A new study found that having your arm in the wrong position during blood pressure checks — either at home or the doctor’s office — can result in readings “markedly higher” than when your arm is in the recommended position: appropriately supported on a table with the middle of the cuff positioned at heart level.

This suggests that not consistently having your arm positioned and supported appropriately during a blood pressure reading might result in a misdiagnosis of high blood pressure, which some experts worry could possibly lead to unnecessary treatment.

The study, published Monday in the journal JAMA Internal Medicine, found that having your arm resting in the lap during a blood pressure reading can lead to an overestimated systolic blood pressure measurement by 3.9 mm Hg and overestimated diastolic reading by 4 mm Hg. And having your arm hang by your side can lead to an overestimated systolic reading by 6.5 mm Hg and overestimated diastolic reading by 4.4 mm Hg.

Blood pressure is measured in units of millimeters of mercury or mm Hg, which consists of two numbers – the upper or systolic reading and the lower or diastolic reading. A person is considered as having high blood pressure or hypertension if they have consistent readings of 130/80 or higher.

“Health care providers need to be reminded about the importance of taking the time to do these steps properly,” said the study’s senior author Dr. Tammy Brady, who serves as vice chair for clinical research in the department of pediatrics at the Johns Hopkins University School of Medicine and medical director of the pediatric hypertension program at Johns Hopkins Children’s Center.

“I just hope that it raises awareness regarding how important things like arm position are to blood pressure measurement accuracy,” Brady said about the study.

“I also hope this study educates patients, empowering them to advocate for proper measurement when they’re in a clinic setting,” she said. “But also, because so many patients rely on home blood pressure measurements for hypertension diagnosis, evaluation, and treatment, I hope this educates them on how to properly position their arms because our findings absolutely extend to measurements in the home.”

The researchers, from Johns Hopkins University, recruited 133 adults from Baltimore, Maryland, between August 2022 and June 2023, to participate in the study. The adults, ages 18 to 80, were randomly assigned to have their blood pressure measured while their arms were positioned in three different ways: supported on a desk, as is the recommended standard practice; supported on their lap; or hanging by their side, unsupported.

Each study participant underwent a total of 12 blood pressure measurements with their arms in each of those various positions.

Among the participants, when their arms were supported on a desk as recommended by the standard clinical guidelines, the average of their blood pressure reading was 126/74, the researchers found.

But when the participants’ arms were resting in their laps during their readings, the average blood pressure reading was 130/78, and when their arms were hanging by their sides, the average of their readings was 133/78.

“I did not expect there to be that much of a difference when the arms were placed in the two alternative positions. We tested those positions because those are the positions that most people have their blood pressure measured, according to data and personal observations,” Brady said. “We thought there’d be a difference, but I was surprised by how much of a difference there was.”

The researchers wrote that there are several physiological reasons why blood pressure measurements could be erroneously higher when the arm is not adequately positioned or supported. A greater vertical distance between the heart and where the cuff is placed can increase hydrostatic pressure in the arteries, or the pressure of the blood against the wall, due to gravity’s pull, leading to an overestimation of blood pressure. And an unsupported arm can lead to the muscle contracting, which may cause an increase in blood pressure.

How to check blood pressure accurately

The new study findings are consistent with some separate previous research that has shown unsupported arm positions or the arm being positioned below heart level can overestimate a blood pressure reading.

“We know that arm position affects the blood pressure reading, what this study does is gives us a more precise measure of just how much this has an impact,” Dr. Nichola Davis, vice president and chief population health officer at NYC Health and Hospitals, who was not involved in the study, said in an email.

“This is seen in participants with all levels of blood pressure, but more pronounced in participants with elevated blood pressure. These are important findings because we diagnose and treat hypertension based on blood pressure readings, and these readings need to be accurate,” Davis said in the email.

The researchers wrote in the study that the “latest clinical practice guidelines emphasize several key steps for accurate measurement,” including:

• Wear the appropriate cuff size.
• Have your back supported.
• Keep feet flat on the floor with legs uncrossed.
• Have the appropriate arm position – the cuff should be positioned at heart level with the arm supported on a desk or table.

Wearing the appropriate cuff size is key because an improperly sized blood pressure cuff can distort blood pressure readings taken by automated blood pressure devices. One study, published last year in the journal JAMA Internal Medicine, found that using standard, regular-sized blood pressure cuffs resulted in “strikingly inaccurate” readings if a patient required differently sized ones, particularly for participants who fit larger cuffs.

There are some steps people should take before they have their blood pressure checked, said Dr. Gail Adler, a cardiovascular endocrinologist and co-specialist in the Hypertension Clinic at Brigham and Women’s Hospital, who was not involved in the new study.

“In the 30 minutes before you take your blood pressure, you want no smoking, no exercise, no caffeine, no alcohol, and … no major arguments. Don’t be psychologically stressed,” Adler said.

“Then what you want to do is you want to sit in the chair, get in the right position, and for five minutes, you relax, you don’t talk, you rest your arm comfortably, you sit upright with your back straight, you keep your legs uncrossed and your feet flat on the floor. Make sure you don’t have to pee, if you have to pee or urinate, that’s going to raise your blood pressure,” she said. “And you’re not supposed to talk when you take your blood pressure, and the cuff should be against your arm. So you don’t want to put it over clothes.”

Because people often feel more calm and comfortable at home, “what we’re finding now is studies suggest that blood pressures measured at home are a better indicator of your usual blood pressure than blood pressures measured in the office,” Adler said.

Blood pressure measurements are often “suboptimally performed” in clinical practice, which can lead to errors that subsequently and inappropriately alter doctors’ decisions around managing a patient’s blood pressure in about 20% to 45% of cases, according to the American Medical Association.

“Anecdotally, I think all of us on the study team have started to pay more attention to how our blood pressures are being measured. I’ve certainly paid attention to how my kids’ blood pressures are being measured,” Brady said.

‘We shouldn’t make hasty judgments’

Having high blood pressure raises the risk for heart disease and stroke, which are leading causes of death in the United States.

It’s estimated that nearly half of adults in the United States have high blood pressure, but only about 1 in 4 adults with high blood pressure has their blood pressure under control, according to the US Centers for Disease Control and Prevention.

“I don’t think that we are aggressive enough with the treatment of hypertension in America,” Dr. Dave Montgomery, a cardiologist at Piedmont Healthcare in Atlanta, who was not involved in the new study, said in an email.

“Hypertension is one of the commonest risk factors for cardiovascular disease and among the easiest to treat and control. Still, hypertension is the culprit or major contributor to a large proportion of cardiovascular morbidity and mortality,” he said. “We too often miss the mark.”

Regarding the new study, he added that he doesn’t see a blood pressure reading that may be off by 4 to 5 mm Hg – due to arm position – leading to overdiagnosis or overtreatment, in the form of a new prescription or dose modification.

“If someone has normal blood pressure, 4 mmHg won’t change that. But in someone with a diagnosis of hypertension, 5-10 points may signal that we don’t quite have the control we seek,” Montgomery said. He added that blood pressure can change minute by minute, due to physical factors and someone’s psycho-emotional states.

“This means that in general we shouldn’t make hasty judgments about one blood pressure reading. But consistently high blood pressure should be treated aggressively,” he said. “This is how we lower the burden of heart disease.”