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State Clarifies How It Counts Coronavirus Cases


On Friday afternoon, the state of Pennsylvania began reporting how many of its diagnosed COVID-19 cases were discovered through viral testing, and how many were discovered by antibody tests.

Of the 66,258 cases among Pennsylvanians, 97.5 percent were confirmed by viral tests, which use a nasal or saliva swab.

The rest of the cases are classified as “presumed” positives. Most of those cases involve individuals who are believed to have the disease because they have COVID-19-like symptoms and a high-risk exposure. But some presumed positives reflect individuals who had a blood test that showed antibodies for the virus.

The state published this information after it was questionedabout its data disclosure practices when the Atlantic magazine reported that the state lumped together results of viral and antibody tests.

The concern is these tests reveal two different things.  Viral tests determine whether a person is currently infected with the coronavirus. Antibody, or serology, tests show whether someone has been infected at some point in time. Conflating the tests can give an inaccurate picture of how the virus is spreading.

Knowing how many people have recently been infected with the coronavirus is important data in policy-making decisions, like easing restrictions in some counties.

“Only confirmed cases are taken into consideration in any of our metrics and our calculations,” said Dr. Rachel Levine, secretary of Pennsylvania’s Department of Health.

But the state has offered conflicting accounts of how it discloses test data. State Department of Health spokesman Nate Wardle originally told WESA that negative antibody tests were included in the state’s count of all negative test results. He later said that only negative results from viral tests are included in the total.

“I had an in-depth conversation with our data team [Friday] to ensure my understanding was correct regarding negative results, and it was not," said Wardle. "I was under the impression that negative results included all negative results. That is not correct.”

Antibody tests are more likely to come back negative. So if the state only includes antibody testing in its totals when the results are positive, that could make it seem like a higher percentage of residents tested have the disease.

On Friday, Levine also said that 57 percent of Pennsylvanians diagnosed with the disease since the outbreak began have recovered.  This is the first time the state has disclosed that metric.

Someone is considered recovered if they haven't been reported dead by health officials, and it's been more than 30 days since the onset of symptoms or a first positive test.

Sarah Boden covers health and science for 90.5 WESA. Before coming to Pittsburgh in November 2017, she was a reporter for Iowa Public Radio. As a contributor to the NPR-Kaiser Health News Member Station Reporting Project on Health Care in the States, Sarah's print and audio reporting frequently appears on NPR and KFF Health News.