Rapid tests an important, if imperfect, tool in potential sixth wave, say experts

By Adina Bresge
THE CANADIAN PRESS

Experts say rapid antigen tests are emerging as an important tool as Canada’s pandemic strategy shifts from public health vigilance to an emphasis on personal responsibility.

But they warn that rapid test results should be read with caution because of limitations on diagnostic accuracy and precision.

A professor of biomedical engineering and immunology at University of Toronto says rapid tests are an easy and convenient way to help Canadians make better choices about their health and the safety of others.

But Omar Khan says rapid tests work best when paired with public surveillance strategies that can track the spread of variants to help inform health policy.

Dr. Christopher Labos, a Montreal cardiologist with a degree in epidemiology, says it takes more than one rapid test to rule out infection because of high rates of false negatives.

Labos says potential sources of error include improper technique, testing too early in the illness for viral levels to be detectable and preliminary data suggesting rapid tests are less sensitive to the Omicron variant.

In February, the Ontario COVID-19 Science Advisory Table published findings from an analysis of preprint studies suggesting that the pooled sensitivity of rapid antigen tests for detecting Omicron infections is about 37 per cent, compared to 81 per cent for the Delta variant.

Health Canada says on its website that it has “no evidence” that variants affect the ability of agency-approved tests to confirm COVID-19 cases, but notes that these new devices are still being investigated.

“Now that we’re into the ‘judge your own risk’ phase of the pandemic … I think people need to realize that the risk of false negatives with rapid tests is very real,” Labos said, noting that evidence suggests the rate of false positives is fairly low.

“If you take your negative result as a license to return to normal, you might be inadvertently infecting other people, so I would continue to isolate, repeat the testing and make very sure you don’t have COVID.”

Ontarians aged 60 and older can start booking appointments for a fourth dose of a COVID-19 vaccine Thursday.

First Nations, Inuit and Métis individuals and their non-Indigenous household members aged 18 and above are also eligible to start booking those shots Thursday.

Fourth doses are being offered at a recommended interval of five months after the initial booster shot.