A simple arm-position error may raise your blood pressure reading by nearly 7 points

Something as basic as where you place your arm can make a blood pressure reading appear far higher than it truly is.

A study led by Johns Hopkins Medicine found that the way a patient positions an arm during a blood pressure check can significantly influence the result. Arm positions commonly used during screenings may make blood pressure seem higher than it really is, which could lead to a mistaken diagnosis of hypertension.

In findings published in JAMA Internal Medicine, researchers compared three positions: an arm supported on a desk, an arm resting on a lap, and an unsupported arm hanging at the side. They found that resting the arm on the lap raised systolic pressure — the top number in a blood pressure reading — by nearly 4 mmHg, while letting the arm hang at the side increased it by nearly 7 mmHg.

Arm position can have a major impact

According to Tammy Brady, M.D., Ph.D., vice chair for clinical research in the Department of Pediatrics at the Johns Hopkins University School of Medicine, medical director of the pediatric hypertension program at Johns Hopkins Children's Center, deputy director of the Welch Center for Prevention, Epidemiology, and Clinical Research, and senior author of the study, arm position makes a “huge difference” in getting an accurate reading.

The research team said the results back clinical guidance that recommends keeping the patient's arm firmly supported on a desk or another surface during blood pressure measurement.

According to the American Heart Association, nearly half of U.S. adults have elevated blood pressure, a condition diagnosed when the force of blood moving through the vessels is higher than normal, on average 120/80. If left untreated, high blood pressure raises the risk of heart attack, stroke and other serious cardiovascular problems.

Hypertension often causes few or no obvious symptoms, which makes early and routine screening during medical visits especially important. In many cases, blood pressure can be managed with lifestyle changes such as weight loss, healthier eating and exercise, along with medication when needed.

How blood pressure should be checked

The American Heart Association's clinical practice guidance lists several steps to improve accuracy. These include using the right cuff size, supporting the back, keeping the feet flat on the floor with the legs uncrossed, and placing the arm correctly. The center of an adjustable cuff should be at mid-heart level while the arm rests on a desk or table.

Even so, the researchers said blood pressure is often taken while patients sit on an exam table with little or no arm support. In some cases, a clinician holds the arm; in others, the patient rests it in the lap.

For the Johns Hopkins study, researchers enrolled 133 adults, including 78% Black participants and 52% women. Participants were between 18 and 80 years old and were randomly assigned to one of six groups that differed in the order in which the three seated arm positions were tested.

All readings were taken during a single visit. Before measurements began, participants emptied their bladders and walked for two minutes to mimic the experience of arriving at a clinic before a screening. They then sat quietly for five minutes with their backs and feet supported.

Each participant wore an upper-arm cuff matched to arm size. Researchers used a digital blood pressure device to collect three sets of three measurements, with 30 seconds between each reading.

Researchers tightly controlled the readings

After each set of three readings, the cuff was removed. Participants then walked for two minutes and rested for another five minutes. During the same visit, each person also completed a fourth set of three measurements with the arm supported on a desk, allowing researchers to account for the natural variation that can occur between readings.

Every measurement was taken in a quiet, private setting. Participants were also told not to speak with researchers or use their phones while their blood pressure was being checked.

The results showed that the two arm positions often seen in clinical practice — resting the arm on the lap or leaving it unsupported at the side — produced noticeably higher readings than the recommended position of supporting the arm on a desk.

Resting the arm on the lap overestimated systolic blood pressure by 3.9 mmHg. It also overestimated diastolic blood pressure, the bottom number, by 4.0 mmHg.

Leaving the arm unsupported at the side produced an even larger difference. Systolic pressure was overestimated by 6.5 mmHg, while diastolic pressure was overestimated by 4.4 mmHg.

A difference that could change a diagnosis

Sherry Liu, M.H.S., an epidemiology research coordinator at the Welch Center for Prevention, Epidemiology, and Clinical Research in the Department of Epidemiology at Johns Hopkins Bloomberg School of Public Health and a study author, said that if blood pressure is consistently measured with an unsupported arm, the resulting overestimate of 6.5 mmHg could mean the difference between readings such as 123 and 130 systolic, or 133 and 140 — which would be classified as stage 2 hypertension.

The researchers noted that the findings may apply only to blood pressure readings taken with automated devices and may not necessarily extend to other kinds of equipment.

Brady said the study still shows why clinicians should follow recommended measurement procedures more carefully. She also said patients should advocate for themselves in clinical settings and when checking their blood pressure at home.

Along with Brady and Liu, Johns Hopkins authors include Di Zhao, Ahmed Sabit, Chathurangi Pathiravasan, Junichi Ishigami, Jeanne Charleston, Edgar Miller III, Kunihiro Matsushita and Lawrence Appel.

The study was supported by Resolve to Save Lives, which is funded by Bloomberg Philanthropies, the Bill and Melinda Gates Foundation and Gates Philanthropy Partners, which is funded with support from the Chan Zuckerberg Foundation.