Ontario's official numbers for positive COVID-19 tests fall short of capturing the true extent of the coronavirus' spread in the province, according to infectious disease experts.

That shortfall is being blamed on the limitations of Ontario's testing regime, including its rationing of tests and a backlog in the province's lab facilities that is causing a four-day lag in results.

The official count of 503 people testing positive is "not the complete number of cases," said Dr. Barbara Yaffe, Ontario's associate chief medical officer of health, in a news conference Monday.

"I think that is undoubtedly a major underestimate," said Dr. David Fisman, an epidemiologist and professor at the University of Toronto's Dalla Lana School of Public Health. "But I would temper that by saying that everybody probably underestimates how many cases they have."

The province's guidelines for who gets tested prioritize those who have travelled out of the country. Fisman believes that is too restrictive because the virus is now being transmitted within the province.

The current guidelines mean the province is "only going to find COVID-19 where we expect to find it," said Fisman in an interview. "That's a problem in a rapidly moving epidemic, because we want to find COVID in the places where we don't expect to find it."

While Ontario's assessment centres have been conducting roughly 3,000 tests per day, the province's labs are currently producing only 2,000 to 2,100 test results each day.

"The fact that we can't keep up with this very constrained volume of testing is a concern," said Fisman.

Ontario currently has a backlog of more than 8,400 tests, with people waiting at least four days between test and result.

"We can't see this epidemic if we don't have the test coming through in a timely way," said Fisman. "We're flying blind right now."

He is urging the province to begin processing tests at university and private laboratories, something that Premier Doug Ford says is in the works.

Experts suggest it is critical for Ontario to ramp up its testing capacity now because of the 200,000-plus travellers returning to the province from the U.S., where the virus has been spreading more rapidly.

Ontario Health Minister Christine Elliott answers questions, as Minister of Finance Rod Phillips, right, and Premier Doug Ford listen in. (Frank Gunn/The Canadian Press)

The province is aiming to process 5,000 tests per day. Health Minister Christine Elliott told CBC News that she expects that target will be achieved "by the end of this week."

For Ontario, running 5,000 tests per day would put the province on par with South Korea, a country that has been praised for slowing the spread of the virus in large part through extensive testing to monitor for outbreaks. South Korea has roughly four times Ontario's population and has been conducting roughly 20,000 COVID-19 tests per day.

In addition to lab capacity, another factor slowing the pace of testing in Ontario is a shortage of nasopharyngeal swabs, considered the best way to detect COVID-19 because they take samples from deep within a patient's nasal passages.

Public Health Ontario is telling doctors they can use other types of swabs to collect COVID-19 test samples, but the province's chief medical officer of health warns this method risks missing positive cases.

"Your chance of detecting a small or light infection may not be as good as it should be," Dr. David Williams told a news conference Monday. He said the shortage "should be ending soon" as the federal government has shipped a large order of the preferred swabs.