More and more experts now recommend that people with high blood pressure regularly check their blood pressure at home. Doing this gives people an idea where their blood pressure stands in between office visits and can motivate them to care more about their health. It also helps doctors make quick medication adjustments to keep blood pressure in the healthy zone. But home blood pressure monitors aren’t always as accurate as they should be. "Home blood pressure monitors may be inaccurate in 5% to 15% of patients, depending on the threshold for accuracy used," according to Dr. Swapnil Hiremath, a kidney specialist at Ottawa Hospital in Canada.
Because many doctors do rely at least in part on home measurements to guide treatment, such inaccuracies could end with some people taking too much or too little blood pressure medication. "We are going down that road of asking people to measure their pressures at home," Dr. Hiremath says. "We want to empower patients, but we also want to make sure the measurements are accurate."
Dr. Hiremath says the finding emerged from a program to teach people with kidney disease on how to use home blood pressure monitors. Untreated or inadequately treated high blood pressure is the main cause of kidney disease today and contributes to its complications.
Starting in 2011, people with kidney disease were asked to bring their home blood pressure monitoring equipment to the kidney clinic at Ottawa Hospital to have it checked for accuracy against a standard office device.
"I was taken aback by how inaccurate some of the machines were," Dr. Hiremath says. "They were sometimes 15 or 20 mm Hg off."
Dr. Hiremath and his colleagues pulled together blood pressure records for 210 clinic patients. For 30% of them, the systolic pressure—the first number of a blood pressure reading—was 5 mm Hg or more different from the office reference measurement. The diastolic pressure (second number) was similarly inaccurate. "In one patient, the pressure was off by 21 mm Hg," Dr. Hiremath says.
The fact that blood pressure measurements vary is no surprise. Blood pressure wanders all over the map throughout the day. It’s generally lowest first thing in the morning, after a person wakes up, and then steadily rises. Blood pressure responds dynamically to movements, meals, and moods.
"Blood pressure is variable even when measured by precise methods," says Dr. Christian Ruff, a cardiologist at Harvard-affiliated Brigham and Women’s Hospital. "To get a more accurate assessment of blood pressure, regardless of blood pressure monitor used, people should perform multiple recordings and average them."
Dr. Ruff strongly encourages home monitoring to help keep a person’s blood pressure within healthy boundaries. "Home monitoring allows patients and their physicians to jointly engage and optimize treatment of hypertension in a more rapid manner."
Before starting routine blood pressure checks at home, Dr. Ruff says, it’s important to bring your home blood pressure monitor into your doctor’s office to test it against a known, accurate instrument. It’s also a good idea to redo the test every year.
The best device for most people is a device with an automatically inflating arm cuff and large digital readout for easy reading. To get the most accurate blood pressure readings at home, follow these steps:
- Avoid caffeinated or alcoholic beverages, and don’t smoke, during the 30 minutes before the test.
- Sit quietly for five minutes with your back supported and feet on the floor.
- When making the measurement, support your arm so your elbow is at the level of your heart.
- Push your sleeves out of the way and wrap the blood pressure cuff over bare skin.
- Measure your blood pressure according to the machine’s instructions. Leave the deflated cuff in place, wait a minute, then take a second reading. If the readings are close, average them. If not, repeat again and average the three readings.
- Don’t be too concerned if a reading is high. Relax for a few minutes and try again.
- Keep a record of your blood pressure readings and the time of day they are made.