WASHINGTON – As it tracks the coronavirus’ spread, the CDC is combining tests that detect active infection with those that detect recovery — a system that muddies the pandemic picture but raises the percentage of Americans tested as President Donald Trump boasts about testing.

Now that serology tests, which look for antibodies in the blood of people who have recovered, are more widespread, CDC officials said Friday they will work to separate them from the results of diagnostic tests, which detect active infection. One of the agency’s data tracker websites has been lumping them together.

Stunned epidemiologists say data from antibody tests and active virus tests should never be mixed because diagnostic testing seeks to quantify the amount of active disease in the population. Serological testing can also be unreliable. And patients who have had both diagnostic and serology tests would be counted twice in the totals.

“It just doesn’t make any sense; all of us are really baffled,” said Natalie Dean, a biostatistician at the University of Florida.

Epidemiologists, state health officials and a CDC spokeswoman said there was no ill intent; they attributed the flawed reporting system to confusion and fatigue in overworked state and local health departments that typically track infections — not tests — during outbreaks. The CDC relies on states to report their data.

If the agency intended to bolster the testing numbers for political purposes, the advantage to Trump would be minimal. The Atlantic reported that on Monday, one of the CDC’s trackers reported that 10.2 million viral tests had been conducted nationwide since the pandemic began. On Wednesday, after the CDC stopped differentiating virus tests, the number went to 10.8 million.

CDC spokeswoman Kristen Nordlund said viral testing was much more common than antibody testing in the pandemic’s early days, and some states combined the virus tests together with the few antibody results they had. “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,” Nordlund said.

State health officials in Virginia, Texas, Georgia and Vermont also said they were beginning to disaggregate their testing data.

“This is not an intentional misuse of information — it’s part of the fog of the infectious disease war,” said Michael Osterholm, a University of Minnesota professor who was sharply critical of the disease control centers early in the pandemic. “We’ve done surveillance for cases, and now we’re all trying to do testing, and it presents unique challenges.”

Whatever the reason, the numbers are fueling Trump’s frequent — and inaccurate — boasts that the United States is doing more testing “than all other countries combined,” a claim that the fact-checking website PolitiFact has declared “pants on fire wrong.” Governors rely on testing in deciding how far to go in reopening their economies. With all 50 states moving to reopen, accurate tracking is essential.

“We’re like the blind epidemiologists trying to understand the elephant,” said Michael Levy, a professor of epidemiology at the University of Pennsylvania. Health officials, he said, need good reporting to “understand the relationship between the epidemic that we can’t see and the data that we can see.”

Scott Becker, executive director of the Association of Public Health Laboratories, said there was another reason states were tracking testing: Trump wants the numbers.

“We’ve never needed to capture test volume. That is what the White House wanted to know, how many tests were being done,” Becker said, adding, “Ordinarily this all works through the public health system, but in this response, there’s been a drive to have data numbers, at multiple levels.”

Like Nordlund at the CDC, health officials around the country say diagnostic tests, which detect the presence of the virus, had initially far outpaced antibody tests, so agencies grouped them together. While both numbers are helpful in assessing the scale of the outbreak, only viral test numbers can indicate a state’s ability to identify people who currently have the virus.

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