People with normal blood pressure at the doctor's office but high blood pressure at other times may have a doubled risk of heart attacks and strokes, according to new research reviews.
In two analyses covering four industrialized countries and more than 5,000 people, researchers also found that home blood pressure monitoring frequently picks up that so-called masked hypertension.
If that leads to more people with hidden high blood pressure getting treatment, it would be affordable and worthwhile to expand home monitoring, they say.
"We know that a lot of cardiovascular complications occur in people who are normotensive if you measure the blood pressure in the regular way in the office," said Dr. Jan Staessen in an email.
Staessen, a researcher in the Division of Cardiovascular Diseases at the University of Leuven in Belgium, is senior author of the studies published in PLOS Medicine and Hypertension.
About 10 percent of the general population has normal or high-normal blood pressure readings at the doctor's office, but they actually have high blood pressure at home or at work. This is known as masked hypertension and it often goes untreated because it's hard for doctors to detect.
To see how masked hypertension affects risk for cardiovascular events like heart attack and stroke, Staessen and colleagues analyzed data from five previous studies of home-monitored blood pressure done in Finland, Japan, Greece and Uruguay.
For the new study, blood pressure readings with a top number below 120 and a bottom number below 80 were considered "optimal." Normal blood pressure was a reading of 120-129/80-84, "high-normal" was 130-139/85-89, mild hypertension was 140-159/90-99 and severe hypertension was a top number of 160 or higher and a bottom number of 100 or more.
The researchers considered a home reading of 130/85 or above to indicate masked hypertension when the person did not meet the criteria for high blood pressure at the doctor's office.
By that standard, they found that 5 percent of those with "optimal" blood pressure at the clinic, 18.4 percent with "normal" clinic readings and 30.4 percent with "high-normal" clinic readings had masked hypertension at home.
During a median 8.3 years of follow-up, participants with masked hypertension had between 2.24 and 2.65 times the risk for cardiovascular events when compared to those with true normal blood pressure.
In a commentary published in PLOS Medicine, Dr. Mark Caulfield of Barts and The London School of Medicine and Dentistry points out that cardiovascular disease is the leading cause of death worldwide.
"With a growing burden of high blood pressure and a growing availability of affordable devices, home blood pressure monitoring could be used to diagnose high blood pressure and help decide whom to treat," he writes. "It empowers patients to take on a role in assessment of their blood pressure."
In a separate analysis, Staessen looked at how home blood pressure monitoring affected the cardiovascular risk profiles of patients with masked hypertension or with the opposite problem - so-called white coat hypertension. That's when patients have high blood pressure readings at the doctor's office but normal blood pressure at home.
The study included 6,458 participants, 5,007 of whom were not being treated for blood pressure problems. Of that untreated group, 404 people had masked hypertension, 695 had white coat hypertension and 924 had sustained hypertension.
Staessen's team found that masked hypertension raised risk for cardiovascular events in the untreated group by 55 percent compared to people with normal blood pressure.
White coat hypertension raised the risk by 42 percent and sustained hypertension more than doubled it.
Even among the participants who were being treated for high blood pressure, masked hypertension diagnosed by home monitoring raised cardiovascular risk by 76 percent.
The patients with masked hypertension were probably undertreated because of their low office blood pressure, Staessen and his colleagues write.
Whether people are already on blood pressure treatment or not, Staessen said, those who have risk factors for masked hypertension, including smokers, obese people and older people, should monitor their blood pressure at home.
"The clinical relevance of masked hypertension is that often our blood pressure assessments are done in the clinic in a sort of an ideal setting where there's a nurse or a medical assistant and provider," said Dr. Edgar Miller, III, an epidemiologist at Johns Hopkins University in Baltimore.
Miller, who was not involved in the studies, said that home monitoring of blood pressure is becoming popular, but it's not standard yet, because current treatment guidelines are built around clinical assessments of blood pressure.
"We need to figure out ways to encourage home blood pressure monitoring, figure out standardized ways to report it to your provider, and there's more research needed into how providers should react to home blood pressure measurements," Miller said.