1 in 3 Americans has a condition doctors aren't screening for

Obesity and diabetes can sometimes go hand in hand.

The higher amount of fat a person has, the higher the chance they can develop diabetes. But new research shows the measurement that doctors are using to calculate that risk may not be so accurate.

Doctors say it's an epidemic. More than 84 million people in the U.S. have prediabetes, but many do not know it.

"There are a lot of people being missed. Six, seven, eight million people are probably being missed," said Arch Mainous, a professor of health services research at the University of Florida.

That's because body mass index, or BMI, the measurement doctors use to determine if someone is overweight or obese, is missing people who are of normal weight but are prediabetic.

"A lot of people who are healthy weight may not be so healthy," Mainous said.

Researchers studied data on adults 20 and over with a healthy BMI, between 18.5 and 24.9. He found that one in five adults between the ages of 20 and 44 who had a healthy weight met the blood glucose criteria for prediabetes. For people 45 or older, it was one in three.

"As people get older, they tend to lose lean muscle mass, and they are still at the exact same BMI," Mainous said. "Over time, they get really skinny arms and really skinny legs and get more weight."

The weight added is around their waist.

"It moves them, so that they really are no different than people who are overweight," Mainous said.

"Once you hit the age of 45, you should be always talking to your doctor about getting screened," said Ryan Sanders, a registered dietitian and nutritionist at UF Health Shands Hospital in Gainesville, Florida.

If you have a family history of diabetes, your waistline is at least 40 inches for men and at least 35 inches for women, it is important to talk to your doctor about getting screened, also if you are a high-risk ethnicity, such as African-American, Hispanic, Asian or Native American.

"If we don't find the people who are at the high-risk of developing diabetes and do something about them, then we are just waiting for a lot of people to develop diabetes," Mainous said.

Fifteen to 30 percent of people with prediabetes will eventually develop diabetes within five years. Those diagnosed with diabetes will spend 2.3 times more on health care than if they didn't have the disease. The U.S. Centers for Disease Control has a free test for anyone who wants to know their risk of prediabetes.