A simple positioning mistake during a blood pressure check could quietly alter the results.

That quick blood pressure check at the doctor’s office may not be as accurate as you think. According to research from Johns Hopkins Medicine, something as simple as where you rest your arm during the test can significantly raise the reading and potentially lead to a hypertension diagnosis that is not actually correct.

The study found that two common arm positions frequently used in clinics can artificially increase blood pressure measurements by enough to shift some patients into a higher risk category. Researchers say the findings are especially important because high blood pressure is one of the leading causes of heart disease, stroke, kidney failure, and preventable death worldwide.

Published in JAMA Internal Medicine, the study examined how different arm positions affect blood pressure readings in adults between 18 and 80 years old. Researchers compared the guideline-recommended position, with the arm supported on a desk at heart level, against two alternatives often seen in real-world settings: resting the arm in the lap and letting the arm hang unsupported at the side.

The differences were striking.

Small Changes, Significant Effects

When participants rested their arm on their lap, systolic blood pressure readings rose by an average of 3.9 mmHg, while diastolic pressure increased by 4.0 mmHg. When the arm hung unsupported at the side, systolic pressure climbed by 6.5 mmHg and diastolic pressure by 4.4 mmHg.

That may sound minor, but even small changes in blood pressure readings can have major consequences. A difference of just a few points can determine whether someone is considered healthy, at risk, or in need of medication.

“If you are consistently measuring blood pressure with an unsupported arm, and that gives you an overestimated BP of 6.5 mmHg, that’s a potential difference between a systolic BP of 123 and 130, or 133 and 140, which is considered stage 2 hypertension,” said Sherry Liu, M.H.S., a study author and epidemiology research coordinator at the Johns Hopkins Bloomberg School of Public Health.

Hypertension affects nearly half of U.S. adults, according to the American Heart Association. Yet many people do not realize they have it because symptoms are often absent until serious damage has already occurred. That makes accurate screening especially important.

Why Proper Positioning Matters

Doctors have long known that talking during a reading, crossing your legs, using the wrong cuff size, or drinking caffeine beforehand can distort results. This study adds more evidence that arm position is another overlooked factor that can quietly skew measurements.

The researchers designed the trial to closely mirror a typical doctor’s appointment. Participants first emptied their bladders, walked for two minutes, and then sat quietly with their backs supported and feet flat on the floor. Each person underwent repeated blood pressure measurements using all three arm positions. To reduce the impact of natural fluctuations in blood pressure, researchers also repeated the standard desk-supported measurement at the end of the session.

The study included 133 adults in Baltimore, Maryland, with a broad mix of ages and health backgrounds. About 36% had systolic blood pressure readings of 130 mmHg or higher, and 41% had obesity. The pattern of inflated readings remained consistent across groups, including older adults and people with obesity.

A Widespread Issue in Clinics and Homes

Researchers say the problem is widespread because many clinics do not strictly follow measurement guidelines. Patients are often seated on exam tables without proper arm support, or they may rest their own arm in their lap while a reading is taken. In some cases, a health care worker holds the patient’s arm during the test, which can also affect positioning.

The standard recommendation is simple: sit upright in a chair with feet flat on the floor, legs uncrossed, and the arm resting on a desk or table so the blood pressure cuff sits at heart level.

Tammy Brady, M.D., Ph.D., senior author of the study and a hypertension specialist at Johns Hopkins Medicine, said the findings reinforce the need for both clinicians and patients to pay closer attention during screenings.

The issue extends beyond doctor’s offices. Millions of people now use home blood pressure monitors, but many may unknowingly take readings while sitting on a couch, reclining, or holding their arm incorrectly. Experts say inaccurate home measurements can create unnecessary anxiety or lead people to believe their treatment is not working.

The researchers caution that the study focused on automated blood pressure devices, so the findings may not apply to every type of monitor. Still, they say the message is clear: proper positioning matters far more than many people realize.

Follow-up Research

The authors later followed up the work with a broader review exploring how blood pressure checks could be streamlined in busy clinics without sacrificing accuracy. The follow-up analysis found that some guideline steps, including long rest periods before measurements, may be shortened or simplified. However, the researchers stressed that proper cuff sizing, correct patient posture, and keeping the arm supported at heart level remain essential because mistakes in those areas can significantly distort readings.

They concluded that a faster, evidence-based approach to blood pressure screening could allow clinics to evaluate far more patients each day while still avoiding the kinds of measurement errors uncovered in the original arm-position study.