Testing for COVID-19: Your questions answered

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.

If you have symptoms, there is a 70 per cent the disease will be detected with a positive result, said Cheng, who recently wrote a scientific article summarizing research so far on COVID-19 testing. On average, symptoms develop five to six days after exposure, but it can take two to 14 days.

The COVID-19 assessment centre run by the Sudbury hospital offers drive-thru virus testing for patients with less serious pre-existing health conditions. (Erik White/CBC)

If I’m asymptomatic, will I still test positive?

If the person is asymptomatic, the chance that they will test positive drops significantly, Cheng said, but researchers don’t know by how much.

According to the World Health Organization, presymptomatic people can test positive one to three days before symptoms start.

How accurate are COVID-19 tests?

There are a few ways to look at this. 

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.

According to Dr. Kelly MacDonald, head of the infectious disease program at the University of Manitoba, the nasal swab test is accurate 99 per cent of the time in a laboratory setting, but in a clinical setting, errors can happen when the sample is taken. For example, the swabbing may not be done properly.

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.

On the other hand, MacDonald said you can’t have the same confidence about a negative test, as it may simply mean you have been tested too early, before viral levels are high enough to be reliably measured. That’s why people with symptoms who get a negative test result are told to self-isolate anyway.

People line up at a COVID-19 testing centre near Toronto Western Hospital on May 11, 2020. (Evan Mitsui/CBC)

How long am I contagious?

Recent evidence suggests that people are only contagious during the first eight days of symptoms.

However, there is now good evidence that even those who are asymptomatic or presymptomatic can pass on the disease. So how long are they contagious?

“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.”

A snapshot of one of the two forms you’re given when you arrive at the testing centre. (Farrah Merali/CBC News)

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.

For a better idea of what procedures are in place to protect those getting tested, you can read a first-hand account by CBC reporter Farrah Merali.

Do I have to self-isolate while I wait for my results?

Generally, most provinces recommend that you do. 

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.”

Muhammad Junayed gets tested for COVID-19 by a health-care worker at a pop-up testing centre at the Islamic Institute of Toronto during the COVID-19 pandemic in Scarborough, Ont., on May 29. (Nathan Denette/The Canadian Press)

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.

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