Health workers dressed in personal protective equipment handle a coronavirus test at a drive-thru testing station at Cummings Park on March 23, in Stamford, Connecticut. John Moore/Getty Images

Demand for coronavirus tests at some US commercial labs continues to exceed capacity, even as millions of tests have become available. Test turnaround times range from 24 hours to a week or more, according to labs contacted by CNN.

Quest Diagnostics: One of the largest US clinical laboratories said current turnaround time for coronavirus tests is on average four to five days, from the time of specimen pickup to the time results are delivered, though some results may not be provided for about a week.

“Although we are rapidly expanding testing capacity, demand for the testing is growing faster, and we cannot accommodate everyone who wants testing and meet tight turnaround time expectations,” Quest said in a statement.

TriCore Reference Laboratories: The New Mexico-based lab said its turnaround time is about five days.

Eurofins: A spokesperson for laboratory network Eurofins said its labs are running up to 5,000 tests a day, many in 24 hours, but also said, “due to the backlog, unprecedented demand, pressures on our supply chain, and shipping delays … our turnaround time has been pushed beyond what we normally experience. At most, we are looking at results in 72 hours.”

BioReference Laboratories: While there had been a backlog in its facilities, the company said it has cleared it within a week of making their testing for Covid-19 available and cut its turnaround time to 24 hours for inpatients and 48 to 72 hours for the general public.

Dr. Alex Greninger, an assistant professor at the University of Washington's Department of Laboratory Medicine, said unprecedented demand is the driving factor of such wait times for test results.

Faster delivery of tests is critical to conserve personal protective equipment, he said.