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Colorado’s COVID-19 test positivity rate, a crucial metric, may be off

John Ingold
The Colorado Sun

When the coronavirus pandemic besieged Colorado last spring, Boulder Mayor Sam Weaver and a few of his friends quickly developed a new hobby — downloading data on the virus’ spread in the state and plugging it into their own spreadsheets for analysis.

Weaver is an engineer by training; data is oxygen for him. He said having numbers for Boulder and other jurisdictions at his fingertips provided fast insight into the pandemic. Where was it spreading? Where was it in decline? What could his city do differently?

But there was one measurement where Weaver and his friends soon noticed something curious: Test positivity. The figure, expressed as a percentage, shows how many coronavirus tests in a given area are coming back positive. Generally, anything below 5% shows that the virus is under control, according to the World Health Organization. Currently, CDPHE says the positivity rate over the past week in Colorado (as of Monday) is 8.1%.

The number comes with particularly high stakes now because Colorado health officials use it as one of three measurements to determine whether restrictions on businesses and other gathering spots should be increased or relaxed in a county. Elevated positivity numbers contributed to the Colorado Department of Public Health and Environment’s decision last week to move several counties to tighter restrictions.

In theory, test positivity is a simple calculation. You take the number of tests that come back positive, divide by the total number of tests administered and multiply by 100. But when Weaver and his friends did that for their own calculations, they could never recreate the positivity figure the state reported for Boulder.

Usually, it was off by just a couple of percentage points. But, during the spike in cases this fall that coincided with the return of students to the University of Colorado campus, Weaver said his calculations put test positivity at 10 percentage points higher in Boulder County than the state’s numbers did, meaning the virus’ spread was worse than what was being reported.

So what was going on?

“It took me awhile to sort all this out,” he said.

This story is being published partially online and in full in select print editions through an agreement with the Colorado Sun. Read the full story online at coloradosun.com.