COVID-19 test: answers to your questions


We answer your questions about the pandemic. Send yours to [email protected], and we will get back to you as much as possible. We post a selection of responses online and also ask questions of the experts during The National and on the CBC News Network.

We have received over 1,500 questions about the COVID-19 tests from Canadians, and many readers are confused about the process and the results.

We asked your most common questions to the experts. Here’s what you wanted to know about testing.

How long after exposure and infection can you expect a positive nasal swab test?

Unfortunately, there is no set number of days.

And you can only really count on a positive test if you have symptoms, according to Dr. Matthew Cheng, microbiologist and infectious disease specialist at the McGill University Health Center.

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

The COVID-19 Assessment Center operated by the Sudbury Hospital provides driving tests for patients with less serious pre-existing health conditions. (Erik White / CBC)

If I am asymptomatic, will I always be positive?

If the person is asymptomatic, the likelihood that they will test positive decreases considerably, said Cheng, but the researchers don’t know to what extent.

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

How accurate are the COVID-19 tests?

There are many ways to look at it.

As mentioned earlier, in people with symptoms, nasal swab tests detect the disease with an accurate result in about 70% of cases. However, Cheng said the accuracy is much higher in hospital patients with breathing tubes, which are buffered 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% of the time in the laboratory, but in a clinical context, errors can occur when the sample is taken. For example, the swab may not be performed correctly.

That said, if you get a positive test, you almost certainly have COVID-19 – the rate of false positives is very low – less than one percent of tests overall, said Dr. Philippe Lagacé-Wiens, medical microbiologist at St. Boniface Hospital in Winnipeg.

On the other hand, MacDonald said that you cannot have the same confidence about a negative test because it may just mean you’ve been tested too soon, before viral levels are high enough to be reliably measured. This is why people with symptoms who get a negative test result are encouraged to self-isolate anyway.

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

How long am I contagious?

Recent evidence suggests that people are not contagious during the first eight days of symptoms.

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

“It’s a great question,” said Cheng. “If you are tested and you are negative and you have no symptoms, it is unlikely that you are contagious. “

If you have a positive test, how long you are contagious depends on the individual, he added. But if you later test negative, you are no longer considered contagious.

Otherwise, you should definitely not be contagious after two weeks – the length of time authorities recommend that you isolate yourself if you have symptoms of COVID-19 or have been exposed to COVID-19.

Why are there nasal swab and mouth or throat swab tests? Which is more reliable?

Cheng said the two types effectively swab the same part of your body – the pharynx – from different openings. “I think you will get a very similar result,” he said, although he suggested that swabbing the nose was “more comfortable”.

A snapshot of one of the two forms given to you when you arrive at the test center. (Farrah Merali / CBC News)

Isn’t it risky to queue in an assessment center with people who might have COVID-19?

Cheng doesn’t think so. Health workers at assessment and testing centers are well trained and wear appropriate personal protective equipment to protect the patient and themselves, Cheng said. “They are experienced professionals. “

He noted that there were no reports of an epidemic in the test centers.

To get a better idea of ​​the procedures in place to protect those tested, you can read a first-hand account of CBC reporter Farrah Merali.

Should I isolate oneself pending my results?

In general, most provinces recommend doing this.

Most provinces still mainly test people with symptoms or people who may be exposed to the virus.

Cheng said the only case in which you might not need to isolate yourself while waiting for the results is if you had no exposure and no symptoms – “in which case you probably wouldn’t normally have a test. “

Muhammad Junayed was tested for COVID-19 by a health care worker at an automatic testing center at the Islamic Institute in Toronto during the COVID-19 pandemic in Scarborough, Ontario, May 29. (Nathan Denette / The Canadian Press)

Will a COVID-19 test tell me if I have had the disease and have recovered since?

A nasal swab test won’t tell you, said Cheng. Swab tests show that the virus is replicating, which shows that you are currently infected.

However, another type of test – a blood antibody test – can detect a previous infection and a certain level of immunity. In people with symptoms, antibodies start to appear after about a week and culminate a week or two later. We know less about people who never show symptoms.


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