The primary type of testing for the novel coronavirus around the world, including Canada, produces “false-negative” results at least 20 per cent of the time, researchers from Johns Hopkins University found.

According to a study published in the Annals of Internal Medicine in May, the false-negative rate of RT-PCR testing used to detect the novel coronavirus changes depending on where a person is in the timeline of the infection cycle.

On day 1 of an infection, the test is completely ineffective at detecting the virus, while on day 8 of infection the test produces false negatives 20 per cent of the time, the study found. The rate of false-negatives then increases every day afterward.

On day five of infection, which is when symptoms of COVID-19 typically begin to appear, and when people are often encouraged to seek out testing, the study found the false-negative rate is 38 per cent.

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“The probability of a false-negative result declines throughout the course of infection,” said Dr. Lauren Kucirka, a resident physician at Johns Hopkins and co-author of the study.

“If you test someone immediately after they’re infected,