Doctors often ask patients to list their race—white, Latino, African American, Asian, Native American—to help them provide better healthcare. They do this because loads of medical research shows that the incidence of certain diseases and treatment success can vary somewhat from race to race.

But the more important question may be: What is your genetic ancestry?

A study released in the New England Journal of Medicine examined the accuracy of a lung function test and how race and ancestry played a role in the test's accuracy. A lung function test measures damage to the lungs caused by asthma or other disease. However, the definition of "normal" lung function is known to vary substantially by race. For example, doctors have long known that vital lung capacity (the maximum amount of air that can be expelled after maximum inhalation) is 6% to 12% lower in blacks compared with whites and Native Americans.

Researchers looked at data from more than 3,000 patients that included their lung function test results, standard information on race and additional information on genetic ancestry that was obtained through genotyping. The study found that standard race categories don't capture the extent of ancestral diversity and, thus, may limit the amount of information available to a doctor in making a diagnosis or ordering treatment. Instead, many people have a rich and diverse genetic background that does not lend itself to a simple classification, such as "white" or "Asian."

For example, when using genetic ancestry data, the study showed a strong link between African ancestry and lung function measurement in both men and women. According to the findings, for 6.4% of people in the United States who identify themselves as African American, the actual percentage of African ancestry would be 15% higher or lower than average—a difference that would result in an incorrect estimation of lung function test, and possibly, mistakes about the severity of lung disease. About 2.1 million self-identified African Americans have asthma. But based on the study conclusions, the severity of the asthma would be misclassified in about 4% of those patients.

"When we force patients into an individual box, such as 'African American' or 'Caucasian,' we're missing a lot of genetic information," senior author of the study Dr. Esteban G. Burchard, of UC San Francisco, said in a news release. "This study provides new evidence that genetic ancestry correlates to physiological measures. With it, we're one step closer to personalized medicine."

On a more practical level, the study points to the need for improvements in measuring lung function in some people. In an editorial accompanying the paper, authors noted: "Refinements are needed for poorly represented or misrepresented populations and for persons of mixed ancestry, who represent an increasing proportion of the U.S. population."