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Combined diagnostic and antibody tests resulted in lower "percent positive"

State changed key COVID-19 calculation
Posted at 9:42 PM, May 06, 2020
and last updated 2020-05-07 23:57:52-04

The ABC15 Investigators discovered Arizona state officials changed how they calculated the percent of positive COVID-19 tests, resulting in a dramatic drop last week.

Gov. Doug Ducey touted the 6 percent positive testing number Monday as part of the reason Arizona satisfied federal gating criteria and could safely reopen salons and restaurants. His announcement came one day before President Donald Trump visited Arizona.

"That downward trajectory is very positive," Ducey said Monday as he showed several charts, including one showing that positive tests for coronavirus dropped from 10 percent to 6 percent of all tests administered in Arizona over a two-week period.

State Health Director Cara Christ admitted the 6 percent positive figure from the week of April 26 was derived from using all coronavirus tests administered that week. For the first time, that included roughly 10,000 serology tests that detect the presence of coronavirus antibodies. This antibody test is used to detect the prevalence of prior coronavirus spread, mostly finding people who carried the virus several weeks earlier who are no longer contagious.

Dr. Christ defended her decision to ABC15's Garrett Archer, saying "We know that they are different tests but a positive serology, under the right circumstances, could lead to a positive presumptive or a probable case." She added that they are still trying to "navigate" the issue.

The Arizona Public Health Association's Will Humble, a former Arizona state health director, said the change in calculations was not disclosed upfront, and he added, it was like "comparing apples and oranges," because only the diagnostic test is a good measure of the current spread of the virus.

"It's fine to use both sets of data, but use them separately," Humble said. "To include them in the same data set is changing the rules in the middle of the game."

The positive rate of the antibody test is about 3.6 percent, but the diagnostic rate has averaged 8.6 percent for all tests administered in Arizona since the pandemic began.

"This is a simple math equation," Archer said. "The number of antibody tests were much smaller, so when you added these in, you got a whole bunch more negatives that brought your percent rate down a lot."

A Ducey spokesman said even without those antibody numbers, the percent positive tests would still have declined, but Archer, a former state data analyst himself, said his calculation shows a smaller decline or only one week.

Some health professionals told ABC15 the reason for that small decline more likely due to the governor's testing blitz than a true decline in rate of coronavirus infection.

The blitz, started last week, opened up testing to all Arizonans, regardless of job or symptoms, who believed they may have been exposed to the virus.

"Before the testing blitz who got tested?" questioned Dr. Randy Friese, who is also a democratic state representative from Tucson. "High-risk people who are exposed or having symptoms, so the number of proportion of positive from the population should be higher than if you're just testing the general population."