For the first time in more than 20 years, a national study that measures the health and nutrition status of Americans across the country has set its sights on the Bronx, which is consistently ranked as one of the unhealthiest counties in the nation.

The National Health and Nutrition Examination Survey (NHANES), conducted by the Centers for Disease Control and Prevention, surveys 6,500 residents from 15 counties across the nation each year. The data gathered is meant to provide a snapshot of the country’s health and is used by legislators and officials to influence health programs and policies.

“It’s kind of like a health report card for the nation,” said senior study manager Nora Martinello.

Staff from the CDC started knocking on doors across the Bronx last week to conduct health interviews at a random sampling of addresses. Starting in the middle of May, about 500 chosen residents will undergo full, confidential medical exams from a team of survey physicians, dentists, nutritionists, and lab technicians.NHANES last came to the Bronx in 1982, Martinello said. The CDC has set up shop in a number of mobile examination trailers on West Kingsbridge Road, on the campus of the James J. Peter VA Medical Center.

“It’s really impressive to see what we can do inside four tractor trailers,” she said.

There, all participants will get a physical exam, take a dietary interview, and get measured for height and body weight. Depending on age, some will receive a scan to evaluate body fat or undergo tests to measure breathing, blood pressure and hearing.

Individuals who are surveyed receive a stipend and a full medical report from their exam.

The data gathered annually from each surveyed county is pooled together to create estimates for the prevalence of major diseases like diabetes, osteoporosis, hypertension, and heart and respiratory disease.

The information gathered helps shape national health policies and programs. First lady Michelle Obama’s “Let’s Move!” program, for example, which focuses on improving the health and fitness of children, was launched after NHANSE data revealed an alarming increase in incidents of childhood obesity, Martinello said.

Other national health policies, like standardized growth charts used by pediatricians across the country to track children’s growth, or the amounts of vitamins and minerals recommended for a healthy diet, are also based on the survey’s findings.

“The survey is a unique resource for health information, and without it we would lack important knowledge about major health conditions,” said CDC Director Thomas Frieden, in a statement.

This year’s survey is placing extra emphasis on the health of older persons, Asian-Americans, African-Americans, and Hispanics.