What do 'probable' COVID-19 cases mean to Illinois numbers?

When the state began reporting probable cases of COVID-19 in June, those figures were always separated from cases that had been confirmed using standard molecular tests that take days to get results.

That changed Friday when the Illinois Department of Public Health spread nearly 8,000 “probable cases” of COVID-19 among all the state's county case totals, sending county infection rates soaring and raising eyebrows about how health officials were reporting data.

The “probable” label was just a way of identifying what type of test determined a new case, and the state added those probable cases under the guidance of the U.S. Centers for Disease Control and Prevention, IDPH Director Dr. Ngozi Ezike explained at Friday's media briefing.

But it wasn't the first time the state had used the term “probable case.”

Unlike cases and deaths that were labeled “probable” in the early stages of the pandemic — where symptomatic patients were treated or died without being tested — these new “probable” cases also cover someone who tests positive using the newer “rapid-result” antigen test. These tests are more susceptible to false positives, the U.S. Food and Drug Administration recently reported.

“In August, the national case definition was changed so that an antigen test alone would be considered a ‘probable case,'” Ezike noted during the Friday news briefing. “With the increased use of antigen tests, we will get more probable cases and we want to make sure we capture all of those tests.”

This change confused many who follow the state's daily COVID-19 figures because the state had previously separated out probable cases.

And while IDPH had been counting antigen tests since Oct. 15, it turns out IDPH had merely been counting the number of antigen tests taken, not the results of those tests.

Adding to the confusion, IDPH dumped “all the probable cases since the beginning of the pandemic” into Friday's case totals, Ezike said. That move amounted to an additional 7,673 cases lumped together with more than 10,000 new cases announced from standard test results, artificially inflating infection rates in every county of the state that day.

That action baffled many observers since state health officials use the state's average infection rate to help guide virus mitigation decisions.

“There just needs to be justification for doing it that way,” said Courtney M. Hughes, a data scientist and associate professor of public health at Northern Illinois University. “It's important to have consistency in how these numbers are derived.”

Hughes and others agree that spreading all the probable cases over the dates they were actually diagnosed or discovered would keep the state's data more accurate.

State officials cite other state health departments that have also shifted accounting practices to include both confirmed and probable cases, but many of those states also provide separate daily tallies of both types of cases. Illinois does not.

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