High blood pressure affects millions of people, but is it possible to have hypertension without even knowing it? According to a new study, this is the case for one in eight Americans.

The condition, known as masked hypertension, occurs when a patient’s blood pressure is normal when measured in a doctor’s office or clinic but high outside of the office. This undiagnosed condition puts the patient at an increased risk for heart disease because patients are unaware that they even have hypertension.

The study, published in the American Journal of Epidemiology, measured the blood pressure of patients who were monitored for 24 hours a day and found 14 percent of participants to have masked hypertension. This is unlike “white-coat hypertension,” which does not put patients at higher risk for heart disease. That’s when patients have higher blood pressure measurements when they are in the doctor’s office but normal rates outside of the office.

Yinka Afolabi-Brown, MD, of the division of Cardiovascular Disease at Einstein Medical Center in Philadelphia, says there are certain groups that are at a higher risk of the hidden condition, including men, patients 45 years or older, or those with diabetes. Dr. Afolabi-Brown says the condition is very difficult to diagnose. “We certainly need to have a higher index of suspicion when a patient offers this history of having high blood pressures at home even if it is normal in the office,” he adds.

In terms of what patients can do, Dr. Afolabi-Brown recommends taking advantage of blood pressure machines that are freely available in pharmacies and other locations on a regular basis and then informing their doctor if they have concerns.