People with high blood pressure who monitor it regularly at home may be getting incorrect readings with devices that take measurements on the wrist, a new study suggests.

Researchers found that self-measurement at home with wrist devices often led to false reports of elevated blood pressure when compared to measurements in a doctor's office. Accurate readings often depended on correct positioning of the wrist, which patients either didn't understand or didn't remember how to do.

"Arterial blood pressure is a silent killer," said lead author Dr. Edoardo Casiglia, a European clinical hypertension specialist at the University of Padua in Italy. "It's important for those with high blood pressure to be aware of their numbers and receive adequate treatment."

High blood pressure, also known as hypertension, can lead to heart attack, stroke and kidney problems. About one in three adults in the U.S., or 70 million people, have high blood pressure, according to the Centers for Disease Control and Prevention.

The American Heart Association recommends that people with high blood pressure take measurements at home with commercially available monitoring devices because blood pressure fluctuates throughout the day and occasional readings at a doctor's office don't provide a true assessment of a person's condition.

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"The only way to monitor blood pressure consistently is to trust patients to measure at home," Casiglia told Reuters Health by email. "Many devices are available, reliable and good. Recently, many wrist devices have appeared on the market, but they require the wrist to be placed exactly at heart level."

Often, as seen in the study, patients rest the wrist device below heart level, which gives false high numbers and may encourage a doctor to increase medication. On the other hand, Casiglia said, holding the wrist at higher than heart level gives false low numbers and may encourage doctors to reduce treatment.

Casiglia and colleagues trained 720 patients to use wrist devices and then measured blood pressure at home and in the doctor's office with both an upper-arm device and a wrist device. The patients were instructed to take upper arm and wrist measurements themselves at home every morning and evening at the same time of day for seven consecutive days.

In general, healthy people have a systolic blood pressure, the measurement when the heart beats, of less than 120 mmHg, and diastolic pressure, when the heart rests, of less than 80 mmHg. Blood pressures above 140 mmHg systolic or 90 mmHg diastolic are considered hypertension.

In the current study, a total of 620 patients had inaccurate measurements at home, with 433 of these having discrepancies of more than 10 mm/Hg.

"Even after a training course, we found that choosing correct wrist position largely depends on cognitive pattern, which can be influenced by age and education," Casiglia said. "This is why the wrist devices are not good for everybody."

Doctors should decide which patients are a good fit for at-home wrist measurement, the study team writes in Hypertension.

"Using upper arm measurement is the gold standard and always our first choice," said Dr. Vincent Canzanello, a clinical hypertension specialist at the Mayo Clinic in Rochester, Minnesota, who wasn't involved with the study. "However, sometimes the cuffs don't fit larger arms or are painful for patients."

One limitation of the study is a potential "white coat effect," in which some people's blood pressure rises simply from the stress of being in a doctor's office, Canzanello told Reuters Health.

"Typically, our clinical assistants do several automated readings before a health care provider ever enters the room to avoid this effect," he said. "Studies show that physicians are notoriously bad at measuring blood pressure in terms of following the proper guidelines," he added.

About a third of Americans with high blood pressure have it controlled correctly, according to the American Heart Association. Proper at-home measurement could help patients obtain the correct treatment and medication.

"A device that measures the upper arm with the appropriately-sized cuff is the best option," Canzanello said. "But for those who need to use a wrist device, this study shows that taking measurements with the proper instructions is key."