Around-the-clock monitoring during daily activities revealed undetected high blood pressure among otherwise healthy adults who had normal readings in the clinic, according to new research.

“Masked hypertension” is normal blood pressure in the doctor’s office, but high readings outside of the office — the reverse of “white coat hypertension” (higher blood pressure readings at the doctor’s office than outside the clinic setting).

Masked hypertension is easy to miss, and can occur during the day or night. To uncover masked hypertension, people wear a blood pressure cuff on their arm, attached to a small, portable device. Compared to clinic blood pressure, ambulatory blood pressure is recognized as a better predictor of future cardiovascular disease.

After comparing clinic blood pressure measurements to ambulatory blood pressure monitoring in 888 healthy, middle-aged participants, researchers found:

  • Almost 16 percent of participants with normal clinic blood pressure had masked hypertension based on ambulatory monitoring, regardless of sex, race or ethnicity.
  • Younger, normal-weight participants were more likely than older, overweight participants to have ambulatory blood pressure readings higher than their clinic readings.

“These findings debunk the widely held belief that ambulatory blood pressure is usually lower than clinic blood pressure,” said Joseph E. Schwartz, Ph.D., study lead author and professor of psychiatry and sociology at Stony Brook University in Stony Brook, New York. “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.”

Study participants had three blood pressure readings taken during each of three clinic visits and completed one 24-hour ambulatory blood pressure recording (with readings about every 30 minutes).

A substantial number of otherwise healthy people who have masked hypertension should have their blood pressure monitored regularly and future research should determine if they would benefit from treatment, Schwartz said.

The study is published in the American Heart Association journal Circulation.