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Polymerase chain reaction tests, which have typically been considered the gold standard for detecting the virus, are typically performed in a laboratory and involve making many copies of the virus’s genetic material. That process helps P.C.R. tests to detect even minute traces of the virus.
Rapid antigen tests, which do not amplify the virus, are less sensitive than P.C.R. tests. If you take one during the earliest phase of an infection, before the virus has replicated widely, the test could return a false negative.
Some of the at-home rapid antigen tests have an overall sensitivity of roughly 85 percent, which means that they are catching roughly 85 percent of people who are infected with the virus and missing 15 percent. In some studies, their real-world performance has been even lower.
But the tests are more sensitive in people with symptoms than without and are most sensitive during the first week of symptoms, studies have found.
And antigen tests are excellent at flagging people who have high viral loads — and who are thus most likely to be actively transmitting the virus to others, experts said.
“The more virus you have in your nose, the more virus you’re breathing out into the air, and the more virus other people can then breathe in,” Dr. Gronvall said. “The tests are very accurate, and correlate very well with P.C.R., when people are most infectious.”