At-home antigen tests are booming — but are they reliable? Here’s the case for and against them

Researchers have long known that rapid [COVID-19] antigen tests, although convenient, sacrifice some accuracy for their art. Compared with PCR-based laboratory tests, they’re not very good at rooting out the coronavirus when it’s present in low amounts. Whereas PCR testing involves repeatedly xeroxing SARS-CoV-2’s genetic material, so that it can be detected even when it’s exceedingly scarce, antigen tests just scan for what’s already floating around—a coarser kind of survey.

[UCLA clinical microbiologist Omai] Garner says the tests will work best for people who already feel ill. “If you’ve got a kid and they come home with a fever, that’s a good way to use it,” he told me. Trials run by test makers show that when antigen tests are taken in the first few days after someone’s symptoms start, their results can match up with those of PCR tests more than 80 percent of the time, though data collected by independent research groups have often produced slightly less stellar results. When antigen tests are used in people who feel healthy, however, their performance tends to take a nosedive.

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