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Health Unit can’t accurately differentiate asymptomatic cases from false positives

June 26, 2020   ·   0 Comments

As testing capacity has increased for COVID-19, its accuracy has remained a problem, so much so that the Simcoe Muskoka District Health Unit (SMDHU) cannot separate asymptomatic cases from false positives.

While false positives are uncommon, they do occur, and are difficult to separate out from asymptomatic carriers of the virus.

“Even a very good test will have a certain degree of false positivity, false negativity from what I’ve read of the COVID-19 PCR tests,” said SMDHU Chief Medical Officer of Health Dr. Charles Gardner.

He noted that even though inaccurate tests are rare, when testing a population with low rates of infection, there’s a “significant likelihood” that many of those testing positive are actually virus-free.

“That’s a potential here, we are treating all tests as positives because that is the safest thing to do,” Dr. Gardner reasoned.

“You’re never able to clearly determine at the end of the day whether or not somebody had a false positive or whether in fact they had a true positive or if it was near the end of their infection and they were clearing it,” he continued.

“If we follow up later and we test them and they’re negative, we’re left with those possibilities and we can never really sort them out.”

While some tests for COVID-19 are only 70 percent accurate, Ontario’s PCR tests are over 90 per cent correct for sensitivity and specificity.

Meanwhile, testing in Ontario saw a big jump two weeks ago when Premier Doug Ford announced people without symptoms should be tested in late May.

This caused a nearly 400 per cent increase in the number of tests done per day or per week across the province.

“It’s dropped a bit since then, but it’s maintained itself very high and in fact we might have had a bit of a jump again when the province made requirements for testing for all workers in long-term care facilities every two weeks and all visitors to long-term care facilities and retirement homes,” Dr. Gardner explained.

“Those requirements have definitely increased the testing further, so it’s quite possible that we’ve gotten levels that are even higher than that.”

Stevenson Memorial Hospital’s (SMH) COVID-19 Assessment Centre saw a threefold increase in foot traffic following Premier Ford’s announcement.

The assessment centre is located in SMH’s lower parking lot and is operating on Mondays, Wednesdays, and Fridays from 3-7 p.m.

“You have to remember and bear with us, we have a dated infrastructure, so it’s an outdoor swabbing process and you bear with our staff who are wearing lots of PPE,” said SMH CEO Jody Levac.

A solid indicator of determining the breadth and depth of testing is the per cent positivity, which is the percentage of tests that come back positive.

Typically, this number is in the range of about two to three per cent.

Last week, however, it’s been down to 0.7 per cent in Simcoe Muskoka and Ontario’s been at 0.8 per cent, so there is more than enough testing happening both locally and provincially.

By Sam Odrowski
Local Journalism Initiative Reporter


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