Seventeen million American adults with blood pressure that appears normal during a visit to the doctor may have masked hypertension, according to a new study from researchers at Columbia University Medical Center (CUMC) and Stony Brook University School of Medicine.

Masked hypertension occurs in people with normal or pre–hypertensive blood pressure readings in the clinic (at or below 140/90 mm Hg) but high daytime ambulatory readings (average above 135/85 mm Hg). Ambulatory blood pressure is measured with a wearable monitor that takes a blood pressure reading every 15 or 30 minutes, for 24 hours, as the individual goes about normal daily activities.

Because national health surveys do not include 24–hour ambulatory blood pressure measurements, it is unknown how many people in the United States have masked hypertension and how they can be identified.

To estimate the prevalence of masked hypertension in the United States, the researchers used data from a previous study to identify predictors of ambulatory hypertension in those with normal clinic blood pressure. They pooled this data with information about 9,316 non–hypertensive adults enrolled in the National Health and Nutrition Examination Survey (NHANES) between 2005–2010.

The researchers used a method called multiple imputation to simulate missing NHANES data regarding masked hypertension and arrived at a U.S. prevalence estimate of 12.3 percent, or 17.1 million adults. They also confirmed prior studies showing that masked hypertension was more common in males, individuals age 45 years or older, and those with diabetes and pre–hypertension (120/80 mm Hg to 139/89 mm Hg).

“While it may not be practical to perform 24–hour blood pressure monitoring on all adults who have normal clinic blood pressure, our study suggests that it may be prudent to screen men 45+ years of age with diabetes or prehypertension,” said lead investigator Joseph E. Schwartz, PhD, professor of psychiatry and sociology at Stony Brook University School of Medicine and a lecturer at CUMC.

“Having a prevalence estimate is the first step in recognizing the extent of the problem,” said Daichi Shimbo, MD, associate professor of medicine at Columbia, cardiologist at NewYork–Presbyterian/Columbia University Medical Center, and an author of the study. “Future research should focus on characterizing the health benefits and net costs of treating people with masked hypertension.”

The paper, titled ‘Prevalence of Masked Hypertension Among US Adults with Nonelevated Clinic Blood Pressure,’ was published online in the American Journal of Epidemiology on Jan. 18, 2017.