CDC messes up, merges two different coronavirus testing data that could create misleading picture of pandemic
The US Centers for Disease Control and Prevention (CDC) has been combining Covid-19 results from two different kinds of tests when reporting the country's testing totals, potentially providing a misleading picture of when and where coronavirus spread as the US eases lockdown.
According to reports, the CDC and several state health departments have been reporting Covid-19 diagnostic tests and antibody tests as one grand tally, rather than keeping their results separate. The two tests reveal different information and are used for different reasons. Viral tests are taken by nose swab or saliva sample and they look for direct evidence that someone currently has Covid-19. In contrast, antibody tests use blood samples to see if a person’s immune system has created antibodies to fight Covid-19. Antibody tests allow doctors to understand if a person has been exposed to the virus in the past.
As the tests serve different purposes, positive or negative results from either cannot be interpreted in the same way. Accordingly, combining the two test results into one total is not appropriate and could provide an inaccurate picture of how many Covid-19 cases have emerged over time and where the virus has spread.
The combined number does not reflect the rate of new infection in a particular state or area. It could also exaggerate a state's ability to test and track active infections as well as inhibit the ability to gauge the country’s actual testing capacity. These are key metrics to help control outbreaks and ease lockdowns. Experts have emphasized many times that adequate testing and tracing is a must for states to reopen safely.
"You've got to be kidding me. How could the CDC make that mistake? This is a mess," Ashish Jha, director of the Harvard Global Health Institute, told The Atlantic. "The viral testing is to understand how many people are getting infected, while antibody testing is like looking in the rearview mirror. The two tests are totally different signals. By combining the two types of results, the CDC has made them both uninterpretable," he said.
A May 14 page on the CDC website explaining the information on the CDC Covid-19 data tracker, said that "these data only represent viral tests" and that "antibody tests are not currently captured in these data". However, on May 17, the two lines were removed. The information on the website currently reads, “These data are compiled from a number of sources. Not all viral tests are reported to the CDC. The number of positive tests in a state is not equal to the number of cases, as one person may be tested more than once."
The numbers also changed. On May 14, the CDC page reported that 10,217,573 viral tests had been conducted nationwide since the pandemic began, with 15% — 1,510,924 — coming back positive. But on May 17, after the CDC changed its terms, it said on the same page that 10,847,778 tests of any type had been conducted across the US, with 14% or 1,563,353, coming back as positive.
CDC spokeswoman Kristen Nordlund told NPR that the "majority of the data is PCR testing" but acknowledged that the agency's tally includes antibody testing because "some states are including serology data" in their testing numbers. "Those numbers still give us an idea of the burden of Covid-19," she added.
Nordlund told The Hill that when the agency began to track coronavirus testing, viral tests were far more commonly used nationwide than serology testing. "Now that serology testing is more widely available, CDC is working to differentiate those tests from the viral tests and will report this information, differentiated by test type, publicly on our Covid data tracker website in the coming weeks," she said.
According to reports, several states, including Georgia, Pennsylvania, Texas, Vermont, Virginia, and Florida, have also been combining the results of the two tests. The Georgia Department of Health recently tweeted that it has received antibody test reports since early April and included them in the “total tests” number on the Covid-19 daily status report, as per CDC methodology. "It is one of DPH's top priorities to provide accurate and timely molecular and serologic testing data on our website," it said.
"I suspect it will artificially lower the percent positive," Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told NPR. The criticism over how testing results are being reported by the CDC is the latest in many controversies related to testing in the US. Many public health experts have criticized the US government for failing to ramp up testing quickly enough to track and control the coronavirus pandemic. As of May 22, more than 1,577,750 coronavirus cases have been reported from across the US, and over 94,720 have died in the Covid-19 pandemic, according to data compiled by Johns Hopkins.