You can trust the positivity rate even more, thanks to boom in COVID-19 testing

North Carolina news

RALEIGH, N.C. (WNCN) — The percent of COVID-19 tests that come back positive hinges on one key factor: How many tests are actually performed.

And now that the state is reporting its highest testing average since March, that positivity rate is even more valuable now.

“Looking at percent positive, it is really important to also be looking at just how much testing is occurring,” said Dr. Pia MacDonald, an epidemiologist at RTI International.

The short answer: Much more, in large part because of the rapidly spreading delta variant.

According to numbers from the state Department of Health and Human Services, the seven-day average number of tests across the state Monday — 43,680 of them — was the highest since March 1.

Not every tests goes into the percent positive calculation, with state health officials saying they only use the subset of PCR tests reported electronically and can be matched up with the specific date they were taken.

Testing levels dropped so low in the months that followed that one leading infectious disease expert said the percent positive “may not be as reliable a number of the number of cases that are really occurring” and that he was taking “some of the testing data with a little bit of grain of salt.”

The rebound in testing numbers — with blocks-long lines spotted at some testing centers in recent weeks — helped the percent positive regain a measure of its value.

“What we’re seeing right now is testing levels that are almost similar to what we saw in the winter when we were at our highest,” MacDonald said. “And that’s a good place to be in terms of testing.”

That’s the good news. The bad news: The rate of tests found to be positive has consistently been high.

DHHS said Wednesday that 13.2 percent of tests performed Monday came back positive, the 17th consecutive day it has been 10 percent or higher — or, twice as high as the state’s target of 5 percent.

“That helps us understand that the infection rate is much greater now than it was last year,” said Dr. Thomas Denny, the chief operating officer of the Duke Human Vaccine Institute.

And an even closer look at the map shows some trouble spots.

“What we want to see is level testing across the geographic area of North Carolina,” MacDonald said.

There were 18 counties that didn’t conduct enough tests to even have a countywide average percent positive listed on the DHHS dashboard. The agency says the two-week average is left off the site if it didn’t average 50 tests per day during that span. Most of those are in the western mountains or in the state’s northeast corner.

(Source: NCDHHS)

“You would have hoped by this late into the game, you would have testing across the state, without any question,” Denny said.

But of even more concern to MacDonald: The 24 counties with an average percent positive that exceeds 15 percent. 

“Where we see the least amount of testing is where we see the highest percent positive,” she said.

She says that could reflect a lack of access to testing sites, with tests perhaps only available at doctor’s offices and not at easily accessible testing sites.

“Sick people go to their healthcare providers rather than finding places in the community where they can easily go test if they’re even suspicious” of being infected, she said.


CBS 17’s Joedy McCreary has been tracking COVID-19 figures since March 2020, compiling data from federal, state and local sources to deliver a clear snapshot of what the coronavirus situation looks like now and what it could look like in the future.


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