Basic math? When it comes to percent positive rate, not exactly

North Carolina news

RALEIGH, N.C. (WNCN) — It’s not as easy as it might seem to figure out how the state calculates one of its four key coronavirus metrics.

The percent positive rate — the percentage of all COVID-19 tests that are determined to be positive — seems pretty straightforward: Divide the number of new cases on a particular day by the number of all tests on that day, then move the decimal point to turn it into a percent.

And while the Department of Health and Human Services has disclosed that figure every day for the past month, it has not released the specific testing numbers it uses to come up with it.

DHHS did not respond to a message from CBS 17 News asking why the counts that factor into that measurement are not released.

The agency does release a daily count of new cases and the number of tests on its data dashboard. But those aren’t always the numbers used to come up with the percent positive rate.

DHHS depends on laboratories to report testing numbers to the electronic disease surveillance system, but some labs don’t always provide the number of negative tests each day. Without those, the percentage is skewed.

So the agency only counts results from labs that report both positive and negative tests for a particular day.

State Epidemiologist Dr. Zack Moore said “a large majority of the labs” provide complete testing results to DHHS, “but it’s not all labs, yet.”

“We’re only including in that percent positive the ones that are already providing the negative and positive results electronically so that we can give out a more accurate proportion,” Moore said. “So when you look at the number of cases and the number of total labs, and see that that on some days (it) doesn’t fit exactly with the percent positive, that’s the reason.”

It has become more of an issue because that statistic is one that has not consistently trended in an encouraging direction in the ongoing public battle against the coronavirus. 

The percent positive calculation is an effective way to determine whether a rise in the number of new cases reflects increased testing (if the percentage is low) or if the virus is spreading further (if the percentage is high).

In two of the past four days, the state has matched an all-time high with 10 percent of all tests coming back positive on those days — including on consecutive days Friday and Saturday.

That number was said to be at 8 percent on Tuesday. But in a fitting example of how the figures don’t always align, the state also publicly reported 676 new cases and 15,598 tests. Dividing those numbers yields just 4.3 percent.

While it’s down from the high, it’s still way above the World Health Organization’s guidelines. 

That group has advised that the rate should be at 5 percent or lower for 14 consecutive days before reopening, but North Carolina is one of 22 states with a percent positive rate that exceeds that recommended benchmark. 

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