With NC’s COVID-19 testing levels so low, how reliable is the positivity test rate?

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RALEIGH, N.C. (WNCN) — One of the key COVID-19 numbers the state wants to be stable before further restrictions are lifted June 1 might be less valuable now than it was a few months ago.

The test positivity rate — the percentage of tests found to be positive — hinges on how frequently people are being tested for the virus.

But with testing levels consistently near multi-month lows, the percent positive “may not be as reliable a number of the number of cases that are really occurring,” said Dr. David Wohl, an infectious disease specialist at the University of North Carolina’s School of Medicine.

North Carolina Department of Health and Human Services Secretary Dr. Mandy Cohen took note of the testing drop during the agency’s last coronavirus task force briefing April 28.

“We are seeing some of our lowest levels of COVID testing,” Cohen said. “And that context is important when interpreting this metric.”

The state averaged fewer than 33,000 tests per day during the final week of April — testing numbers from more recent days tend to fluctuate. They were roughly at that level throughout the month, and were roughly half of what they were at the peak in mid-January.

And an even smaller number of tests go into the percent positive calculation. State health officials say they only use the subset of PCR tests that are reported electronically and can be matched up with the specific date they were taken.

Those low testing numbers can lead to volatility — the percent positive jumped by a full percentage point either way 11 times during the past month.

“I would take some of the testing data with a little bit of grain of salt,” Wohl said.

At a more granular level, 21 counties — more than one in five — didn’t conduct enough tests for DHHS to even have a countywide average percent positive listed on its dashboard.

Source: NCDHHS data dashboard.

The state says counties must average 50 submittable tests per day over two weeks for that number to be reliable enough to be posted. The counties without those figures are located either in the western mountains or along the coast.

A high percent positive tends to indicate not enough tests are being conducted because usually only people who feel sick show up for tests.

“We still need to work on increased testing,” said Dr. Pia MacDonald, an epidemiologist at RTI International. “We need to get to a place where that number is at least below 5 percent.”

The number is worth tracking — “I still think it’s one of the very important metrics to follow,” MacDonald said — but it’s important to know its limitations.

“We should be looking at testing data. I think that gives us a sense of the canary in the coal mine, just how much COVID is out there,” Wohl said. “These are people who are generally coming in with symptoms. And the proportion of them that are positive — even if the numbers are down — does tell us something about how much COVID is being circulating and where. So I think it’s important that people recognize that if you still have symptoms of COVID-19, despite being vaccinated, it does make sense to get tested.”

Of course, the vaccine push has played a part in the decline in testing. But as the state and nation move closer to the end of the pandemic, Wohl says there will be room in the collective toolbox for both.

Vaccination “is a really powerful tool. That’s why we talk a lot about it. But it’s not the only tool,” Wohl said.

“And we really do need to still concentrate on the fundamentals. And the fundamentals are identifying cases, do testing, and then quarantining those contacts, isolating those people who are positive, quarantining their contacts and doing just good public health measures.

“So it’s a combination: The vaccination effort is key, but it’s not the magic wand alone,” Wohl added. “We need these other elements to be working in conjunction with vaccines, so that we could really make a difference.”

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