The positivity of the test will look lower, so ‘we have to change the scale in our heads,’ says the state epidemiologist.
(Trent Nelson | The Salt Lake Tribune) Kylie Archuleta and Joshua Brimhall conduct 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 reports to the public one of the metrics that track the spread of COVID-19.
The test positivity rate is the percentage of tests that come back 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 has risen to 32.71% on Jan. 7, during the post-holiday peak of the state, but recently it has fallen, hovering around 16%.
The percentage appears lower because of the new method that the Utah Department of Health will use. It will help public health officials “see the benefit of our increased testing capacity,” and it will make comparisons with other states more valid, Dr. Angela Dunn, the state epidemiologist, recently addressed reporters in explaining the change.
In Monday’s daily report, the department cited both the previous measurement – a seven-day moving average of 15.4%, and the new one, with a seven-day moving average of 7.3%.
This is how these methods differ.
What does the positivity rate of the test show?
The percentage of tests for COVID-19 that come back positive is “a leading indicator of the spread of COVID,” Dunn said. “It also measures how many tests we do.”
How did Utah calculate it?
Since the start of the pandemic, Utah used a method called “ people over people, ” Dunn said. That means the number of people who tested positive divided by the number of people who tested.
While people were being tested, the state counted their first positive test within a 90-day period – or, if they never tested positive, their first negative test. All other test results in that window are not included. Health officials use a 90-day window, Dunn said, because that’s the period of immunity for someone after they contract the coronavirus.
So, by this method, a person who is tested multiple times in three months is counted once – the subsequent results are “duplicated” in the count of the number of people tested within that 90-day period.
What changes in the way Utah calculates it?
The new method is called ‘test over test’. It takes the number of positive tests in a day or week and divides it by the number of tests performed in the same period.
“Every single test reported to the [Utah] The Department of Health is included in this calculation, ”said Dunn.
The number of tests conducted in Utah is now dramatically higher as testing has expanded with the addition of rapid tests and regular tests are widespread among employees, students, nursing home residents and others.
Those tests are generally from healthy people; they have no symptoms and do not believe they have the virus. They are tested to stay in class or continue to work.
So when all the results are counted, when so many supposedly healthy people are tested, the positivity rate will drop.
The new method, Dunn said, “will allow us to see the benefit of our increased testing capability.”
With all of those tests, people who contract the virus will soon know, she said. Health officials can respond with contact tracking and act faster to stop the spread. And the positivity rate will reflect that environment.
The “people over people” method made sense at the start of the pandemic, she said. At the time, the federal Centers for Disease Control and Prevention required a COVID-19 patient to have two negative tests in a row before they could be considered recovered from the virus.
A person could do a lot of positive tests before getting two negative tests, artificially inflating the “ test over test ” calculation, she said. The CDC later changed its guideline, but Utah continued to use the “people over people” method.
The CDC uses both methods and a third, she noted.
Public health officials are looking at both calculations, Dunn said. The “people over people” method, she said, “reflects our rising cases.”
The ‘test-over-test’ formula is very useful because it allows us to see that we are testing more and how well that is helping us to reduce the number of cases. ‘
What is the difference in practical terms?
The new positivity rate statistic will be smaller.
Monday’s reports showed that difference, with 15.4% versus 7.3% for a seven-day moving average among the two methods.
The “people over people” method, Dunn said, “skews the positivity rate a little higher.” The “test over test” method “skews the positivity rate a bit lower,” she said. “The truth is somewhere in the middle.”
“You will find that the trends for both methods are actually very similar,” said Dunn. Looking at graphs for both calculations, they are practically parallel over time.
Is this a political move to make the numbers look better?
Dunn played down that perception. She noted that at least 37 other states are using “test over test” – and that switching makes it easier to compare apples to apples to other states.
If the daily figure seems lower, will people lower their guard?
“We have to change the scale in our heads,” said Dunn.
The post-Christmas positivity rate of over 30% “was high because that was the scale we used,” 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 bad it is or when it will end,” Dunn said. “We have to look at everything.”
She recommends that people look at three key metrics: the test’s positivity rate, the number of new cases, and the number of hospitalizations or capacity remaining.
“All of these three metrics together give us a really good idea of where we are in the pandemic right now and where we are headed,” said Dunn.