As omicron spreads, here's the best and most accurate way to use rapid antigen tests

AUDIE CORNISH, HOST:

Throughout the U.S., many have noticed it's pretty hard right now to get your hands on those over-the-counter rapid COVID tests. But the Biden administration is hoping to offer some relief. Today, it announced the government has awarded the first in a series of contracts for home tests with plans of purchasing 500 million in all. Now, if you do manage to get your hands on a test kit, we have NPR health correspondent Maria Godoy with us now to talk about how best to use them.

Hey there, Maria.

MARIA GODOY, BYLINE: Hi, Audie.

CORNISH: We're going to be talking about rapid antigen tests, not PCR tests. Remind us the difference.

GODOY: So PCR tests are done in a lab. They're the most accurate because they can detect even trace amounts of the virus. But it can take days to get results back, especially when there's high demand. The benefit of rapid antigen tests is you can take them at home, and you can get results in minutes. But they are less sensitive.

CORNISH: And there has been concern that they're even less sensitive - right? - in detecting the omicron variant. What more have you learned about that?

GODOY: Well - so there is some preliminary evidence that rapid tests might not be as sensitive at detecting omicron. Government studies are looking at that now, but that doesn't mean they don't work. Here's Bruce Tromberg of the National Institutes of Health. He led recent testing of rapid antigen tests.

BRUCE TROMBERG: We should expect the tests to work, and they do work. But they're not perfect, (laughter) so they're guides. They help us understand how to make some decisions.

GODOY: He and others say the tests are still useful. You just have to realize how to use them properly.

CORNISH: What is the proper way to use the test?

GODOY: You know, I asked that of Dr. Yuka Manabe. She runs the Center For Innovative Diagnostics for infectious diseases at Johns Hopkins. She says because the incubation period with omicron seems to be shorter, if you know you've been exposed to the virus, take a test about three days later. If you take a rapid test too soon, you might get a false negative because there's not enough virus in your body yet. And if you have symptoms, definitely test.

YUKA MANABE: They are the most accurate, and they have the highest sensitivity when you have symptoms.

GODOY: And if you do have symptoms and you're high risk, she says tests sooner rather than later because you need to be within five days of the start of symptoms to be eligible for new prescription COVID treatments that are just becoming available. They can help lessen the severity.

CORNISH: So scenario here - someone is exposed to someone else with COVID maybe three days prior, and you want to get a test. What's the best way to get a good sample?

GODOY: Well - so follow the directions that come with the tests very carefully. Now, some public health experts have suggested recently that in addition to taking a nasal swab, you do a throat swab as well. And that's because there's some indication that omicron might be replicating faster or earlier in the throat or mouth before it's in the nose. So a throat swab might pick up a positive sooner. The experts I spoke with say it's plausible. But the tests weren't designed to be used that way, so the sample from the throat swab might not move along the testing filter paper in the same way. We don't know. But if you do have two tests, you could conceivably take one test from your nose and another using a throat sample.

CORNISH: What does a negative result mean at this point?

GODOY: Well, if you have one negative test, realize there is a chance that you might still be infectious and not showing up positive yet. If you're heading over to have dinner with a vaccinated friend, you're probably still OK. But Anne Wyllie, a microbiologist at Yale, says if the person you're seeing is high risk, it's a different story.

ANNE WYLLIE: With antigen tests, you really need to be cautious with interpreting negative results. So if you've had a high-risk exposure, if you are experiencing symptoms and an antigen test comes back negative, you need to keep testing.

GODOY: And that means you take one rapid test and then wait 24 to 36 hours and take a second test. Research shows rapid tests are highly accurate when taken sequentially like that.

CORNISH: That's NPR health correspondent Maria Godoy.

Thanks so much.

GODOY: My pleasure, Audie. Transcript provided by NPR, Copyright NPR.