Blood pressure monitoring done at home can be highly inaccurate for up to 15 percent of patients, according to a new study out of Canada.

Home monitoring is commonly recommended for patients dealing with high blood pressure. Little testing of home systems has been carried out before this study.

Blood pressure measurements were taken from a total of 210 patients, using both home systems, as well as calibrated mercury sphygmomanometers, the standard for health care offices. In 63 of these tests, the consumer-level device differed from the professional instrument by more than 5 mm/Hg, and 16 of the tests showed a difference of 10 mm/Hg for systolic blood pressure. This is the pressure measured as the heart contracts, pushing blood through the body. The diastolic (resting) pressure was off by 5 mm/Hg 32 percent of the time an 10 mm/Hg nine percent of the time.

The American Heart Association considers normal blood pressure to be numbers below 120/80 mm/Hg. Prehypertension is considered to range from 120 to 139 over 80 to 89. Stage one hypertension is diagnosed in patients, starting at 140/90 mm/Hg, and stage 2 at 160/100. A hypertensive crisis, readings over 180/110, require emergency medical care.

"Home blood pressure monitors may be inaccurate in 5 percent to 15 percent of patients, depending on the threshold for accuracy used. We recommend all patients with home monitors get them validated with their health care providers at least once," Swapnil Hiremath of Ottawa Hospital and the University of Ottawa said.

One in three Americans experience high blood pressure, a total of 67 million people, according to the Centers for Disease Control and Prevention. Across all age groups, men and women are affected by hypertension in roughly equal numbers. However, for people under the age of 45, men are more likely than women to exhibit high blood pressure. For seniors aged 65 or more, females are more likely than their male counterparts to have hypertension. The condition costs the nation $47.5 billion dollars each year in medical costs, medicine and missed time at work.

"More than 348,000 American deaths in 2009 included high blood pressure as a primary or contributing cause. That is 1,000 deaths each day... Unfortunately, about one of five U.S. adults with high blood pressure still do not know that they have it," CDC officials report on their Web site.

Study of the accuracy of home blood pressure monitors will be detailed in a talk to be delivered at the ASN Kidney Week 2014, to be held in November, the Pennsylvania Convention Center in Philadelphia,  sponsored by the American Society of Nephrology.