How Confident Can You Be in a Coronavirus Test?

How Confident Can You Be in a Coronavirus Test?

Things like which kind of test it was, and the reason for taking it, should factor into how much credence to give a positive or negative result.

Swabbing for a coronavirus test in San Diego this week.Credit…Ariana Drehsler for The New York Times
In mid-November, David Piegaro tested positive for the coronavirus. His results came too late.

The night before, Mr. Piegaro, a member of the National Guard, drove to New Jersey to visit his family after receiving two negative rapid test results, two days in a row. By the next morning, he had left. But the single overnight stay was enough to spread the virus Mr. Piegaro was unknowingly carrying to multiple members of his family, including his grandfather, who ended up spending two weeks in the hospital.

“Those two negative results gave me more confidence than I should’ve had that I could see my grandfather,” Mr. Piegaro said.

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How Confident Can You Be in a Coronavirus Test?

Since the start of the pandemic, the Food and Drug Administration has issued emergency green lights to more than 200 types of coronavirus tests, each with its own curiosities and quirks. Yet we tend to talk about all of them in the same binary way, with identical terms: positive, negative, true, false.

But when it comes to interpreting results, not all positives and negatives are equally reliable. Factors like whether you had symptoms, or the number of people in your neighborhood who are infected, can influence how confident you should be in your test results.

“It’s about context,” said Andrea Prinzi, a clinical microbiologist and diagnostics researcher at the University of Colorado Anschutz Graduate School. “Your test doesn’t end when you get your result.”

Why ‘positive’ and ‘negative’ are misleading
Experts say these words imply a deceptive permanence. “Negative” can mislead people into thinking they are safer from the virus than they actually are.

Some people may mistakenly believe that testing negative gives them a free pass to socialize — “I got a negative test and now I can go visit grandma,” said Dr. Ashish Jha, the dean of the Brown University School of Public Health.

But certain types of tests, especially the rapid ones, aren’t reliable at picking up on low levels of the virus and might mislabel infected people as “negative.” And no test can capture a person’s status in the future. People who test negative one morning might be positive by the next, either simply because the test missed the virus, or because they were newly infected.