What Is Arizona's COVID-19 Positivity Rate? That's Harder To Answer Than You Might Think

By Katherine Davis-Young
Published: Friday, August 14, 2020 - 5:05am
Updated: Friday, August 14, 2020 - 1:37pm

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Governor Doug Ducey
Howard Fischer/Capitol Media Services
Governor Doug Ducey discusses Arizona's COVID-19 testing positivity rate during a July 2020 press conference.

When Gov. Doug Ducey or the state’s public health officials talk about benchmarks for reopening schools or businesses, they talk a lot about the percentage of COVID-19 tests that turn up positive.

The state’s Department of Health Services reports diagnostic test positivity, antibody test positivity, daily rates, weekly rates, and an all-time rate. But the positivity rates the state reports are consistently lower than what’s reported by other sources.

“There’s this difference between what the state says at those weekly press conferences and what you can just go to Johns Hopkins and see," said Will Humble, former director of Arizona’s Department of Health Services. Humble said there’s a lot of confusion surrounding these figures.

Take the first week of August, for example. The state reports about 8% of tests that week were positive. The state’s largest lab, Sonora Quest, shows 11% positivity among its tests that week. And for the same time period, ASU’s Biodesign Institute reports about a 13% positivity rate.

“The data sourcing environment is evolving, complicated and frequently challenging," said Jonathan Kreiss-Tomkins, one of the founders of COVID Act Now — that’s a data tracking website that reports on the virus across all 50 states with the goal of informing policy decisions related to the pandemic. 

Like many data aggregators, Kreiss-Tomkins' site is measuring positivity by taking the daily number of new positive cases reported and comparing that to the total daily number of tests reported. But he said each state has a slightly different way of reporting these numbers. 

“The fact that we don’t really have a truly coordinated national or federal response, unlike every other developed country in the world, makes it much, much more challenging," Kreiss-Tomkins said. 

So state-by-state comparisons can be apples-to-oranges. But if COVID Act Now uses numbers reported by each state, why doesn’t its Arizona positivity calculation at least match what Arizona calculates?

One difference: the state looks at test results by the day a test was performed. COVID Act Now, and many other COVID-19 tracking sites look at tests by the day results were reported. Since Arizona has had some long test reporting delays, that means those figures come out differently.

But Humble said, the biggest difference in calculations comes from what’s actually getting counted.

“[DHS] is saying, ‘oh, let’s just throw out half the data set,’” Humble said.  

Arizona’s Department of Health Services did not provide an interview for this story, but in response to questions via email, the department confirmed it is excluding some reported cases from positivity rate calculations.

The department said some laboratories have only been reporting positive results to the state. Those results get reported as new cases, but the state doesn’t count them toward the positivity rate, since more positives without more negatives would skew the positivity rate to appear higher.

“There’s a transparency issue and there’s a management issue," Humble said. 

Humble said the state’s reporting method has some merit in terms of getting an accurate positivity rate, but he said the department hasn’t been upfront about its missing data.

Will Humble
Bret Jaspers/KJZZ
Will Humble

Previously, the raw numbers DHS was using to make these calculations were not publicly available. In the time since KJZZ began asking about this, the department added a more detailed breakdown of its math to its online data dashboard. The department also updated language on the data dashboard to indicate that the percent positive it displays is a rate of tests reported through the state’s electronic record system, not a rate of all Arizonans who have been tested, as the website previously stated.

What is not clear is which labs are not fully disclosing data and why. All state labs are required by a March executive order to report all tests. KJZZ requested a list of the laboratories not in compliance, but the DHS did not provide it.

“To me, that doesn’t make good management sense when [DHS] regulates those labs," Humble said. 

Humble said it's up to DHS to oversee labs. Five months after the state started compiling this data, he questions why reporting is still so incomplete that it creates this discrepancy between the state’s calculated positivity rate and what other sources calculate. 

“The percent positivity is a really important number now–way more important than it used to be — because decisions that have lots of money attached to them are being made using percent positive," Humble said. 

There is, at least, one consistent trend. Whether you look at numbers from the state, or from ASU's Biodesign institute, or from Sonora Quest Laboratories, the positivity rate in Arizona is declining. According to any of those sources, the rate it is now less than half what was reported in late-June and early July. But week-by-week, those 2-to-5 percentage point differences among reporting sources remain. And Humble said, in a public health emergency when data is driving critical policy decisions, a few percentage points can have real-life consequences.

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