U.S. Virus Data Muddied by Mixing of Diagnostic and Antibody Tests


(Bloomberg) -- The U.S. Centers for Disease Control and Prevention and some states reported both completed diagnostic tests and antibody tests in public datasets, mixing results that show current infections with past infections and presenting an image of testing capacity that’s greater than the on-the-ground reality.

In measuring testing and urging improved capacity, decision-makers and public-health experts have largely focused on diagnostic testing, which indicates whether a patient has an active Covid-19 infection. That presents a picture of current cases. Antibody tests, by contrast, look for markers in the blood that indicate past exposure and that often take weeks after an infection to develop.

The mixing of the test results was first reported by the Atlantic.

When the CDC began reporting laboratory test data, diagnostic testing was far more widely used than antibody testing, which first became available in late March, agency spokeswoman Kristen Nordlund said in an email. The agency is now working to distinguish the two types of tests and will report the differentiated data online “in the coming weeks,” Nordlund said. States often have their own forms of the data that drive decision-making, she said.

Texas, Georgia, Pennsylvania and Vermont confirmed they’ve released data combining the two types of tests, but said that either they now disclose that they do it, didn’t rely on it for reopening decision-making, or had recently changed the practice. Pennsylvania, Texas and Vermont said antibody tests made up a small share of their counts.

U.S. Virus Data Muddied by Mixing of Diagnostic and Antibody Tests

Starting to Separate

Texas, which is expanding its reopening to allow restaurants and bars to resume with reduced capacity, began breaking out antibody tests from diagnostic tests in its online data dashboard on Thursday, according to Chris Van Deusen, media director at the state’s Health Services agency.

“Throughout the Covid-19 response, we’ve been requiring labs to report all tests to us, and that’s how we’ve been reporting them out,” Van Deusen said in an email. “We’ve had a small number of antibody tests reported, but with that kind of testing becoming more available, we wanted to start separating those numbers out to provide more detailed information about the virus in Texas.”

In Pennsylvania, less than 1% of total cases had a positive antibody test, while 97.5% were confirmed with a “polymerase chain reaction” diagnostic test that’s considered the best way to tell if someone is infected, according to a department of health spokesman. The state counts positive antibody tests with symptoms or a high-risk exposure as probable cases, but isn’t relying on that for reopening-related decisions, he said.

Pennsylvania began combining both tests for a public-facing total test count once it began getting results for antibody tests, which was in line with the CDC guidelines published in mid-April, he said.

‘Point is Being Accurate’

Georgia said that about 57,000 of the 408,000 tests reported having been done were for antibodies. It’s working to provide greater transparency in its online data report, a spokeswoman for the department said. The state has also clarified its reporting to say that total tests includes both types of tests, she said.

U.S. Virus Data Muddied by Mixing of Diagnostic and Antibody Tests

Vermont stopped counting antibody tests over the weekend as the level increased and “we realized this is impacting the number and needed to correct it,” Department of Health Public Health Communication Officer Ben Truman said in an email.

Antibody tests only made up about 4% of the state’s tests and removing them had a “minimal” effect on the data, he said.

“I want to emphasize the point is being accurate about cases of active Covid-19 disease,” which only diagnostic testing can reflect, Truman said.

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