Texas officials change how the state reports positivity rate after testing backlogs skewed coronavirus data

Kroger pharmacists explain and collect self-swab COVID-19 tests at a testing site in the Fifth Ward in Houston on June 27, 2020.                    Credit: Annie Mulligan for The Texas Tribune
Kroger pharmacists explain and collect self-swab COVID-19 tests at a testing site in the Fifth Ward in Houston on June 27, 2020. Credit: Annie Mulligan for The Texas Tribune

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Texas health officials announced Monday they are changing the way the state reports a key metric used to evaluate the extent of coronavirus infection, a move that conceded that the state’s previous method of calculating the “positivity rate” muddied the extent of viral transmission by mixing old data with new.

The Texas Department of State Health Services said it will now “primarily rely” on a new calculation of the daily positivity rate — defined as the share of tests that yield positive results — which takes into account the date on which a coronavirus test was administered. Officials said the new metric will give a more accurate representation of viral transmission in Texas on a given day.

It also means that each day’s positivity rate will be an oft-changing number, fluctuating as officials collect lab results over time. Labs and hospitals report their test results to the state with varying degrees of timeliness, and state officials will have to recalculate the positivity rates for previous days as more test results from those dates pour in.

That’s a departure from the current system, which calculates the positivity rate based on the date the health agency receives test results, which can be weeks or even months after the tests were actually administered.

It marks the latest in a series of data methodology changes and corrections health officials have issued over the course of the pandemic.

While touting the new reporting method as an improvement, state officials defended the old system as providing the best information that was available at the time. The new positivity rate for the most part closely resembles the old metric, particularly when viewed as a trend line over time, officials said.

Health department spokesperson Chris Van Deusen said in an email that the different positivity rates generally moved in parallel until about August, when the state reported dumps of test results. “As long as the test results and new cases were reported fairly close in time, the case reported date was working as a metric,” he said.