A study of healthy adults found that the mean awake ambulatory blood pressure was often higher than clinic blood pressure and that 15.7 percent of adults with nonelevated clinical blood pressure had masked hypertension.

Lead researcher Joseph E. Schwartz, PhD, of Stony Brook University in New York, and colleagues published their results online Dec. 5 in Circulation.

“These findings debunk the widely held belief that ambulatory blood pressure is usually lower than clinic blood pressure,” Schwartz said in a news releae. “It is important for healthcare providers to know that there is a systematic tendency for ambulatory blood pressure to exceed clinic blood pressure in healthy, untreated individuals evaluated for hypertension during well-patient visits. Our study’s results may not apply to those who have previously been diagnosed as having hypertension or are already being treated for hypertension.”

The researchers mentioned that 24-hour ambulatory blood pressure monitoring has been common since 1983. They added that during the past 30-plus years, studies have shown ambulatory blood pressure to be better than clinic blood pressure to determine organ damage and predict cardiovascular morbidity and mortality risk.

In this trial, known as the Masked Hypertension Study, the researchers enrolled 888 adults between  2005 and 2012. The participants were employees of Stony Brook University and Columbia University, their medical schools and affiliated hospitals and a private hedge fund management organization.

The adults were at least 21 years old, worked at least 17.5 hours per week, spoke and read English, had blood pressure of less than 160/105 mmHg before enrolling and were not take blood pressure lowering medications. Each of the participants had their blood pressure taken three times at each of three separate visits in accordance with American Heart Association guidelines. They also completed a 24-hour ambulatory blood pressure recording, in which they had readings taken approximately every 30 minutes.

The mean age of the participants was 45 years old, while 59 percent were females and 80.6 percent were white.

The researchers found that the mean systolic/diastolic blood pressure reading was 123.0/77/4 mmHg for ambulatory blood pressure and 116.0/75.4 mmHg for clinic blood pressure. They also mentioned that ambulatory blood pressure exceeded clinic blood pressure by more than 10 mmHg more frequently than vice versa.

The frequency of ambulatory blood pressure being higher than clinic blood pressure was more often found in young adults and people with a normal body mass index.

On average, systolic ambulatory blood pressure was 7 mmHg higher than clinic blood pressure and diastolic ambulatory blood pressure was 2 mmHg higher than clinic blood pressure.

The researchers defined masked hypertension as adults having normal blood pressure in the doctor’s office but high readings outside of the office.

“Although the view that [ambulatory blood pressure] is usually lower than [clinic blood pressure] may well hold for older individuals with elevated [clinic blood pressure], especially those being treated for hypertension and those wearing an [ambulatory blood pressure monitoring to rule out white-coat hypertension, it is not true in this sample or, we suspect, in the general US adult population,” the researchers wrote.

The researchers acknowledged a few limitations of the study, including that the sample was not representative of the general population. They also noted that they excluded nonemployed people, which meant that there were few people older than 65 years old, which is the age group where adults are most likely to have hypertension.

“Given the substantial evidence that an elevated average awake [blood pressure] over the course of the day increases one’s risk for a cardiovascular event, regardless of one’s [blood pressure] in the clinic setting, it is likely that many adults, especially those with prehypertension in the clinic, would benefit from completing a 24-hour ambulatory [blood pressure] monitoring,” the researchers wrote.