Explainer: Why you should be concerned about Utah’s COVID-19 test positivity rate

Utah ranked ninth among the 50 states over the weekend for its high percentage of tests that come back as positive.

One of the most watched — and often most alarming — data points of the COVID-19 pandemic is the test positivity rate.

It’s a simple metric: Out of all the COVID-19 tests processed, how many of them come back positive?

But Utah’s true rate — which hit 32.2% on Monday — is more complicated to calculate, as some amateur data analysts have discovered.

For example, the Utah Department of Health reported Monday that the rolling seven-day average for the test positivity rate was 30.8%. (Public health officials use seven-day averages for many metrics, because they reveal broad trends rather than the wider up-and-down swings of the daily reports, leveling out such factors as weekends and holidays.)

But if you took the number of positive cases during the previous seven days — 19,043 — and divide into the number of total tests processed, which was 54,219 for the seven days ending Monday, you’d get a figure of 35.1%.

David Nierenberg, an airline pilot living in Salt Lake City, found similar discrepancies as he examined UDOH numbers last week.

“The last several days of positivity percentage reporting has been off,” Nierenberg wrote to The Salt Lake Tribune last week. “Would you please post the actual number tested over the past week in the next article to correct the record?”

So why is the state’s official positivity rate different, and lower? With the daily numbers released by UDOH, “you don’t have all the negative results factored in,” explained Tom Hudachko, a department spokesman.

Positive test results get reported faster than negative ones, so UDOH sets in some lag time — usually around five days — for the negative results to catch up, Hudachko said. Without that lag, the positivity rate would be artificially high, he said.

A positivity rate hovering around 30% is alarmingly high as it is. Public health experts say such a high positivity rate is an indicator that there are lots of people who have the virus and aren’t getting tested.

Dr. Todd Vento, infectious diseases physician at Intermountain Healthcare, noted Monday that Utah ranked ninth among the 50 states this weekend for its test positivity rate.

“We still have quite a bit of positive transmission,” Vento said in an online media briefing Monday.

Targeting areas of Utah with high positivity rates, Utah is offering free antigen testing for COVID-19 this week in 15 counties, with funding from the federal CARES Act. The goal is to identify people who are now infectious and help slow the spread of the virus, according to a UDOH spokesman.

“Testing is important, but that’s not how you get yourself out of a pandemic. You don’t test your way out of a pandemic,” Vento warned. “You prevent, and stop disease from spreading. That’s how you get yourself out of a pandemic.”

Government officials, Vento said, “fell in this trap early on” that “we just have to test more to show we have more negative [tests]. That’s really the wrong approach. The right approach is we have to test more to find out what the truth is.”

UDOH has a target of a 5% test positivity rate, Hudachko said. That low rate, Vento said, would “show excellent control” of the virus’s spread. “We haven’t been 5% for months and months, nor have many states in the United States,” Vento said.

The way to bring the test positivity rate down, Vento said, is to get people vaccinated and, until that happens, keep doing what health experts have been saying for months: “You have to wear a mask, 100% of the time when out in public and in contact with others. You have to stop gathering, and [you have to be] keeping your physical distancing.”

Vento added, “The message isn’t going to change because the calendar year changed. The message is the same.”