The positivity rate of the test will look lower, so “we need to change the scale of our head,” says the state epidemiologist.
(Trent Nelson | The Salt Lake Tribune) Kylie Archuleta and Joshua Brimhall perform COVID-19 testing at the University of Utah Health’s Farmington Health Center on Friday, July 31, 2020.
The state of Utah is changing the way it informs the public of one of the statistics that track the spread of COVID-19.
The test positivity rate is the percentage of tests that turn positive for COVID-19, and Utah health officials have been explaining for months that a 3 to 5 percent rate indicates the virus is under control.
It rose to 32.71% on Jan. 7, during the state’s holiday hike, but has recently dropped, to around 16%.
The new method the Utah Department of Health will use will make the percentage appear lower. It will help public health officials “see the benefits of our greater testing capacity” and make comparisons with other states more valid, Dr. Angela Dunn, the state’s epidemiologist, told reporters recently. exchange.
In its daily report on Monday, the department listed both the previous measure: a seven-day average of 15.4%, and the new one, which has a seven-day average of 7.3%.
The difference between these methods is explained below.
What does the test positivity rate show?
The percentage of COVID-19 tests that return positive is “a major indicator of the spread of COVID,” Dunn said. “In addition, it also measures the amount of testing we’re doing.”
How was Utah calculating it?
Since the beginning of the pandemic, Utah had used a method called “people over people,” Dunn said. This means the number of people who have tested positive, divided by the number of people who have been tested.
As the people test was done, the state counted its first positive test in a 90-day period or, if they never tested positive, the first negative test. No other test results were included in this window. Health officials use a 90-day window, Dunn said, because this is the period of immunity for someone after catching the coronavirus.
Thus, according to this method, a person is counted once who is tested several times in three months; subsequent results are “split” in the health department’s calculation of how many people have been tested in this 90-day period.
What changes in the way Utah calculates it?
The new method is called “test over test”. Take the number of positive tests in a day or a week and divide it by the number of tests performed in that same period of time.
“Each test reported to the [Utah] That calculation includes the Department of Health, ”Dunn said.
The number of tests being conducted in Utah is now drastically larger, as tests have expanded with the addition of rapid testing and periodic testing is widespread among employees, students, residents of Utah. residences and others.
These tests are usually of healthy people; they have no symptoms or believe they have the virus. They are being tested to stay in class or keep working.
Therefore, counting all the results, when so many presumably healthy people are tested, the positivity rate will decrease.
The new method, Dunn said, “will allow us to see the benefit of our greater testing ability.”
With all this testing, people who catch the virus will know quickly, he said. Health officials can respond by tracking contacts and moving faster to stop the spread. And the positivity rate will reflect that environment.
He said the “people over people” method made sense at the beginning of the pandemic. At the time, federal disease control and prevention centers required a COVID-19 patient to take two consecutive negative tests before they could be considered recovered from the virus.
A person could accumulate a lot of positive tests before doing two negative tests, artificially inflating the “test over test” calculation, he said. Later, the CDC changed its pattern, but Utah continued to use the “people on people” method.
The CDC uses both methods and a third, he noted.
Public health officials look at both calculations, Dunn said. She said the “people above people” method “reflects our growing cases.”
The “test over test” formula is really useful because it allows us to see our increase in evidence and to what extent it is helping us decrease our cases. “
What is the difference in practical terms?
The new positivity rate statistic will be lower.
Monday’s report showed that difference, with 15.4% versus 7.3% for an average of seven days in both methods.
Dunn said the “people over people” method, “biases in slightly higher percentage of positivity.” The “test over test” method “skews the positivity percentage a little lower,” he said. “The truth is somewhere in the middle.”
“You’ll notice that the trends in these methods are really similar,” Dunn said. Looking at the graphs for the two calculations, over time, they are virtually parallel.
Is this a political move to make the numbers look better?
Dunn downplayed this perception. He noted that at least 37 other states use “test over test,” and switching facilitates apple-to-apple comparison with other states.
If the daily figure seems lower, will people lower their guard?
“We need to change the scale of our head,” Dunn said.
The positivity rate of the post-Christmas test above 30% “was high because that was the scale we were using,” Dunn said. “Now, 15% will be very high, 10% will be very high.”
How should people read the data?
“We know that no metric will tell us how wrong it is or when it will end,” Dunn said. “We have to look at it all.”
She recommends that people look at three key statistics: the test positivity rate, the number of new cases, and the number of hospitalizations or capacity remaining.
“The three metrics together give us a very nice picture of where we are currently in the pandemic and where we are going,” Dunn said.