TORONTO -- Some parents have been flocking to COVID-19 testing centres, looking to rule out the possibility their children are carrying the infection after reports of exposures happening in schools across Canada.

While it seems like a responsible move, one expert says a negative result from being tested too soon in the virus’ incubation period may give people the false impression that they cannot spread the virus.

Dr. Matthew Oughton, an infectious diseases specialist at the Jewish General Hospital in Montreal, says getting a negative test result one day, for example, doesn't necessarily mean you're risk-free in the following days.

"Yes, you're negative in terms of it can't detect any virus in your body at the time the test is taken, but several days from that point, you may still go on to develop symptoms. That forms the basis for why we tell people that if they test negative after what is deemed to be a significant exposure, they should still self-isolate because of the risk of false negatives," Oughton said in a telephone interview on Monday.

The period between contracting the novel coronavirus and displaying symptoms -- for those who do show symptoms -- can be anywhere from zero to five days or more, Oughton said.

He added that public health measures, such as wearing a mask and physical distancing, are still needed when interacting with others, rather than relying on a negative test that may have been taken during the virus' incubation period.


While the current PCR diagnostic nasal swab can pick up very small amounts of the virus, Oughton said the risk of receiving a false negative result during that incubation period is much higher.

"If you were to be tested on the same day of exposure, quite bluntly, there is probably far too little virus in you, if you really were exposed, to be detectable by the usual test. So being tested on the same day of exposure or even one or two days afterwards comes with it a high rate of false negatives," he said.

Oughton explained that a person could test negative and develop symptoms days later, potentially leading to an infected person spreading the virus.

However, he acknowledged that the same is not true for false positives.

"The rate of false positives with this particular test is quite low. In other words, if the test comes back saying positive, then believe it, it's a real positive," Oughton said.

While a false result can occur at any point in the incubation period, Oughton says it is "worth waiting for testing for at least a few days after the exposure event" or until symptoms develop. He said people should self-isolate while they wait.

"No matter how many days you wait, there is always going to be an appreciable rate of false negatives, which is what forms the basis for the recommendations of isolating, even if you do get a negative test," Oughton said.

It gets tricky, Oughton said, when schools require a negative test result for children to return to the classroom.

He explained that parents have to do what is required of them by local public health authorities and by their respective school boards.

"If the instructions are go home, get tested and come back assuming your test is negative and you don't have any symptoms, then do that, but at least from the science that still comes with a risk of false negatives," he said.


Canada's testing criteria was limited in early stages of the pandemic, partly to ensure access for those most at risk of contracting COVID-19. But some provinces began expanding criteria over the summer to include anyone who wanted a test.

Ontario and Alberta are among those that say they will test anyone worried about COVID-19 exposure, regardless of whether they have symptoms.

Manitoba, meanwhile, asks people to use an online self-assessment tool "to be certain they meet the necessary criteria" but says anyone with symptoms should get tested.

The B.C. Centre for Disease Control says on its website that testing is "not recommended for asymptomatic individuals" yet may be encouraged regardless of symptoms as part of an outbreak investigation.

Infectious disease specialist Dr. Isaac Bogoch told CTV News Channel last week that policymakers should not impede Canadians' ability to get tested, but also not encourage over-testing.

As long lines are being reported at COVID-19 testing centres across the country, the federal government has pledged billions in funding to address the issue and improve other pandemic measures.

Bogoch explained that testing wait times are affected by a combination of factors, including limited testing capacity and increased demand.

"The capacity currently is significantly better than what it was in for example March or April of this year, but clearly it's not where it needs to be," he said.

In an effort to meet the increasing demand for tests, provinces are scrambling to introduce more testing sites, such as at pharmacies in Ontario and Alberta, mobile clinics in Quebec, and an easier method of testing using a "mouth rinse gargle" in B.C.

However, Bogoch said this still might not be enough. To address the capacity issue, he said provinces may have to change their messaging around testing.

"Given the snapshot that we're in right now, maybe it's best for messaging to focus on people to get tested if they're either at risk for getting this infection, if they have any signs or symptoms of infection regardless of how mild, or if they've had any possible exposures to this infection," Bogoch said.

"Certainly those individuals should be prioritized, but in the same breath of course, you shouldn't be turned away from a testing centre," he added.

With files from The Canadian Press