What percent positivity can — and can’t — tell us about Virginia’s COVID-19 epidemic

As Gov. Ralph Northam prepares to gradually reopen parts of Virginia on Friday, a new health metric has emerged as a key factor in his decision-making. While the state has seen a growth in total COVID-19 cases as testing increases, Northam said he’s looking more closely at what the state is calling “percent positivity”— the percentage of total tests that return positive.

The metric has only recently surfaced as an important marker in the coronavirus pandemic, driven by recommendations from the World Health Organization and Centers for Disease Control and Prevention. And, as case numbers continue to rise in Virginia, health experts say it’s also important to look at other data points to determine how effectively the state’s distancing guidelines are slowing the spread of disease.

For one, test results are often reported several days after patients develop symptoms, making them less useful in tracking the development of disease in real-time. A rise in case numbers is also the expected result of more tests being performed. That can be a good thing, experts say — indicating that the state is doing a better job of picking up infections.

For Northam, the decision to launch the first phase of his reopening plan was at least partially informed by a 14-day decline in Virginia’s percent positivity rate. While the metric can be a good data point to consider, experts say there are also drawbacks to relying on it too heavily. Here’s what Virginia’s positive-test rate can tell us about the status of coronavirus in the state.

What is percent positivity and what does it measure?

On the surface, percent positivity is fairly straightforward, said Dr. William Petri, an infectious disease specialist at the University of Virginia.

“That’s the easiest part of the equation,” he continued. “It’s just the number of tests that have been performed versus the percent that come back positive.”

How it’s interpreted is a little more complicated. Percent positivity is often used to gauge the amount of testing being conducted in a particular area and who’s being screened. “It’s telling you who’s got the biggest testing game in town,” said Dr. Barry Bloom, a professor and global health expert at the Harvard T.H. Chan School of Public Health.

That’s because as the total number of tests goes up, the percent of positive tests typically goes down relative to the overall number. Both Bloom and Petri pointed to South Korea, a country that’s become internationally renowned for its rapid deployment of tests to a large segment of its population.