Testing positivity chart

Screenshot of a chart presented at Friday's White House Coronavirus Task Force briefing showing Arizona second to Texas for highest 7-day average for testing positivity.

Arizona has made national news for a sharp spike in coronavirus cases but that number — up 40 percent in the past week — isn’t the only or best indicator of what the virus is doing here. 

The state had 66,458 COVID-19 cases as of Friday, and nearly 20,000 of those were reported in the past week. But Arizona has dramatically increased its testing, as have most states, which means a high number of new cases shouldn’t be a surprise. 

What does matter? The percentage of positive test results.

And those numbers give the state cause for concern.

Jennifer Nuzzo, an epidemiologist with Johns Hopkins Bloomberg School of Public Health, said in a Washington Post opinion column Monday that increased testing alone does not explain the rise in COVID-19 cases

“We have to look not just at the number of tests each state is doing, but at the positivity rate of those tests,” Nuzzo said. “In combination, those two metrics give a sense of whether infections are rising, declining or holding steady.”

She said the World Health Organization recommends that countries seeking to lift social distancing restrictions should be doing enough testing to maintain a test positivity of less than five percent.

Last week Vice President Mike Pence said in a blog post, “Every state, territory and major metropolitan area, with the exception of three, have positive test rates under 10%.”

Nuzzo said positivity nationwide is just under five percent but that more than 20 states are over that mark, Arizona included. Statewide, 67,235 people were tested last week and 14,107 cases were reported, Nuzzo said. That’s just under 21 percent. 

“At a glance, it may look as if Arizona found more cases last week because it tested more people, but the reality is that new cases outpace the spread of testing,” she wrote.

Friday, Dr.Deborah Birx, a member of the White House Coronavirus Task Force, displayed a chart at a briefing that showed Arizona second to Texas for the highest seven-day average for testing positivity in the nation, at higher than 16 percent.  

The Arizona Department of Health Services data dashboard indicates, as of Friday, 479,330 viral tests have been administered statewide with 66,458 confirmed cases. What is not clear is whether the AZDHS considers a positive antibody test a confirmed case.

The website reports 11.1 percent positivity for the viral test but if the number of cases is divided by the total number of viral tests it results in a different number than 11.1 percent. 

The Green Valley News reached out to the agency for clarification on how they calculate positivity but did not receive a response.

In a phone interview Friday, Nuzzo told the Green Valley News that health agencies should not be lumping serology and viral tests together when calculating positivity.

"They shouldn't because they are not a diagnostic test."

She said it's possible that doing so could distort positivity. "It's not a huge difference in some places but that could change with an increased use of [serology] tests."

The AZDHS data dashboard reports 157,620 serology tests have been administered as of Friday.

Nuzzo said she supports health departments allowing others to access the data they collect as California has recently done.

“I think it’s a great idea. Transparency is really key for battling this pandemic. People need to be able to understand what the trends are in their communities so that they know what level of protective action to take," she said.

"I think it’s only positive for public health for there to be greater transparency," Nuzzo said.

There is a two- to nine-day delay between the time someone is tested and the results come back. 

Tracking the number of cases in Pima County, the Green Valley News found that between June 8 and June 12, 530 cases and 5,786 tests were reported. Between June 15 and June 19, there were 1,109 cases and 6,201 tests reported for the work week.

As of Friday a total of 6,836 cases and 61,271 tests were reported.

Total positivity for the nation since the start of the pandemic is 10 percent, according to the CDC website.