When it comes to taking an accurate blood pressure (BP) measurement, arm position makes a “huge difference,” says Tammy Brady, medical director of the pediatric hypertension program at Johns Hopkins Children’s Center. She is senior author of a new study showing that commonly used ways of positioning a patient’s arm during blood pressure screenings can substantially overestimate test results and may lead to a misdiagnosis of hypertension.

In a report on the study, published in JAMA Internal Medicine, researchers examined the effects of three different arm positions: an arm supported on a desk, an arm supported on a lap, and an unsupported arm hanging at the patient’s side. Researchers found that lap support overestimated systolic pressure (the top number in a BP reading) by nearly 4 mmHg, and an unsupported arm hanging at the side overestimated systolic pressure by nearly 7 mmHg.

The findings underscore the importance of adhering to clinical guidelines calling for firm support on a desk or other surface when measuring blood pressure, the investigators say.

“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,” says study author Sherry Liu, an epidemiology research coordinator at the Welch Center for Prevention, Epidemiology, and Clinical Research.

The researchers caution that their study results may only apply during screenings with automated BP devices, and may not apply to readings done with other BP devices. However, Brady says the findings suggest that clinicians need to pay better attention to best practice guidelines, and that patients “must advocate for themselves in the clinical setting and when measuring their BP at home.”