Ontario began “enhancing” the pandemic data it provides to the public earlier this week, but provincial health officials are not reporting more than 1,000 cases that are very likely to be COVID-19 but lack lab confirmation because of test rationing, backlogs or flawed results.

Ontario’s official tally of COVID-19 cases — 2,392 as of Wednesday — only counts laboratory-confirmed cases. Experts have warned that this number significantly underestimates the true spread of disease, both because constraints on lab capacity means not everyone with symptoms is being tested and because testing backlogs mean today’s results reflect infections that occurred weeks ago.

Local public health units, however, also report “probable” cases to the province. Ontario’s Ministry of Health would not provide the total number of probable cases, but the Star’s survey of health units this week counted more than 1,100. That number is likely to fluctuate given the rapidly evolving epidemic.

Probable cases are people who have symptoms and whose travel history or close contact with another case makes them very likely to have COVID-19, but whose laboratory test results are unavailable, inconclusive or negative.

Some don’t have lab confirmation because with limited capacity, tests are being prioritized for sicker patients and for high-risk groups like health-care workers. Others are awaiting their test results, though not everyone with a pending test result will count as “probable.” Some received a negative result that seems suspect, given their history or contacts.

Ontario does not include these probable cases in its daily total or anywhere in its public information. Some local public health units have begun to report probable cases totals on their own websites, however, including Toronto.

On Tuesday, Toronto Public Health reported a total of 793 COVID-19 cases. Of those, 628 cases are laboratory-confirmed, and 165 are probable.

A day earlier, Dr. Barbara Yaffe, Ontario’s Associate Chief Medical Officer of Health said that these probable cases are not a “high priority.”

“We know some public health units are entering them and some are not. And they’re not as high priority as the confirmed cases,” Yaffe said at a press conference Monday in response to the Star’s questions.

“There should not be too many probable cases … it’s related to lab testing capacity, and as you hear, lab testing capacity is going up. So hopefully we’ll be dealing mostly with confirmed cases.”

Experts disagreed.

“This is vital information we need to know,” said Isaac Bogoch, an infectious disease expert at Toronto General Hospital Research Institute and the University of Toronto.

“This gives us a much better understanding of what the true burden of illness is in Ontario. This can be factored into some of the equations that we use to determine whether or not our curve is truly being flattened. It seems unusual that this information is available but not being shared.”

Bogoch said it was “great” Toronto was sharing the probable case count on its website.

“We’re all for honesty, openness and transparency of data. That includes sharing the good news but also sharing the bad news as well.”

The Star contacted public health units across the province to gather the number of probable cases. Local health units, which are charged with managing the COVID-19 response in their region, report new COVID-19 cases to the province through an integrated tracking system. Ontario’s official definition of a COVID-19 case includes both lab-confirmed and probable patients.

A small number of public health units said they didn’t have any cases that met the “probable” criteria, or didn’t use this definition, focusing on lab-confirmed cases only. Some didn’t answer the Star’s requests, and one — Grey Bruce — claimed that the information was “proprietary to the province.”

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Others reported the number online or provided it to the Star. Earlier this week, Southwestern had five lab-confirmed and 30 probable cases, Sudbury had seven lab-confirmed and three probable cases, Peel had 197 lab-confirmed and eight probable cases, and Wellington-Dufferin-Guelph had 40 lab-confirmed and 15 probable cases. Those numbers have likely since shifted, given how quickly the outbreak is evolving.

Hastings-Prince Edward now releases information about its probable case total online as well. The region, which has 11 lab-confirmed cases, has 253 probable cases as of Wednesday, up from 219 from two days earlier.

On Tuesday, Middlesex-London said it had 640 probable cases on top of its 52 lab-confirmed cases. A spokesperson for the health unit emphasized that this number was dynamic, changing daily or even hourly. While the number of lab-confirmed cases would continue to rise, the number of probable cases could decrease, as people with pending test results are either moved into the “confirmed” category or are ruled out.

Some of those “probable” cases would never be tested for clinical reasons, the spokesperson said.

Doctors may choose not to test patients with mild respiratory symptoms: if a patient is well enough to recover at home, and if they, say, just got back from northern Italy or lives with their brother who is a confirmed case, it would not change the clinical management of the case to get what is very likely to be a positive test result. A test of insignificant clinical value might be considered a waste of limited laboratory resources.

However, experts say that widespread testing is still important because it gives us a truer sense of how wide the scope of the outbreak is here.

Provincial labs are scaling up testing, with new hospital-based and other facilities coming online to add to the volume processed by Public Health Ontario’s primary lab. Ontario’s top doctors say the volume of tests processed daily will rise above 15,000 by later in April, and the testing backlog is shrinking daily.

“On the one hand, we should be happy that we’re expanding diagnostic testing now. But on the other hand, during a critical time in Ontario’s epidemic we were still restricting diagnostic testing when we really need to have a much better understanding of what the true burden of illness is in the province, and that’s disappointing.”

On Wednesday, Yaffe confirmed that the province wouldn’t include the probable cases in its tally.

“Until we know that all the health units are actually counting probable cases and inputting them into (the province’s integrated case-tracking database), we would not be able to count them because it would not give us a complete picture.”

“As the lab capacity goes up, there will be less probable cases. That is my hope. So it’s good that the health units have decided, those that are entering the data, to put it out. That’s fine. But until they’re all entering it we really can’t put it in.”

Even the probable and confirmed cases combined still don’t capture the true burden of illness in the province, however, since some people with mild symptoms never seek health care at all.

Bogoch credited Ontario for adding more data to its case information website earlier this week, calling it a good start. But he called the probable case counts necessary information.

“How many confirmed cases, how many probable cases? This is textbook epidemiology. This is textbook epidemiology. We need to know.”