologist at Rutgers University. “It wasn’t detected today. That doesn’t mean I won’t have it tomorrow.”

Some tests, called molecular tests, look for the coronavirus’s genetic material, or RNA. (Whether done by a nose swab or saliva, these tests are generally considered to be reliable, especially if processed in a lab with a technique called polymerase chain reaction, or P.C.R.) Others, called antigen tests, hunt instead for pieces of coronavirus proteins, or antigens. Antigen tests tend to be faster, but are worse than molecular tests at identifying coronavirus cases, especially when the virus is present at relatively low levels, making it possible for someone to be RNA “positive,” but antigen “negative.”

This may occur because the virus is on its way out of the body, or because it has largely failed to gain a foothold. But it’s also possible that antigen tests might not identify the virus on its upswing, in the early days of infection.

Such discordant results have forced the word “negative” to play a multitude of roles: “Negative” for antigen doesn’t always mean “negative” for the virus, confusing and frustrating people who’d hoped to declare themselves infection-free.

Using terms like “positive” and “negative,” which might evoke outdated stereotypes about sexually transmitted infections, could also dissuade people from disclosing their status to others, or following guidelines around isolation, said Hannah Getachew-Smith, a health communication expert at Northwestern University. A detection mind-set, however, might help “detach testing from my life and my person.”

No test is perfect. But the likelihood of a false positive or a false negative can vary drastically depending on the circumstances under which a test was taken. The two most important of these are people’s recent health history, and how much the virus has spread in their community. Scientists call the confluence of these factors pretest probability.


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