June 1, 2020 — Public health experts have criticized the CDC and several states for mixing antibody tests with diagnostic tests in their reporting of COVID-19 test results. Using both can inflate testing numbers. This also complicates the picture for state officials who rely on these metrics to make decisions about reopening society.
The two tests serve different purposes, says Harvard’s Global Health Institute. “Virus testing is diagnostic, indicating whether a person is currently infected. It tells you the state of play of the infection in the community: how many people currently have the disease and can potentially spread it. The antibody tests, on the other hand, assess immune system response. They are the ‘rearview mirror’ that tells you where you’ve been: how many people were previously infected.”
“Mixing the two tests makes it much harder to understand the meaning of positive tests, and it clouds important information about the U.S. response to the pandemic,” says Ashish Jha, MD, MPH, faculty director of Harvard’s Global Health Institute.
During this first phase of the epidemic, the assumption was the CDC and state departments of health were only reporting diagnostic tests and their positive and negative results. But The Atlantic reported last week that several states including Virginia, Pennsylvania, Georgia, Texas, Maine, and Vermont were also including antibody tests and their results in their numbers.
This widespread practice has raised concerns about overstating testing numbers. The CDC, which confirmed to The Atlantic that it was including antibody tests in its test reporting, also said that was the reason the agency’s testing numbers for Florida were hundreds of thousands higher than what the state government has.
Because the CDC has reported that the percentage of positive tests has improved substantially, “this has been seen as good news — that we are advancing toward the level of testing we need to open up our economy safely — when in fact we’re not,” says Harvard’s Global Health Institute.
After local newspapers publicized the blending practice, Virginia and Maine reversed course in mid-May and are now separating the test results. The CDC told The Atlantic that it “hopes to separate the viral and antibody test results in the next few weeks.”