Why getting tested too early for COVID-19 can give a false sense of security


TORONTO – Some parents have flocked to COVID-19 testing centers, seeking to rule out the possibility of their children carrying the infection after reports of exposures in schools across Canada. Although it seems like a responsible decision, an expert says that a negative result after being tested too early during the incubation period of the virus can give people the false impression that they cannot spread the virus.

Dr. Matthew Oughton, an infectious disease specialist at the McGill University Health Center and the Jewish General Hospital in Montreal, says that getting a negative test result one day, for example, doesn’t necessarily mean you’re safe for them. following days.

“Yes, you are negative because you cannot detect any viruses in your body at the time the test is taken, but several days from that time you may continue to develop symptoms. This is why we tell people that if they test negative after what is considered significant exposure, they still have to self-isolate due to the risk of false negatives, ”Oughton said during ‘a telephone interview on Monday.

The period between contracting the novel coronavirus and the onset of symptoms – for those with symptoms – can range from zero to five days or more, Oughton said.

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


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

“If you were to be tested on the same day of exposure, in a fairly straightforward fashion, there is probably far too little virus in you, if you have really been exposed, to be detectable by the usual test. So being tested on the same day of exposure or even a day or two after that comes with a high false negative rate, ”he said.

Oughton explained that a person could test negative and develop symptoms a few days later, which could lead to the virus being spread from an infected person.

However, he admitted that the same was not true for false positives.

“The false positive rate 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 time during the incubation period, Mr Oughton says it “is worth waiting for testing for at least a few days after the exposure event” or until until symptoms develop. He said people should isolate themselves while they wait.

“No matter how many days you wait, there will always be an appreciable rate of false negatives, which is the basis for the isolation recommendations, 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 class.

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

“If the instructions have gone home, get tested and come back assuming your test is negative and you have no symptoms, then do it, but at least from the science that still carries a risk of false negatives, ”he said. .


Canada’s screening criteria were limited to the early stages of the pandemic, in part to ensure access to those most at risk of contracting COVID-19. But some provinces started broadening the criteria over the summer to include anyone wishing to take a test.

Ontario and Alberta are among those saying they will test anyone worried about exposure to COVID-19, whether or not they have symptoms.

Manitoba, meanwhile, is asking people to use an online self-assessment tool “to make sure they meet the necessary criteria,” but says anyone with symptoms should get tested.

The BC Center for Disease Control says on its website that testing “is not recommended for asymptomatic people,” but can 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 hamper the ability of Canadians to get tested, but also not encourage over-testing.

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

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

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

In an effort to meet the growing demand for testing, provinces are working to introduce more testing sites, such as pharmacies in Ontario and Alberta, mobile clinics in Quebec and a simpler method. test using a “mouthwash gargle” in British Columbia

However, Bogoch said that might not be enough. To address the capacity issue, he said provinces may need to change their message regarding testing.

“Given the snapshot we’re in right now, maybe it’s best for the messaging to focus on people to be tested if they are at risk for this infection, if they are showing signs or symptoms of infection. , regardless of their severity or whether they have had possible exposures to this infection, ”Bogoch said.

“Certainly these people should be given priority, but in the same breath, of course, you shouldn’t be fired from a testing center,” he added.

With files from the Canadian Press


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