"A positive test is almost always true," Colgrove says. "So in a person with an exposure or a person with suggestive symptoms, if they do a test and it's positive, you're done. You have your diagnosis."

It's a slightly different story if you are getting over COVID-19 and are testing to see whether you're still positive.

But a negative "does not rule out" a COVID-19 infection, according to the Centers for Disease Control and Prevention. If someone tests negative, they're supposed to take another antigen test 48 hours later to see if it turns positive. And if that person has a known COVID exposure or symptoms, the FDA recommends a third test 48 hours after that.