The way you place your arm to take blood pressure makes a "huge difference" in your hypertension diagnosis, warns a new study. Researchers examined common arm placements and discovered that incorrect positioning often leads to inflated readings, potentially misdiagnosing millions of Americans with hypertension.

According to the guidelines, the patient's arm must be supported on a desk during assessment with the mid cuff positioned at the heart level. The researchers of the latest study examined how other common non-standard practices, such as resting the arm on the lap or allowing it to hang unsupported at the side, affect blood pressure readings.

The results published in Jama Internal Medicine suggest that resting the arm on the lap overestimated systolic pressure by nearly 4 mmHg, while an unsupported arm hanging down inflated systolic pressure by almost 7 mmHg. The diastolic readings were higher by 4.0 mmHg for the arm in the lap position and 4.4 mmHg for the unsupported arm position.

"The findings confirm that arm position makes a huge difference when it comes to an accurate blood pressure measurement," said Dr. Tammy Brady, senior author of the study.

The study evaluated the blood pressure assessment of 133 adult participants between the ages of 18 to 80. During the trial, participants were randomly assigned to one of six groups, each with a different order of three seated arm positions: arm supported on a desk, arm on a lap, or unsupported arm hanging at the side.

Measurements were taken during a single visit between 9 a.m. and 6 p.m. Before taking the blood pressure readings, participants emptied their bladders and walked for two minutes to mimic a typical clinic visit. They then rested for five minutes while seated with their backs and feet supported.

Each participant wore a properly sized upper arm blood pressure cuff, and three sets of triplicate measurements were taken with a digital device, 30 seconds apart. After each set, the cuff was removed, and participants walked for another two minutes before resting for five minutes again.

Finally, a fourth set of triplicate measurements was taken with the recommended arm supported on a desk position. All measurements were done in a quiet, private space, with participants instructed not to talk or use their phones.

The measurements taken with arm positions resting on the lap or hanging unsupported were significantly higher than the recommended position.

The study also noted that these improper arm positioning could lead to 16% of U.S. adults, approximately 40 million individuals, being misclassified as hypertensive when using a systolic blood pressure cutoff of 140 mm Hg or higher. The misclassification rate rises to 22% (around 54 million individuals) when a 130 mm Hg or higher cutoff is applied.

"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, a study author, explaining how incorrect arm position leads to misdiagnosis.

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