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CDC's Approach To Collecting Testing Data May Muddy The Picture Of Current Infections


How much the coronavirus is circulating in the country is a vital question right now. Diagnostic tests can tell us how many people are actively infected with the virus. And now the Centers for Disease Control and Prevention have a national dashboard to collect testing data from all over the U.S. But it turns out that dashboard is mixing together different kinds of data, and public health experts warn that that can muddy the picture.

Here to explain is NPR's Selena Simmons-Duffin. Hey, Selena.


CHANG: So first, what does that even mean, that the CDC is combining different kinds of tests into this dashboard?

SIMMONS-DUFFIN: Right. So the CDC is mushing together diagnostic tests with serology, or antibody tests. So if you're trying to understand how much virus is out there in your community, tracking the number of diagnostic tests done in a place gives us a lot of information. There's just the raw number. Are we completing enough tests? And that has been a big issue since widespread testing was so slow to get going in the U.S.

And then there's the information from how many come back positive, which is the positivity rate. So that's one metric communities look at to figure out how much virus is circulating and whether enough tests are being conducted. So then, antibody tests tell us something totally different - how many people in your community were once infected and now have antibodies? If you add antibody tests to your diagnostic tests, then those metrics - the positivity rate and the number of tests completed - are much less accurate and potentially misleading.

CHANG: OK. So how did all of this come to light, that the CDC was mixing these things together?

SIMMONS-DUFFIN: The CDC launched its national testing dashboard about two weeks ago. And a team running the COVID tracking project at The Atlantic did an analysis this week, which found that the CDC's testing numbers for some states were really different than the state's own numbers. So reporter Daniel Rivero at NPR member station WLRN in Miami asked the CDC about the discrepancy in Florida. And the CDC told him that the higher number that it found included Florida's antibody tests. And a CDC spokeswoman confirmed to NPR that this was true for Florida and several other states and explained it was because, quote, "some states are including serology data in their testing numbers," but added that the agency plans to separate those numbers out again in the coming weeks.

CHANG: So what's been the reaction to this development in the public health world? How are people looking at this mixing of data that the CDC has been doing?

SIMMONS-DUFFIN: Well, epidemiologists and other public health experts have unequivocally said that it does not make sense to lump these two kinds of tests together. And the risk is that it might look like you're doing more diagnostic testing than you really are. And the positivity rate might look lower than it really is. And these numbers really matter. Policymakers and even members of the public are using them to make decisions about how safe it is to do certain things. So the integrity of the data and having the public trust that it's accurate and clear is incredibly important.

CHANG: That is NPR's Selena Simmons-Duffin. Thank you, Selena.


(SOUNDBITE OF POOLSIDE SONG, "NEXT TO YOU") Transcript provided by NPR, Copyright NPR.

Selena Simmons-Duffin reports on health policy for NPR.