AILSA CHANG, HOST:
The federal government's website offering free COVID-19 rapid tests officially launched today. Each home is allowed to order four test kits, and millions have already visited the site. Now, rapid tests have not been easy to find in recent months, just as schools and businesses are relying more on them. Still, there have been questions about the accuracy of these at-home antigen tests, especially when it comes to detecting the omicron variant.
Well, we have asked NPR's health correspondent Maria Godoy to help answer some of these lingering questions, and she joins us now. Hey, Maria.
MARIA GODOY, BYLINE: Hey, Ailsa.
CHANG: So what does the latest research say about the accuracy of these tests?
GODOY: Well, you know, there's been several studies out recently looking at this, both in the lab and in the real world. And the bottom line is this - rapid antigen tests are a very useful tool, but they aren't perfect. No test is perfect.
I talked with Dr. Wilbur Lam. He's a physician and biomedical engineer at Emory University. And he's one of the lead investigators assessing COVID-19 tests for the government. He says in the lab, they found that some rapid antigen tests do seem to be less sensitive with omicron. So in other words, they didn't detect the virus at lower concentrations where they would have expected to catch it with other variants. But in the clinical setting - the real-world setting - he says, these tests, they work.
WILBUR LAM: When patients come in and they have symptoms, we test them against the gold standard PCR test, and then we test with the rapid tests. And by and large with omicron, we see that they're performing almost as good as the other variants.
CHANG: Wait, wait, almost as good - what does Dr. Lam mean by almost here?
GODOY: Well, you know, there are still questions that researchers need to figure out. For example, there have been lots of reports of people who have symptoms. They take a rapid test, and they come up negative, sometimes for a couple of days. And then eventually, they get a positive test. And that's puzzling because you'd think if you have symptoms, that should mean you have enough virus to show up on a test, so the test shouldn't be negative. So Lam and his colleagues are trying to figure out what's going on.
CHANG: Well, what do they think might be happening?
GODOY: So this is all still theoretical, but it could be, for example, that if a person is vaccinated, their immune system is mounting a response that's keeping the viral load too low for the rapid antigen test to pick up. It's also possible that omicron might be showing up in different parts of the body first. So while the rapid tests require a nose swab, it could be that the virus is more heavily concentrated in your throat. Or maybe some subvariants of omicron produce fewer antigens, and that would make the tests less sensitive.
Lam says these are all questions that they're exploring. But the bottom line is if you have symptoms and you test positive, you should absolutely consider yourself positive. That's when these tests are most accurate.
CHANG: OK, but what about if I don't have symptoms and I test negative? How much can I trust that negative?
GODOY: Yeah. So David Walt is a medical diagnostics expert at Harvard. And he says if you have a reasonably high chance of being infected because you've got symptoms or you've taken a long flight or been in a big crowd or been expected to - exposed to someone with COVID, and you take, you know, a test and test negative at first, then you should test again two days later.
DAVID WALT: Because there's a good chance, by that time, that the virus will have replicated, and your viral load will then be high enough to give you a positive result.
GODOY: But if you are negative on two tests and you take them a day or two apart and you have no symptoms and no known exposures, then it's likely you really are negative.
CHANG: That is NPR health correspondent Maria Godoy. Thank you, Maria.
GODOY: My pleasure.
(SOUNDBITE OF ALBERT HAMMOND JR.'S "SPOOKY COUCH") Transcript provided by NPR, Copyright NPR.
