The enormous number of Americans getting swabbed for the coronavirus has overwhelmed every step of the COVID-19 testing process, creating shortages of critical supplies in laboratories and inundating them with more specimens than they can process.

The testing spike, which experts say has been particularly dramatic over the past month as more coronavirus hot spots have popped up, has meant waits of up to two weeks or more for test results in some cases a far from ideal turnaround time.

Laboratories say they are working as fast as they can.

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"The challenges that labs are facing are complex," said Louise Serio, a spokesperson for the American Clinical Laboratory Association, a trade group that represents companies such as Quest Diagnostics and LabCorp. "There is a significant strain on the global supply chain."

Since the beginning of the pandemic, the American Clinical Laboratory Association's members have performed more than 23 million COVID-19 tests, Serio said. Testing capacity is increasing every week, she said, and employees are working round-the-clock to run as many tests as possible, but they are running out of necessary chemicals and other products faster than their manufacturers can replenish them.

"What we have consistently heard from members is that reagents, test kits, pipettes and platforms are all in great demand right now," she said.

The backlog could get worse if the outbreak continues on its current trajectory. On Thursday, the United States reached a somber milestone, surpassing 4 million confirmed coronavirus cases nationwide, according to data compiled by NBC News, just 15 days after hitting the 3 million mark.

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Dr. Scott Gottlieb, the former commissioner of the Food and Drug Administration, said on MSNBC's "Morning Joe" that access to testing and quick results is "likely to continue to be a challenging issue heading into the fall."

Testing lags will make it significantly harder to stop transmission of the virus, he said.

"After about 48 hours, the test really isn't that useful for the clinical management of the patient," Gottlieb said. "At that point, you're not going to be able to use the result to do effective contact tracing. Too much time has passed, and you're not giving information back to the provider and the patient that they could use."

His comments echoed those of Dr. Brett Giroir, the assistant secretary of health and human services for health, who is overseeing U.S. coronavirus testing and who has said three days is a "reasonable turnaround time" for results.

But given the surge in demand, that has been difficult to attain.

While some COVID-19 tests are done by a more rapid, cartridge-based method in hospitals for patients who are admitted, a much larger percentage in the U.S. are done by commercial labs.

Mike Geller, a spokesman for LabCorp, said LabCorp has so far performed 7.5 million molecular tests for COVID-19 and is processing 165,000 tests a day, with plans to increase capacity further. The average time to deliver results is three to five days from when a specimen was picked up; before the surge, the turnaround time was one to two days, he said.

Jim Davis, executive vice president of general diagnostics at Quest Diagnostics, said Quest has performed more than 8.5 million tests and is processing 130,000 tests a day. Quest anticipates that by the end of the month, it will have a daily capacity of 150,000 tests, which should help with the delays: Average turnaround time for test results right now is at least seven days, up from two to three days until several weeks ago, Davis said.

While Quest's test capacity has doubled over the last two months, demand has tripled, Davis said, with the last three to four weeks representing a steep increase. Labs are staffed 24/7, which they were not before the pandemic.

"We're asking everyone right now to work overtime," Davis said.

Federal funding and clearer guidance on testing from the federal government could help get turnaround times down. This week, the American Clinical Laboratory Association was among 50 health care organizations to call on Congress to dedicate more funding for testing.

"If the demand keeps increasing like it's been increasing over the last couple of weeks, the lab industry will never be able to keep up with it."

In the meantime, to best accommodate the backlogs, many testing sites are prioritizing certain patients.

At Northwell Health, New York's largest health care provider, screening tests for employment, travel or summer camp are considered lowest priority; those tests are typically sent out to the national commercial laboratories, while symptomatic patients awaiting clinical treatments are put at the top of the list, with their tests performed at Northwell's regional laboratories, said Dr. Dwayne Breining, executive director of Northwell Health Laboratories.

The turnaround time at the regional laboratories is one to two days, he said. (Northwell's hospitals, meanwhile, get results within thee hours using their cartridge tests.)

Breining said the U.S.'s maxed-out testing capacity reflects the country's struggle to contain the spread of the virus, and he urged Americans to take precautions.

"If the demand keeps increasing like it's been increasing over the last couple of weeks, the lab industry will never be able to keep up with it," he said. "I think the top priority is going to be mitigating the clinical spread of this virus by doing things we know work: things like social distancing, masking and monitoring.

"All those things make a huge difference," he added. "That would allow you to slow it down enough so that not only the lab testing industry, but the entire medical system, can catch up."

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'The challenges that labs are facing are complex': Why COVID-19 test results are so delayed - NBC News