//Test positivity rates rise across California. What does that mean?

Test positivity rates rise across California. What does that mean?

a screenshot of a cell phone: The Chronicle's Coronavirus Tracker includes trends in California's coronavirus positive test rate.

As coronavirus cases surge again across California, many counties have paused or rolled back their reopening plans. Among the key factors driving officials’ decisions are record-breaking days of new confirmed cases and higher positive test rates.

On Monday, Gov. Gavin Newsom announced that the state’s percentage of people who tested positive had climbed to its highest level in months: 5.9%.

Here is what you need to know about what officials call the positivity rate: What it means and how it is measured, what it tells health officials, and how it has changed in California over time.

Follow these and other trends in The Chronicle’s Coronavirus Tracker.

What is a coronavirus positivity rate?

The positivity rate indicates the percentage of total coronavirus tests conducted that come back positive.

The World Health Organization’s recommended positivity rate for reopening is 5% or below over a period of 14 days, according to John Hopkins University’s Coronavirus Resource Center.

California’s standard for its counties to move ahead with reopening is 8% or below over seven days.

What does the rate tell us?

According to Johns Hopkins, if the positivity rate is higher than expected, it might mean that there’s an oversaturation of positive cases in testing, meaning that only those who feel sick enough are the ones being tested, or that the state isn’t casting a broad enough net in its testing efforts to know how widespread the virus really is.

A low positivity in testing data might indicate that the state has sufficient testing capacity, and is testing enough of its community to weigh the costs and benefits of reopening.

California’s positivity rate over time

An analysis of a positivity rate must take into account the number of daily total tests over time. As more people get tested, the positive test rate provides a more clear picture of how many people are carrying the virus, including those without symptoms, and the possibility for a second surge in cases.

At the beginning of California’s shelter-in-place in early March, when fewer tests were available — less than 1,000 were being administered every day — the state’s positivity rate was 2.9%. Within two weeks, it spiked to 10.7%. By the end of March, as testing ramped up, it had risen to 23.4%, with a couple of drops, but then spiked to 28.8%. By the beginning of April, the positivity rate was at almost 50%.

By mid-April, the rate was hovering below 10% and eventually leveled out to below 5% as reporting became more consistent.

For most of June, positivity rates hovered below 5% but jumped to 5.3% on June 23, about a month after parts of the state moved forward with next phases of reopening: outdoor and some indoor dining at restaurants and bars.

In his news briefing on Tuesday, Gov. Gavin Newsom said the state’s positivity rate had jumped from 4.4% to 5.6% in the two weeks ending last week. And in the past seven days, he noted, the rate rose further to 5.9%.

“That’s a point of caution, point of consideration, and obviously a point of concern,” Newsom said.

Why say ‘test positivity rate’ instead of ‘rate of positive tests’ or ‘positive test rate’?

There is no stated reason to use the term “positivity rate” instead of those terms, but it has recently become popular among health officials.

Annie Vainshtein is a San Francisco Chronicle staff writer. Email: avainshtein@sfchronicle.com Twitter: @annievain