(CNN) Health officials in at least three states have been combining Covid-19 data on diagnostic tests and antibody tests, skewing results and potentially giving a muddied picture of the virus's spread over time.
Texas, Virginia and Vermont each said they recognized the data issue and moved to fix it in the past few days.
Diagnostic PCR tests, which use saliva or sputum, check for current infection, while antibody tests, which rely on blood, check for past infection. Combining the two into one result could provide an inaccurate picture of where and when the virus spread and mislead policy-makers and the public.
"Public health officials need to know how many people in my state or my community currently have Covid-19. They also need to learn how many people had it in the past and potentially are immune to it," said CNN's senior medical correspondent Elizabeth Cohen. "Those are two completely different things."
Combining the two types of tests could also overstate a state's ability to test and track active infections.
"You only know how many cases you have if you do a lot of testing," Cohen said. "If you put the two tests together, you fool yourself into thinking you've done more testing than you have."
Virginia and Vermont health officials said they fixed the data issue in the past week. Chris Van Duysen, spokesman for the Texas Department of State Health Services, said in an email Monday that the state "will be separating the numbers out this week" and is "working to integrate that into our online reporting.
The issue is just one of the ways that coronavirus testing data can mislead or obscure when not used consistently and appropriately.
For example, some countries and research institutions, including Johns Hopkins University and its widely cited coronavirus dashboard, have relied on data from a mysterious data aggregation site called Worldometer. And in the US in recent days, Georgia and Florida have faced questions about the transparency of their coronavirus data reporting.
The issue in Texas came after the Texas Observer reported the two testing results were being combined. Gov. Greg Abbott said Monday the state is not presently combining the numbers but did not acknowledge that it had been in the past.
"The answer is no. They are not comingling those numbers," Abbott said. "Those numbers will be provided separately."
Texas State Rep. Chris Turner, who leads the House Democratic Caucus, found Abbott's statement puzzling and reached out to the health department after Monday's press conference.
"They confirmed to my staff that in fact the testing total does include the antibody test. They said they would be breaking those out later in the week," Turner said. "But for now the total number of tests reported includes both types of test."
Turner argued that combining the numbers "really muddies the data" and "indicates that our testing total is inflated."
The Department of State Health Services did not provide CNN more specifics Tuesday on what percentage of the total tests being reported are antibody tests.
As of Wednesday morning, the department's website said the state had a total of 744,937 tests, 23,601 of those are from public labs, while 721,336 are from private labs. The state is reporting a total of 49,912 positive cases. The state's coronavirus dashboard did not yet reflect a distinction in antibody tests versus active virus tests.
Abbott has been touting the low positivity rate in the state. It's unclear how many of those positive cases are from antibody tests or active infection tests.
Virginia similarly had been combining its antibody and diagnostic tests in its reported testing data, the Virginia Department of Health said last week. The department said separating out the results made "minimal change in the percent positive of tests and no difference in overall trends."
Virginia Gov. Ralph Northam, a pediatric neurologist, said he directed the Department of Health to separate out the testing results last week.
"I am a doctor, and I have said all along that I will act based on science and data—and the data must be reliable and up-to-date, so we can make informed decisions based on the facts," he wrote on Twitter. "Going forward, the (Virginia Department of Health) website will break out the number of diagnostic tests."
Vermont, too, began separating its antibody test results from its diagnostic test results on May 16, according to the Department of Health.
"As we began seeing an increase in serology tests, we realized this is impacting the number and needed to correct it," said Ben Truman, spokesman for Vermont's Department of Health.