We’re answering your questions about the pandemic. Send yours to COVID@cbc.ca, and we’ll answer as many as we can. We publish a selection of answers online and also put some questions to the experts during The National and on CBC News Network.
We’ve received more than 1,500 questions about COVID-19 testing from Canadians, and many readers are confused about the process and the results.
We took your most common questions to the experts. Here’s what you wanted to know about testing.
How soon after exposure and infection can you reliably expect a positive nasal swab test?
Unfortunately, there isn’t a set number of days.
And you can only really count on a positive test if you have symptoms, according Dr. Matthew Cheng, a microbiologist and infectious disease specialist at the McGill University Health Centre.
As mentioned before, in people with symptoms, the nasal swab tests will detect the disease with an accurate result about 70 per cent of the time. However, Cheng said the accuracy is much higher in hospital patients with breathing tubes, who are swabbed in their trachea or lungs.
That said, if you get a positive test, you almost certainly have COVID-19 — the false positive rate is very low — less than one per cent of tests overall, estimates Dr. Philippe Lagacé-Wiens, a medical microbiologist at St. Boniface Hospital in Winnipeg.
“That’s a great question,” Cheng said. “If you are tested and you are found to be negative and you have no symptoms it is not likely that you’re contagious.”
If you have a positive test, how long you’re contagious depends on the individual, he added. But if you subsequently test negative, you are no longer considered contagious.
Otherwise, you should definitely not be contagious after two weeks — the amount of time officials recommend you self-isolate if you have COVID-19 symptoms or may have been exposed to COVID-19.
Why do both nasal swab and mouth or throat swab tests exist? Which is more reliable?
Cheng said both types effectively swab the same part of your body — the pharynx — from different openings. “I think you’ll get a very similar result,” he said, although he suggested nose swabbing may be “more comfortable.”
Isn’t it risky to line up at an assessment centre with people who might have COVID-19?
Cheng doesn’t think so. Health-care workers at assessment and testing centres are well trained and wear appropriate personal protective equipment to protect the patient and themselves, Cheng said. “These are experienced professionals.”
He noted that there are no reports of outbreaks at testing centres.
Most provinces still mainly test people with symptoms or people who had a probable exposure to the virus.
Cheng said the only case in which you might not need to self-isolate while waiting for results is if you had no exposure and no symptoms — “in which case you probably wouldn’t normally get a test.”
Will a COVID-19 test tell me if I have had the disease and have since recovered?
A nasal swab test will not tell you that, Cheng said. The swab tests provide evidence that the virus is replicating, so it shows you’re currently infected.
However, a different kind of test — a blood antibody test — can detect a previous infection and some level of immunity. In people with symptoms, antibodies start to appear after about a week and peak a week or two later. Less is known about people who never show symptoms.