Positivity rate emerges as a key metric, but what is it?

Coronavirus science and data is plentiful six months into the pandemic.

Knowing what’s behind the numbers is key to understanding how to move forward.

Positivity rate or percent positive is a term that has often been used to determine what should reopen and what should remain closed.

“It ’ s tricky because there’s a lot of places that are kind of billing it as the be-all, end-all metric and it’s really not,’’ said Kayleigh Blaney, an Oakland County epidemiologist.

The rate itself is thenumber of positive tests compared to thenumber of tests given.

The equation is simple, but the meaning behind it is not.

“Percent positive is a semi-decent metric when used with other metrics, but it should never be used alone to determine anything,’’ Blaney said.

She calls it one of the pieces of the puzzle used to determine what’s going on with COVID-19.

The state of Michigan uses positivity rate, along with cases per million, to determine risk levels for different regions. The lowest risk is defined as a testing positivity rate between 3 and 7 percent, and seven to 20 cases per million.

“Percent positivity with the number of cases being reported is one of the better ways to access what’s going on,’’ Blaney said.

When the pandemic first hit the positivity rate was extremely high because tests were limited to those with symptoms.

According to Michigan. gov, the state’s positivity rate on March 10 was 66.67. On Sept. 23 it was 3 percent statewide and 2.93 percent in Oakland County.

Six months in, widespread tests are available so more people are getting tested including those without symptoms.

“Positivity rate is one of the tools that we use, but it really is something that needs to be used in conjunction with how many cases you’re seeing — you have to look at the numerator— the number on top— howmany positive tests are we seeing and then look at the denominator howmany people are being tested,’’ Blaney said.

The example she used is if there were 300 positive tests this week with 3,000 tests given that would be a 10-percent positivity rate. But if next week there are 300 positive tests and 5,000 were tested that would be a 6-percent rate. In that case the positivity rate would be lower even though the positive tests were the same.

She thinks there’s a reason that positivity rate has become a standout stat in discussing the pandemic.

“I think it’s one of those things we live in a very technological, a very high science society. We think we can put numbers on everything,’’ Blaney said. “We’re very used to, if this is a threshold you can do this. So if your chlorine threshold in a pool is too high, there’s a number and we fix it. Or if there’s too many people doing something — we’re a very numbersdriven society. So I think in this outbreak everybody wants to put a number on something and say, ‘OK we cango back tonormal when X happens.’’’

Percent positive is something that people have latched onto a little bit, maybe too much.

“OK if the percent positivity is low that means we’re doing good and that’s not necessarily untrue. Again, even if you have a ton of cases — even if we were getting 1,000 cases a day which God forbid that ever happened — if you’ve got 30,000 people getting tested that’s no different than 10 cases with 300 people getting tested. It’s very subject to volume.’’

Daniel Ortiz, associate director of microbiology and molecular pathology at BeaumontHealth, agrees that positivity should be just one factor in looking at the pandemic’s status.

“You want to look at all the numbers, if you are just tracking positivity rate and you see that go up there could be various reasons for that — are you increasing your testing? What kind of patient population are you testing? As soon as you see an increase in that positivity rate then you want to start looking at those other numbers such as hospital admits and deaths,’’ Ortiz said. “So as long as your hospital admits and deaths are staying kind of consistent and not increasing, that’s basically saying a lot of these patients are having mild or possibly even asymptomatic infections, they’re not severe cases of COVID which is what we’re trying to avoid.’’

He emphasizes looking at the number of hospitalizations.

“We did flatten the curve and for hospital admissions. We’re no longer at our capacity, that’s been flat since June,’’ Ortiz said. “We haven’t seen an increase with hospitalized patients with COVID which was great. We were seeing slightly more cases, that could be due to more testing and more testing available and the type of patients that we’re testing as

well. We’re not seeing more hospital admits and we’re also seeing no increase in the death rate either.’’

Positivity rate is just one of the factors that helps the county in working with Oakland County schools to determine which schools should be open for face-toface learning. It’s not a onesize-fits-all situation. Buildings and building sizes are different, and so is the student population. Some can do social distancing, others are too small.

“Every school district is ina unique situation. We do help them evaluate that on a case-by-case basis. There is data and metrics and things that go into that and we provide those things,’’ Blaney said. “Schools are better equipped to understand their buildings, understand their communities and understand how they can provide the best education to students with the restrictions that have to be in place because of COVID.’’