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What it means when the Covid-19 positivity rate is rising in your area

(CNN) As Covid-19 spreads to nearly every community in the country, you may hear the terms "percent positive," "test positivity rate" or "positivity rate" use to describe how dire the circumstances are in your area.

It's not a measure of how many cases. It's the percentage of all coronavirus tests performed that show someone has Covid-19.

Here's what you need to know about percent-positive rates and what they mean for your community.

What is percent positive?

"It tells you, at a basic level, how hard you have to look to find a case," said Dr. Amesh Adalja, a fellow with the Infectious Diseases Society of America and a senior scholar at the Johns Hopkins Center for Health Security.

There are multiple ways to calculate percent positivity. The US Centers for Disease Control and Prevention calculates it by taking the number of all positive tests and dividing it by the number of total tests -- both positive and negative -- and then multiplying it by 100 to get a percentage.

Sometimes it is also called the "positivity rate" or "percent positive."

What does a percent positive tell you about cases in your area?

If the percent positive was more than 50% in an area, doctors would have to do only two tests to find one case. If an area's percent positivity rate is below 1%, you would have to do over 100 tests to find one case.

Explained another way, Adalja thinks about the rate like hunting for crayfish.

"If you were looking in a creek for crayfish, you find them by picking up rocks," Adalja said. "You don't have to turn over a lot of rocks to find them if the rate is high."

What does a percent positive not tell you?

When the percent positive rate in your area is 58%, that does not mean that 58% of the population has Covid-19. It mostly means that 58% of the population who got a test actually had Covid-19.

Not everyone who has Covid-19 gets a test.

"With the positivity rate, the denominator is always the total number of tests performed, and the numerator is the number of tests that turn positive, but the catch occurs is how to interpret that number," said Dr. George Abraham, the chair of the American Board of Internal Medicine's infectious disease board. "The number of tests performed could be on the same person, for example, so each number may not represent a different person."

The number of actual cases in your area is likely higher than that percent positive number, but the number is one measure public health leaders can use to gauge how much disease is in the area and determine if they need to take further action to slow the spread.

There is no uniform percent positive number at which things shut down.

In New York, for instance, if the percent positive number goes above 3%, schools go to remote learning. In Iowa, it's 15%.

"Either way the percent positive becomes a very important number to watch with schools. It can change a lot of lives," said scientist William Haseltine, who has written several books about infectious disease.

Why should I care about the percent positivity?

You want to know the rate in your area, Haseltine said, so you know your Covid-19 exposure risk.

Thinking about it another way, Haseltine said, you could check the percent-positive rate in your area before you decide to go out, kind of like how you would check the weather before you go out for the day.

"With the weather, if it is a light rain, you'd put on a raincoat, right? If the number of cases is low in your area, wear a mask and you are probably going to be OK," said Haseltine, the chair and president of the global health think tank ACCESS Health International.

"If it's at 10 or 15%, think of it like a heavy rain or thunderstorm. You need to be even more careful. Start limiting your activities," Haseltine said. "If the rate gets higher than that, think of it like a tornado. With a tornado you'd get in the basement and make sure your kids are safe."

How should the percent-positive rate figure into my decisions?

When the Covid-19 percent-positive rate is high in your area, the chance that you will run into someone with the virus at the grocery store, for example, is much higher than if the rate is low in your area, Adalja said.

The percent-positive rate should be one factor in deciding if you should go out.

If you are at high risk because you are overweight, you have diabetes, or asthma or another underlying condition "that's a recipe for getting really sick," Haseltine said.

"Remember, it's not just older people that are killed by this," Haseltine said. "If you have any underlying condition, or you live with older people, or you are in an area that isn't taking precautions, you need to factor that all in, along with the percent-positive rate."

"It's pretty simple," Haseltine said. "But it's something everybody has to think about now."