Eleven states also confirmed the combination of viral and antibody test results.
The CDC says it is planning to separate the numbers in the coming weeks, but experts say the current scheme is prudent and potentially misleading.
That’s because antibody tests are not used to diagnose current infections or determine if a person is potentially contagious. Instead, they indicate if a person has been exposed to the virus in the past.
Combining numbers from antibody and viral tests has driven the total number of trials being conducted in the US. But antibody tests are often intended for the general public – not just for people with suspected infections ̵1; so they can bypass a key indicator of how pandemics develop: the percentage of tests that come back positive.
The CDC’s approach appears to be that the US has a greater capacity to test than it actually does, at least when it comes to identifying current infections.
“It’s not useful information, unless you have a political topic you’re trying to back up. That’s really the only reason to do that,” CNN’s medical analyst Dr. Celine Gounder, a professor of medicine and infectious diseases at the New York University School of Medicine.
“This is another example of why we are concerned that science has been suppressed by the CDC and stands behind the political priorities of this administration,” he said.
If the aggregate numbers come from the states and are published by the CDC – or if the CDC misinterprets the numbers themselves – Gounder says the data is about. The numbers can make it look like states with enough testing capacity and are willing to lift restrictions, if that’s not the case.
“It’s pretty basic biological biology 101 that any college student who has studied these things will have some basic understanding,” he said. “So the CDC epidemics know this.”
The news of the agency combining viral and antibody tests was first reported by the Miami-based public radio station WLRN and the Atlantic magazine.
CDC spokeswoman Kristen Nordlund described the agency’s practice on CNN on Thursday and confirmed it the next day.
“Initially, when the CDC launched its website and reported laboratory testing, viral testing (tests for current infection) was used more frequently nationwide than serology testing (tests for in the previous infection), “he said in an email. “Now that serology testing is more widely available, CDC is working to identify tests from virus tests and report this information, which differs by type of test, publicly to our COVID Data Tracker website. in the coming weeks. “
Two very different types of coronavirus tests
Viral tests, or diagnostic tests, identify current infections. Often called a PCR or antigen test, they look for signs of active infection. A positive test result means that a person is potentially contagious – even if some tests have accuracy problems.
Antibody tests, by contrast, identify if a person has been infected with the virus in the past, not if they are currently infected. As the CDC says on its website: “Antibody tests should not be used to diagnose a person currently suffering from COVID-19.”
To date, the CDC says nearly 12.9 million tests have been reported in the US, though it cautions that not all tests are reported to the agency. Still, data from independent labs suggest that a large number of tests conducted in the US are antibody tests.
Quest Diagnostics said Monday that it had conducted about 1 million antibody tests. LabCorp, another clinical laboratory, said it performed more than 700,000 by Friday.
Both companies say they have a far greater ability to conduct antibody tests than viral tests.
Quest says it can run 70,000 tests per day, compared to 200,000 antibody tests. LabCorp says it can run more than 75,000 viral tests a day, compared to more than 200,000 antibody tests.
The difference between trials is important, as the percentage of positive return tests – the so-called positivity rate – is a critical indicator for whether states are ready to open again. It’s a sign of development that the country is, or isn’t, virus-producing – and that antibody tests can complicate the picture.
The White House’s own reopening guidelines, for example, call for a “[downward] trajectory of positive trials as a percentage of total trials over a 14-day period (flat or increasing number of trials). “
While antibody tests are useful for understanding how widespread the virus is – and whether humans can have some form of immunity in the future – they do not reflect the current state of pandemic.
“Many states use the number of diagnostic tests per capita and the proportion of positive test results as the key metric for reopening decisions, thus adding a whole bunch of other tests that not measuring the current state of pandemic in the mix is not good and creates confusion, “Bill Hanage, a professor of epidemiology at Harvard University’s School of Public Health, told CNN.
“Fortunately, most, though not all, states report these results separately, which is the data that should be used when making decisions about reopening,” he said.
Some states recognize mixing the numbers together
Contacting CNN, public health officials in most states said they had not compiled figures from antibody and diagnostic tests. But 11 states have reported mixing numbers together at some point.
Colorado, Delaware, Georgia, Maine, Mississippi, Missouri, New Hampshire, Pennsylvania, Texas, Vermont and Virginia have all done so, but some states have stopped practicing.
Officials in Virginia and Vermont say they’ve fixed the issue. New Hampshire said it only reported combined numbers for a day, and Colorado said it had done so for about a week. Maine is now cutting its numbers, too.
Texas said it would cut the numbers this week, and Georgia said it is working to provide greater transparency. In 50 states and the District of Columbia, CNN has not received answers from Alaska, Arizona, Florida, Iowa, or Kentucky.
CNN’s Renee Baharaeen, Shawn Nottingham and Konstantin Toropin contributed to this report.