Health·CBC Explains

With COVID-19 test numbers down, what does the test positivity rate mean?

COVID-19 testing rates are way down. But the daily case numbers can still reveal some important information, such as the test positivity rate – which is the percentage of the total COVID-19 tests performed that produce a positive result.

Rate provides an idea of the amount of community transmission

A woman wearing a mask, gloves and other protective gear carries out a nose swab on a man.
A nurse administers a COVID-19 test to a man on Feb. 1, 2021. Amid soaring Omicron numbers, provinces and territories no longer have the resources to test every one who has symptoms or has come into contact with a person who tested positive. (Evan Mitsui/CBC)

Amid soaring COVID-19 case numbers fuelled by the highly transmissible Omicron variant, many provinces no longer have the capacity to test everyone who has symptoms or has been in contact with someone who tested positive. 

That's led health officials to warn that the true number of COVID-19 cases in Canada is likely far higher than what is being reported by the provinces and territories.

But another, related statistic can still reveal some important information; the test positivity rate is the percentage of the total COVID-19 tests performed that produced a positive result.

It's a useful way to understand the level of community transmission. A high test positivity rate — even when the overall number of tests done is low — shows that there's a high level of community transmission.

But it's important to remember that a test positivity rate of 50 per cent does not mean that half of the population has COVID-19. 

On Friday, for example, the tiny Bearskin Lake First Nation in northern Ontario reported a test positivity rate of more than 50 per cent. But the portion of the population that tested positive was just somewhere over 10 per cent. 

While the test positivity rate can give someone a good idea about the level of risk in their area, it doesn't tell the entire story. 

It's important to keep in mind that when a province isn't testing enough people and people aren't seeking out tests, the test positivity rate will not reflect the true picture, just like the case count. The rate also does not include rapid test results, which are normally done by individuals and not reported to officials.

Omicron appears to lead to less severe illness, which means many cases could go untested. So it is important to also look at local hospitalization numbers, ICU capacity and even wastewater testing to get a true sense of the size of the Omicron surge. 

When the percentage of cases that are testing positive starts to level out and come down, that will be the indicator that the Omicron wave has peaked.

The test positivity rate does not take into account rapid tests, like those seen here, which are normally done by individuals and not reported to officials. (David Horemans/CBC)