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

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

COLUMBIA, S.C. (GRAY) - 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.

It’s important to note that even though the new method of calculation ends up with a lower number, 20.1% in the old method and 8.8% in the new method mean the same thing: high community spread of COVID-19.

So what accounts for the difference in numbers?

DHEC now calculates the state’s percent positive using the test-over-test method. That divides the number of positive 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 positive by the total number of people who got tested.

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,” she said. “So we would have a better understanding of where we fit in with percent positivity compared to others.”

Kelly added this new method, test-over-test, allows state leaders to better assess how South Carolina 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%.

The formula is 10 ÷ 100 = 0.1. Then that number is multiplied by 100 to get the percentage: 0.1 × 100 = 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 then 20 of those 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 positive people and tests. But, the bottom number will be different: 120 total tests taken compared to 100 people tested.

Therefore, the percent positives end up about 3.4% apart.

MATH:

People over people:

  • 20 positive people ÷ 100 people tested = 0.2
  • 0.2 × 100 = 20%

Tests over tests:

  • 20 positive tests ÷ 120 total tests = 0.166
  • 0.166 × 100 = 16.6%

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.

MATH:

People over people:

  • 25 positive people ÷ 100 people tested = 0.25
  • 0.25 × 100 = 25%

Tests over tests:

  • 25 positive tests ÷ 200 total tests = 0.125
  • 0.125 × 100 = 12.5%

So what do the numbers mean?

According to the CDC (which uses the tests-over-tests method), a percent positive below 5% is considered low and anything above 8% is considered high community spread.

It’s important to note that this new method of calculating percent positive DOES NOT affect the way DHEC counts new COVID-19 cases. No matter how many tests a person gets, their infection will only be counted once in the new cases.

If people 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 1,000 new cases to be okay, 1,000 to 2,000 to be bad, and any number over 2,000 to be an indication of an increase in the spread of the virus in the state.

Looking at that data point, South Carolina has been doing better lately, reporting around 1,000 or slightly fewer cases in recent days. Back around the holidays, the state was consistently seeing new case counts of 2,000 and even 3,000 or more a day.

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

Here’s DHEC’s Key Indicators dashboard, which does just that:

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