A common mistake during blood pressure checks can make readings appear substantially higher than they really are, Johns Hopkins researchers have found, potentially leading to misdiagnosis.

The study of 133 adults found that resting the arm on the lap overestimated systolic pressure by 3.9 mmHg and diastolic by 4.0 mmHg. An unsupported arm hanging at the side overestimated systolic by 6.5 mmHg and diastolic by 4.4 mmHg, compared with the recommended position of the arm supported on a desk.

‘A 6.5 mmHg overestimate is the potential difference between a systolic reading of 123 and 130, or 133 and 140, which is considered stage 2 hypertension,’ said study author Sherry Liu of the Johns Hopkins Bloomberg School of Public Health.

The findings, published in JAMA Internal Medicine, apply to automated blood pressure devices. Researchers advise patients to advocate for proper arm support both in clinics and when measuring blood pressure at home.