The turnaround time for non-priority testing was now at least seven days but could be reduced by boosting capacity, the company said during an earnings conference call. It aims to increase COVID-19 diagnostic testing to 150,000 per day by next month and ultimately to exceed that level.

A shortfall of adequate diagnostic testing has hampered the U.S. response to the pandemic, which has so far infected more than 4 million Americans.

"We believe, as we get through August and into September, will have a higher level of capacity," Quest's Chief Executive Steve Rusckowski said on the call. "We'll be able to get the turnaround times that are in the acceptable levels that we've had in the past."

A spokeswoman for Quest said its goal is to get test turnaround times back to the 3- to 4-day range for most patients and one day for top priority patients.

Quest's turnaround time for COVID-19 tests was one to two days as recently as April. The U.S. Department of Health and Human Services said last week a turnaround time of up to three days is acceptable.

Quest said it currently has capacity for about 130,000 tests per day, double its capacity since May.

Quest's rival Laboratory Corporation of America Holdings has also struggled to meet demand for tests. Its chief executive, Adam Schechter, told CNBC on Tuesday that turnaround times have climbed to three to five days for patients who are not in the hospital. It is currently processing about 165,000 tests for COVID-19 per day, he said.

The National Institutes of Health said it aims to create capacity for 6 million COVID-19 tests per day by the end of 2020, considerably in excess of the roughly 300,000 daily tests currently conducted by the U.S.'s two largest lab testing companies. Much of that ramp up will require the production of fast and inexpensive antigen tests that do not require a laboratory.

Earlier this month, the FDA authorized a rapid antigen test from Beckton Dickinson & Co. The company said it could make 2 million per week by September.

Testing will be especially important as the U.S. moves towards reopening schools in fall, amid pressure from the Trump administration.

The U.S. Centers for Disease Control and Prevention recently issued guidance to reduce what a senior U.S. health official described as "unnecessary" testing for COVID-19 as it works to cut turnaround times for tests.

"There are some views that maybe that second test to verify your negative is not of great value. And therefore, this might help reduce the demand," Rusckowski said.

Earlier on Thursday, the company reported a slight miss in quarterly revenue, hurt by a decline in non-COVID testing volume versus 2019 due to the pandemic. Second-quarter revenue fell 6.4% to $1.83 billion.

Non-COVID testing volumes are expected to remain below 2019 levels for the remainder of year, the company said on the conference call.

(Reporting by Dania Nadeem and Carl O' Donnell; Editing by Sriraj Kalluvila, Bernadette Baum and David Gregorio)

By Dania Nadeem and Carl O'Donnell