White Coat, Black Art·THE DOSE

If I test negative for COVID-19, am I clear?

We’ve heard a lot of talk about coronavirus tests. But what do the results of a test for such a new virus really tell you? Will there be tests to help determine potential immunity? Infectious disease expert Dr. Jeffrey Pernica joins host Dr. Brian Goldman on this week's episode of The Dose.
A health-care worker collects a sample at a drive-through COVID-19 testing facility in Alberta. (Alberta Health Services)
Listen to the full episode26:45

A negative COVID-19 test result doesn't mean you can afford to ease up on handwashing and physical distancing measures, or that it's safe to go back to work, infectious disease specialists say.

With limited supplies of test materials — as well as the people to both administer the test and process the results in labs — not everybody who has wanted to know their COVID-19 status has been able to get tested.

But if you do receive a test, does a negative result mean you can truly rest easy? 

No it doesn't, said Dr. Jeffrey Pernica, an infectious disease specialist and pediatrician at McMaster University in Hamilton, Ont. 

Pernica told Dr. Brian Goldman, host of the CBC podcast The Dose, that while COVID-19 tests have a high degree of accuracy, what matters most is how much the disease is circulating in the community.

"Don't forget, there's always chances of picking it up from somebody at the grocery store. We are not going to be able to fully eliminate transmission of COVID-19 over the next number of months." 

How important is timing?

You could receive a negative test result one day, but come in contact with the virus the next time you're picking up milk, eggs or a prescription.

Microbiologist Craig Jenn said proposals for widespread testing to allow people to return to work don't take into account the fact that tests are time sensitive. 

"Somebody could, for example, test negative today, be exposed at lunch, and then be infected by tomorrow," said Jenn, associate professor in the department of microbiology, immunology and infectious diseases at the University of Calgary. 

He said people in the earliest stages of the infection can still transmit the virus, even if they test negative and don't have symptoms.

Most COVID-19 testing is done through nasopharyngeal swab, said Pernica. A long Q-tip-like swab is inserted through the nostril, way back into the passage where the back of the nose meets the very top of the throat.

Dr. Jeffrey Pernica, an infectious disease specialist and pediatrician with McMaster University in Hamilton, Ont., says Social distancing and self isolation are critical, 'because there will always be people with false negatives.' (Craig Chivers/CBC News)

"What we're trying to do is get a swab of the lining ... as well as, you know, little secretions that are there, to be able to see if we can detect the nucleic acid, the RNA of the COVID virus," he said. The RNA of a virus is similar to DNA in human beings.

There's a peak window for getting this sample. "With COVID in particular, it seems as though the amount of virus in the human nose is very high in … the first five to seven days … Unfortunately, as people get sicker, it has been observed that there's less virus in the nose and more virus lower down in the respiratory tract."

People with more advanced disease can be diagnosed clinically based on presentation of symptoms, chest scans or through other tests.

How accurate are the tests?

Earlier on, there was some concern — based on initial studies from China — that the nasopharyngeal swab for COVID-19 wasn't as accurate as it is for other respiratory illnesses, said Pernica.

Since then tests have been refined. "The test that we are now using at the Hamilton Regional Laboratory Medicine Program is estimated by the microbiologist there to be around 95 per cent sensitive, which is really quite good," he said. That means the test would detect 95 out of every 100 COVID-19 infections.

WATCH | See what it's like to get tested for COVID-19.

A look at what patients could expect if they end up in an emergency room with COVID-19 symptoms. 1:59

But because of this margin of error — and since people can be asymptomatic for days after contracting the virus, sometimes never becoming ill — following public health directives is essential regardless of your testing status.

"Social distancing, self isolation, are critical, because there will always be people with false negatives," said Pernica. "And if we adhere to these recommendations, the ramifications of occasionally getting it wrong will be much, much reduced."

What about these rapid blood tests we've been hearing about?

Outside of Canada, some jurisdictions are using pin-prick blood tests that detect the presence of COVID-19 antibodies in the bloodstream, an indicator that the body has developed immunity to the illness. Results are shown in about 15 minutes.

One Markham, Ont.-based company, BTNX, has been selling the tests to the U.S., but is not yet permitted to do so in Canada.

Health Canada told The Dose that while serological (blood) testing "will help increase diagnostic capacity, provide a means for studying community transmission and exposure rates, and the efficacy of new treatments," developing a reliable one for COVID-19 "has its challenges."

"The National Microbiology Laboratory and its partners are in the process of assessing a number of serological tests and collecting samples to evaluate them," a Health Canada spokesperson said in an email to CBC. 

Pernica said the development and approval of a reliable blood test will be key to managing future waves of Canada's outbreak in the months to come, and that he has faith that talented researchers are working to make that happen. "It would be extremely useful to figure out who definitely had COVID and is likely to be immune, and who didn't have COVID and is likely to be susceptible."

"We will need to know what healthcare people to deploy where… I think it'll be important to know what firefighters can actually attend to all of these medical calls, which paramedics can, which people can work in grocery stores and … for all of these essential services."


Written by Brandie Weikle. Interview produced by Nicole Ireland and Dawna Dingwall. With files from Eva Uguen-Csenge.
 

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