SCDHEC changes the way “Percent Positive” Calculated for COVID tests


The South Carolina Department of Health and Environmental Control (DHEC) has changed the way percent positive is calculated for COVID-19 cases. According to DHEC officials, the change will allow South Carolina percent positive calculations to be more easily compared to those used by federal entities, including the Center for Disease Control and Prevention (CDC).

DHEC is now reporting percent positive using the tests-over-tests method. Percent positive is now calculated by dividing all positive COVID-19 tests by the total number of COVID-19 tests (positive and negative), and then multiplying the result by 100 to get a percent.

With the change, the public will notice a big drop in the number representing percent positive. That does not mean the level of spread in the community has decreased. Percent positive will appear to be lower only because it is calculated differently.

Not only will DHEC use this new method going forward, but it will go back and recalculate the percent positive for the entire time COVID-19 has been tracked in South Carolina. Anyone wishing to see what the old data looked like can visit this link:

State Epidemiologist Dr. Linda Bell said it is important to note that when changing methods, you cannot compare information from the old method (people over people) with the new method (tests over tests). “It is important that people understand that while percent positive appears as a smaller number under the new way of calculating the rate, COVID-19 continues to spread at an elevated level in our state. That smaller number is solely based on the change in the way we calculate the rate,” Dr. Bell said.

“It is important that South Carolinians continue to take steps we know to protect us all from this deadly disease: wear a mask consistently and correctly, stay six feet away from others, wash your hands frequently, and avoid crowds. And when your time comes, get vaccinated,” Dr. Bell added.

New Calculation Allows for Greater Data Comparison

With many federal agencies and state health departments using the tests-over-tests method, this change by DHEC will allow comparisons with percent positivity calculations provided by the CDC, The White House Coronavirus Task Force, Centers for Medicare and Medicaid Services (CMS), other academic institutions and many states. In December, CMS began requiring health care facilities to use test over test; DHEC began preparing for the change at that time.

Understanding How Precent Positivity is Calculated

Previously, DHEC had been calculating percent positive using the people-over-tests method. That required dividing the number of people with positive results by the number of people who had taken tests overall, which included positive and negative results.

DHEC decided to use this method early in the pandemic because the focus was on identifying new people who were testing positive. This becomes harder to do when much of the population has been tested. With the continued recommendation for South Carolinians to be tested frequently, the shift from people to tests also becomes a more effective method to monitor active cases.

“The positivity rate helps public health officials determine the level at which COVID-19 is spreading in the community and whether enough testing is taking place,” said Dr.Bell. “The rate provides us a snapshot of how much COVID-19 is circulating in the community at a given period of time.”

What Does Percent Positivity Tell Us about Community Spread?

Percent positive will be high if the number of positive tests is high, or if the number of total tests is low. A higher percent positive suggests higher spread and that there are likely more people with COVID-19 in the community who have not been tested.

Percent positivity does not reflect a complete picture of COVID-19 in South Carolina. While it tells us some information about disease spread, other factors are at work as well, including access to testing and how quickly results come back from the lab. No single measure can give a complete picture of COVID-19 spread in our state and our counties.

For information on COVID-19 percent positive rates in South Carolina, visit the testing tab in the county-level dashboard.