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Almost a third of COVID-19 test results could be false negatives, B.C.’s top doctor says

B.C.’s provincial health officer says that mass testing continues to be an ineffective strategy to slow transmission of COVID-19 because it has a false negative rate as high as 30 per cent.

At her daily briefing on Monday, Dr. Bonnie Henry said testing is not as sensitive as health officials expected it to be earlier in the crisis.

“The testing, unfortunately, doesn’t tell us the whole story. People can be negative one minute and positive within an hour.”

Testing in B.C. is currently being limited to those who develop coronavirus symptoms who are hospitalized, residents or staff of long-term care facilities, part of an outbreak investigation, or health-care workers.

“The norm is not to test people who do not have symptoms because we know the test doesn’t perform very well and we can have false negatives,” Henry said.

However, Henry did say that there is a low threshold for testing anybody with symptoms in a care home to try to get ahead of outbreaks.

Tests are reliable, says expert

While a false negative rate of 30 per cent can raise concerns about the reliability of COVID-19 testing in Canada, microbiologist Craig Jenne says that’s the worst-case scenario.

“You’re probably talking [about] a 30 per cent failure rate with a non co-operative patient, and somebody who has never done a nasal swab before, and the sample then ended up in somebody’s car being transported to a lab for three days before it got processed,” he said. 

Microbiologist Craig Jenne says throat swabs are subject to human error. (Paul Chiasson/The Canadian Press)

According to Jenne, associate professor in the department of microbiology, immunology and infectious diseases at the University of Calgary, the reality is that labs won’t accept anything with a false negative rate greater than five per cent.

He says there are several reasons testing can be unreliable. For example, someone in the early stages of infection may not have enough viral replication in their body to be detected yet.

In other cases, a throat swab may not reach the virus.

Testing, he says, is most useful to give health officials a snapshot of how far into the community the virus has spread.

“We can’t guarantee that the test is going to catch 100 per cent of the infected people so we’re using it really as a confirmation of people with symptoms,” Jenne said.

False negatives can still be infectious

Jenne says people in the early stages of infection can, unfortunately, still transmit the virus, even if they test negative and don’t have symptoms.

He says 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.”

‘The norm is not to test people who do not have symptoms because we know the test doesn’t perform very well and we can have false negatives,’ provincial health officer Dr. Bonnie Henry said Monday. (Darryl Dyck/The Canadian Press)

Both Jenne and Henry maintain that without physical distancing measures, testing is an ineffective way of slowing the spread of the coronavirus.

“We’re still seeing viral spread even in the absence of testing,” said Jenne. “So I don’t know how large levels of testing is going to get us back to a normal life any quicker because everyone’s still having to follow the same infection.”

Henry says the strategy in B.C. prioritizes prevention, whether its by assessing health-care workers for symptoms on a daily basis, wearing protective equipment, or general physical distancing.

If you have a COVID-19-related story we should pursue that affects British Columbians, please email us at impact@cbc.ca. 

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