The head of Ontario's public health agency is temporarily stepping away from his job, a day after the premier called the province's low rate of COVID-19 testing "unacceptable."

Dr. Peter Donnelly, the president and chief executive of Public Health Ontario, took a medical leave Thursday. The reasons have not been disclosed, but the agency says it is not related to the coronavirus.

At a news conference Thursday afternoon, Premier Doug Ford said that Donnelly's abrupt departure had nothing to do with his job performance.

"I want to make it very clear I'm not frustrated with any individual or any group," said Ford. He praised Donnelly as an "absolute champion," and said that he can't wait to welcome him back to the team once his health improves.

Colleen Geiger, who currently serves as the agency's chief of strategy, stakeholder relations, information and knowledge will take over as acting president and CEO, while the rest of the senior leadership team remains in place.

Donnelly took over the public health body in November 2014, leaving a position as a professor of public health medicine at the University of St. Andrews in Scotland. At the time, his strong background in violence reduction and anti-gang strategies was touted as a significant plus for a province that was dealing with a surge in gun crime.

When Canada's first COVID-19 case was diagnosed in Ontario on Jan. 25, Donnelly was front and centre at the announcement, expressing his confidence in the province's pandemic preparations and its plans to test for the disease. The coming storm would be nothing like the 2003 SARS outbreak that killed 44 people in Toronto, he predicted.

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"This was a disease unknown to science only two weeks ago and we now have the full genetic fingerprint of the virus and we have a test, which is specific and reliable," he said. "That really is a game changer, because it means that you can very quickly find out whether people have this or not. These are the tools you need to control this and to stop its spread."

But in recent weeks, testing has turned out to be a significant weak spot in Ontario's response to COVID-19. First, a lack of materials, staff and capacity led to a 10,000-sample backlog, and long waits for diagnosis. Then, as the logjam eased, testing rates plummeted for reasons that have yet to be explained, with fewer than 4,000 samples a day being processed, well below the stated daily capacity of 13,000.

That discrepancy is what set Ford off on Wednesday, as he publicly demanded an immediate improvement.

"My patience has run thin and no more excuses," Ford said during his daily briefing. "It's unacceptable … We say we can do 13,000 a day, then we need to start doing 13,000 every single day."

Later that day, the province appointed a former Toronto public health head, Dr. David McKeown, as a troubleshooter, charged with rethinking the province's response to the pandemic.

As of Thursday, Ontario has tallied 5,759 confirmed cases of COVID-19, and 223 deaths.