The US Centers for Disease Control and Prevention acknowledged on Friday it had been mixing together results from viral and antibody tests on its website. The CDC says it’s planning to separate those numbers in the coming weeks, but experts say the current method is unhelpful and potentially misleading.
That’s because antibody tests aren’t used to diagnose current infections or determine whether someone is potentially contagious. Instead, they indicate whether someone has been exposed to the virus in the past.
CDC spokesperson Kristen Nordlund described the agency’s practice to CNN on Thursday and confirmed it the next day. “Initially, when CDC launched its website and its laboratory test reporting, viral testing (tests for current infection) were far more commonly used nationwide than serology testing (tests for past infection),” she said in an email.
“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.”
Combining numbers from antibody and viral tests pushes up the total number of tests conducted in the US. But antibody tests are often intended for the general public – not just people with suspected infections – so they can skew a key indicator of how the pandemic is progressing: the percent of tests that come back positive.
The CDC’s method also makes it appear that the US has greater capacity to test than it really does, at least when it comes to identifying current infections.
“It’s not useful information, unless you have a political agenda that you’re trying to back up. That’s really the only reason to do that,” said CNN Medical Analyst Dr. Celine Gounder, a professor of medicine and infectious diseases at NYU School of Medicine.