Did You Know?

Your BP Reading Could Be Off by nearly 7 mmHg – Here’s Why

September 22, 2025

When it comes to measuring blood pressure (BP), small details can make a big difference. Clinical guidelines recommend that the arm be supported on a desk with the cuff positioned at heart level. Yet in everyday practice, this standard is often overlooked—patients may rest their arm on their lap or let it hang unsupported at their side. But how much does this really affect the reading?

A recent crossover randomized clinical trial, known as the ARMS study, set out to answer that question. Conducted in Baltimore between August 2022 and June 2023, the study enrolled 133 adults aged 18 to 80. Each participant had their BP measured multiple times under three different conditions:

  1. Proper technique – arm supported on a desk at heart level
  2. Lap position – hand resting on the lap
  3. Side position – arm hanging unsupported at the side

To account for natural fluctuations in BP, a fourth reading was taken using the proper technique again.

The results were striking. When the arm was placed on the lap, systolic and diastolic readings were on average 3.9 and 4.0 mm Hg higher, respectively, than when measured correctly. When the arm was left dangling at the side, the overestimation was even greater—6.5 mm Hg systolic and 4.4 mm Hg diastolic. These differences held steady across various subgroups, including age, obesity status, and baseline BP.

The takeaway? Even seemingly minor deviations from proper technique can lead to clinically significant overestimations of blood pressure. This could result in misdiagnosis, unnecessary treatment, or failure to recognize successful BP control. “If you consistently measure your blood pressure with your arm unsupported and end up with a reading that is 6.5 mmHg higher than it should be, that could mean the difference between a systolic blood pressure of 123 and 130, or 133 and 140 — which is considered stage 2 hypertension,” said study author Sherry Liu. In short, how we measure blood pressure matters. This study reinforces the importance of following established Guidelines — not just for accuracy, but for the safety and well-being of patients.

Just as we demand precision in lab tests and imaging, BP measurement must be treated as a serious medical test. Proper technique isn’t optional — it’s essential.

How to Take Your Blood Pressure Accurately:

Best Position for BP Measurement

  • Sit in a chair with back support
  • Keep feet flat on the floor, legs uncrossed
  • Rest your arm on a table, with the elbow at heart level
  • Place the cuff on bare skin, not over clothing
  • Relax and stay quiet during the reading


Which Arm to Use?

  • Measure BP in both arms at least once
  • Use the arm with the higher reading for future checks
  • A consistent difference of 10 mm Hg or more between arms may signal arterial issues – talk to your doctor


Before You Measure

  • Avoid caffeine, smoking, or exercise for 30 minutes
  • Sit quietly for 5 minutes
  • Take two readings, and a third if the first two differ by 5+ points

Sources:

Liu H, Zhao D, Sabit A, et al. Arm Position and Blood Pressure Readings: The ARMS Crossover Randomized Clinical Trial. JAMA Intern Med. 2024;184(12):1436–1442. doi:10.1001/jamainternmed.2024.5213
https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2824754

Johns Hopkins Medicine Study Finds Commonly Used Arm Positions Can Substantially Overestimate Blood Pressure Readings
https://www.hopkinsmedicine.org/news/newsroom/news-releases/2024/10/johns-hopkins-medicine-study-finds-commonly-used-arm-positions-can-substantially-overestimate-blood-pressure-readings