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 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.”
Now that fully at-home rapid tests are trickling into the market, Americans may need to confront these testing conundrums regularly. Two of the three home tests cleared by the F.D.A. to date are antigen tests, which hunt for pieces of coronavirus proteins, or antigens. Antigen tests tend to be faster, but are worse than molecular tests — which look for the coronavirus’s genetic material, or RNA, and are often processed in a lab with a technique called polymerase chain reaction, or P.C.R. — at identifying cases, especially when the virus is present at relatively low levels.
Wherever you get a test result, think about how you’ve been feeling and where you’ve been. If there’s reason to think your test should detect the virus, such as symptoms, a recent exposure or a current outbreak in your community, a positive result is probably correct. That probability increases if the test you’re using has a reputation of being very accurate, such as a laboratory test.
A surprise positive shouldn’t be dismissed, though, especially as the number of coronavirus cases continues to balloon — increasing the pretest probability for millions of people nationwide.
If you’ve truly been cloistered away, and you’re not feeling sick, a negative is more likely a negative. Still, no single test result should clear a person’s path to travel, mingle unmasked or shirk other measures like physical distancing, said Dr. Ashish Jha, the dean of the Brown University School of Public Health.
Tests are there to spot the virus after it’s already taken hold, and can’t by themselves thwart its ability to spread, said Hannah Getachew-Smith, a health communication expert at Northwestern University. While testing is powerful, she said, “it has to be coupled with other mitigation strategies.”
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