Even though the federal government has reassured the public that millions of tests for the coronavirus are on the way, many private laboratories in the U.S. still aren't able to conduct their own tests, in part because of a demanding government approval process, leading lab experts and industry groups say.

One hospital lab says it could have performed thousands of tests by now. The delays — which have also been driven by a shortage of materials and lack of information from the federal government about how much labs will be reimbursed — have prolonged waiting times to diagnose infected patients while the virus has spread further, according to lab directors and public health experts.

Private labs have been in touch with the Department of Health and Human Services since mid-January about developing their own tests, according to the American Clinical Laboratory Association, which represents commercial and hospital labs. But the federal government didn't issue new rules speeding the approval process for commercial, research and academic labs until Feb. 29.

Even after that, however, private labs have continued to face hurdles. It could take weeks for some laboratories to get up and running, said Mark Birenbaum, administrator of the National Independent Laboratory Association, which represents labs in smaller cities and rural areas.

"Our labs want to do the testing — the testing is needed," Birenbaum said. "But we've been waiting to get the OK to either create the tests or buy them from test kit manufacturers. It will still take time."

Some labs have spent weeks developing their own tests but remain frustrated with a federal approval process they say is too time-consuming and expensive.

"I could have tested over 1,000 patients by now instead of checking boxes," said Dr. Melissa Miller, director of the Clinical Microbiology Laboratory at UNC Medical Center in North Carolina. Miller says her lab had developed a test based on the World Health Organization's protocol before the state lab even had its own. But the UNC lab hasn't begun testing patients, as it is still in the process of meeting the requirements for expedited authorization by the Food and Drug Administration, which must sign off on all new tests.

"The necessary validations for the emergency authorization are time- and cost-prohibitive," Miller said. "We are way behind."

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The FDA says that its requirements help to ensure that tests are based on "sound science" before they reach patients and that it is working closely with companies and labs that are developing diagnostic tests.

Several versions of a test are in use in the U.S., including the one developed by the Centers for Disease Control and Prevention. The CDC has made its test available to qualified laboratories, but some private labs lack the equipment to run it, Birenbaum said.

Until recent days, the vast majority of tests were being conducted by the CDC or public health labs, which have limited capacities. On Monday, CDC Director Robert Redfield said about 4,900 people had been tested by government labs. But there still aren't enough tests to meet demand in many parts of the country, local officials and public health experts say. (South Korea, by comparison, says it has tested more than 200,000 people since early January, with the capacity to test 15,000 people a day.)

That's why it's so critical for private labs to ramp up quickly, said Dr. John Glenn Morris, director of the Emerging Pathogens Institute at the University of Florida.

"The solution is not counting the number of test kits that CDC is distributing — it's getting the large commercial companies on board so that we can really start testing everybody that comes in," Morris said. "That's when we're going to start discovering how prevalent the virus is."

In the past week, Quest and LabCorp — two of the country's biggest clinical testing providers — began rolling out their own commercial tests to doctors and medical providers. But results typically take three to four days from the time specimens are picked up, the companies said.

Quest warned that "timing may be impacted by high demand" and that it would take weeks to ramp up testing. "We expect to be able to perform tens of thousands of tests a week within the next six weeks," the company said in a statement.

The turnaround is usually far quicker for smaller private labs and academic labs, because the testing is conducted on site instead of shipped to a central facility. Several private labs said they would be able to produce results within four hours once they were cleared to begin testing.

Quick turnaround is critical to slowing the virus, as an infected person may expose others before getting results back.

"The risk is for further spread," said Dr. Christina Wojewoda, a pathologist at the University of Vermont Medical Center. "There are people who are positive and don't know they are positive, and they aren't staying away from elderly patients and those who are immunocompromised."

So far, the FDA has received requests from 14 private labs for expedited authorization, and 10 of them have begun patient testing, according to a spokesperson for the FDA. Many more labs and diagnostic device companies haven't even gotten to the point of requesting authorization because they are still developing their tests and trying to meet the FDA's requirements, industry representatives and lab experts said.

"We are talking to them around the clock, and our door is open for any developers who want to have a test for use in the U.S.," FDA Commissioner Stephen Hahn said Saturday.