How did our percent positive get cut in half? Explaining DHEC’s new method

How did our percent positive get cut in half? Explaining DHEC’s new method
One of the key measures people use to track the spread of COVID-19 is percent positive, but on Feb. 2 the South Carolina Department of Health and Environmental Control announced a new way of calculating this number that caused a lot of confusion. (Source: Houston Dept. of Health via CNN)

COLUMBIA, S.C. (WCSC) - One of the key measures people use to track the spread of COVID-19 is percent positive, but on Feb. 2 the South Carolina Department of Health and Environmental Control announced a new way of calculating this number that caused a lot of confusion.

According to DHEC data from Jan. 31, the percent positive was 20.1% using the previous method but 8.8% using the new system.

DHEC now calculates the state’s percent positive using the test-over-test method. That divides the number of negative test results by the number of all positive and negative tests during the same time period.

Previously, DHEC was using the people-over-people method, which divides the number of people who test negative by the number of people who test positive.

Assistant State Epidemiologist Dr. Jane Kelly said using the test-over-test method allows South Carolina to compare its percent positive rate to national data, because that is the method used by the Centers for Disease Control and the White House Coronavirus Task Force.

“We changed so we could match their methodology, so that it would be fair to compare across states and with the federal statistics. So, we would have a better understanding of where we fit in with percent positivity compared to others,” Kelly said.

Kelly added this new method, test-over-test, allows state leaders to better assess how the state is doing on testing while also getting a better idea of how many of those tests are coming back positive.

The people-over-people method allows the community to have a rough idea of the likelihood someone is positive, but it doesn’t account for someone getting multiple tests.

For example, if 100 people got tested and 10 people were positive and 90 were negative, both methods would conclude the percent positive is 10%.

However, as more people get tested multiple times the two methods start to look different.

If 100 tests come back negative and 20 come back positive after 100 new people get tested and 20 people who have already been tested get tested again, the new method will produce a lower percent positive.

In both calculations, the top number remains the same: 20 negative people and tests. But, the bottom number will be different: 120 total tests taken compared to 100 people tested.

Therefore, the percent positives will be about 3.4% apart.

The gap between the percent positive will only increase as more people get retested, which Dr. Kelly said is happening more frequently as COVID-19 testing gets easier to access in South Carolina.

“If we have 5 million people in South Carolina. If all of them got tested once and some came back and got tested again, they would all were included in that bottom number and they’ll never get added again,” Kelly said.

According to the CDC, a percent positive below five percent is considered low and anything above eight percent is considered high community spread.

However, for people who want to gain a better understanding of South Carolina’s COVID response, Dr. Kelly recommends looking at multiple data points.

For example, someone can look at new coronavirus daily case counts and then compare that number to the percent positive.

Kelly considers a new case number below 500 to be a positive sign for the state’s COVID-19 response, 500 to 1000 new cases to be okay, 1000 to 2000 to be bad, and any number over 2000 to be an indication of an increase in the spread of the virus in the state.

If the number of new cases dramatically changes in a given day, someone trying to figure out what is causing the shift can then look at the percent positive to figure out if there has simply been more or less testing in the community.

However Kelly said it is important for anyone trying to understand how well SC is fighting the virus looks at trends over time, and multiple other data points like deaths and hospitalizations to get a holistic view of how the state is doing.

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