Your doctor has probably checked your body mass index – or BMI – at one time or another. But according to a new report from UCLA and UC Santa Barbara, there's a good chance your health was misclassified based on your BMI. 

The researchers analyzed the link between BMI and several health markers, like blood pressure and glucose, cholesterol and triglyceride levels, using data from the most recent National Health and Nutrition Examination Survey.

They found that nearly half of people considered overweight on the BMI scale and 29 percent of those considered obese were metabolically healthy. On the flip side, more than 30 percent of people considered to be normal weight on the BMI scale were metabolically unhealthy.

Overall, the study found some 74 million Americans' health has been misclassified based on their BMI.

"These are not trivial numbers here," says Janet Tomiyama, an assistant professor of psychology at UCLA and a study co-author. That's a lot of people, she adds, "who are being told that they're unhealthy when they're actually not and I think that's a problem."

Let's step back for a minute to explain how BMI is calculated and why it's used as a screening tool. Then I'll explain how all of this could affect you.

How do doctors calculate BMI?

BMI is a pretty straight-forward calculation: Take your weight and divide it by your height, in inches squared, then multiply it by 703. You can also use an online BMI calculator, like this one.

You'll get a number, which will place you within four categories: Underweight, healthy weight, overweight or obese.

How is BMI used?

The Centers for Disease Control and Prevention says BMI is an inexpensive and easy-to-use tool to screen for weight and conditions associated with obesity.

But it's not a diagnostic tool, the CDC says. It says doctors should perform more tests to determine if a high BMI is a health risk. These tests, it says, could include skinfold thickness measurements and evaluations of diet, physical activity and family history.

Why does BMI misclassify some people's health status?

The BMI calculation doesn't take into account some important factors, like age, gender, ethnicity and race. This is important because, as the CDC points out, in general, at the same BMI:

• Women tend to have more body fat than men
• African-Americans tend to have less body fat than white people
• Asians tend to have more body fat than white people
• Older people tend to have more body fat than younger adults

On top of all of that, the BMI scale doesn't take into account muscle mass. The CDC points out that at the same BMI, athletes tend to have less body fat than non-athletes.

Besides being a screening tool, is BMI used for anything else?

Yes: Your BMI could affect how much you pay for your health insurance premium.

Here's why: Under the Affordable Care Act, group health plans can adopt wellness program that base people's premiums on their health status. The law says these so-called "health-contingent wellness programs" can give people discounts on their premiums – or charge them more – depending on whether they meet certain health goals, like normal weight or blood pressure, according to a recent report from the Kaiser Family Foundation.

The Foundation finds that in 2015, a small number of large employers – just 5 percent – offered wellness programs that included financial incentives for people to complete biometric screening and meet one or more health outcomes, including some based on BMI.

But that number could increase, the Foundation says, now that the Equal Employment Opportunity Commission has proposed new rules that clarify what's legal for wellness programs under the Americans with Disabilities Act and the Genetic Information Nondiscrimination Act.

Tomiyama of UCLA worries that will entice more companies to offer wellness programs with financial incentives based, at least in part, on BMI.

"All of a sudden, we're now talking about money – we're talking about charging people more money, based on this faulty measure," she says.

For more on BMI, check out this segment on Take Two. I chat with co-host A Martinez about the hullaballoo surrounding BMI and co-host Alex Cohen interviews Deborah Burnet, a professor of medicine and pediatrics at the University of Chicago school of Medicine.