WASHINGTON - Chances are you know your blood pressure. What about your BMI?

Body mass index signals if you're overweight, obese or just right considering your height. Some doctors have begun calling it a vital sign, as crucial to monitor as blood pressure.

But apparently not enough doctors check: A government panel renewed a call Monday for every adult to be screened for obesity during checkups, suggesting more physicians should be routinely calculating their patients' BMIs.

And when someone crosses the line into obesity, the doctor needs to do more than mention a diet. It's time to refer those patients for intensive nutrition-and-fitness help, say the guidelines issued by the U.S. Preventive Services Task Force.

Don't assume your weight's OK if the doctor doesn't bring it up.

Patients "should be asking what their BMI is, and tracking that over time," says task force member Dr. David Grossman, medical director for preventive care at the Group Health Cooperative in Seattle.

By the numbers: A normal BMI is less than 25. Obesity begins at 30. In between is considered overweight. To calculate yours: http://www.nhlbisupport.com/bmi/.

The advice sounds like a no-brainer, considering the national anxiety about our growing waistlines. Two-thirds of adults are either overweight or obese. Some 17 percent of children and teens are obese, on the road to diabetes, heart disease and other ailments before they're even grown.

The task force has recommended adult obesity screening previously, and similar guidelines urge tracking whether youngsters are putting on too many pounds.

Yet BMI remains a mystery for many people. A 2010 survey of members of the American Academy of Family Physicians found up to 40 percent of those primary care doctors were computing their patients' BMIs. Surveys show only about a third of obese patients recall their doctor counseling them about weight loss, even though people whose doctors discuss the problem are more likely to do something about it.

Doctors can struggle with the pounds, too, and Johns Hopkins University researchers recently reported that overweight physicians were less likely than skinnier ones to advise their patients about weight loss.

Why the reluctance? One reason: Few doctors are trained to treat obesity, they're discouraged by yo-yo dieting but they don't know what to advise, says Dr. Glen Stream, president of the physicians' group. His Spokane, Wash., practice uses electronic medical records that automatically calculate BMI when a patient's height and weight is entered.

"Our American culture is always looking for an easy fix, a pill for every problem," Stream says. "The updated recommendation is important because it makes clear exactly what doctors should do to help."

In Monday's Annals of Internal Medicine, the task force concluded high-intensity behavioral interventions are the best non-surgical advice for the obese, citing insufficient evidence about lasting effects from weight-loss medications.

The task force's Grossman says a good program:

• Includes 12 to 26 face-to-face meetings over a year, most in the first few months.

• Makes patients set realistic weight-loss goals. Losing just 5 percent of your initial weight - 10 pounds for a 200-pound person - can significantly improve health.

• Analyzes what blocks each patient from reaching those goals. Do they eat high-calorie comfort foods to deal with depression? Spend too much time at a desk job?

• Tailors ways to help people integrate physical activity into their daily routine.

• Requires self-monitoring, such as a food diary or a pedometer to track activity.

Last year, Medicare started paying primary care doctors for obesity screening and weight-loss counseling for seniors for a year, including weekly meetings for the first month.

But many insurance companies don't pay for all the suggested interventions, and comprehensive programs aren't available everywhere, says Dr. Scott Kahan of George Washington University and the STOP Obesity Alliance. He runs a clinic that provides a medical, psychological and nutritional evaluation before tailoring a plan. In other programs, primary care doctors may offer some counseling and send patients to nutritionists or other specialists for extra help.

Another problem: "Doctors tend to shoo away people who have obesity. They say, 'Don't come back to me and tell me your back hurts or you have acid reflux or high cholesterol until you will do something about it,'" laments Kahan, who is teaching medical school students to motivate patients.

What about the overweight? The task force said more study is needed on how best to help them.

But in Reno, Nev., Dr. Andy Pasternak calculates BMI for every patient at his family medicine practice - and particularly targets the overweight in their 40s and younger for fitness counseling. He says if they wait until they're heavier or older to get active, arthritis exacerbated by the pounds will be another barrier.

Patients seldom know what their BMI should be, but "at least twice a day people say, 'What should be my optimal weight?'" Pasternak says.

He thinks saying to lose 60 pounds is too discouraging: "What I try to get them to focus on is: How much are you working out? How many servings of vegetables do you get a day?"

Lauran Neergaard covers health and medical issues for The Associated Press in Washington.