Why your high blood pressure reading is probably wrong

Have you been told you have high blood pressure (hypertension)?  Get a second opinion—the reading could well have been wrong.

Blood pressure (BP) screening can “substantially overestimate” levels, and it’s all down to the positioning of the patient’s arm when the reading is done.

Researchers from John Hopkins assessed the different blood pressure readings that are achieved when the patient’s arm is in different positions—when the arm is supported on a desk, when it is on a lap, and when it is unsupported and is at the patient’s side.  

In the lap position, the reading overestimated systolic pressure (the top number in a BP score) by nearly 4 mmHg, while an unsupported arm reading overestimated it by nearly 7 mmHg.

The position of the arm makes a “huge difference” in determining a blood pressure reading, said Tammy Brady, one of the researchers.

The American Heart Association estimates that half of all adults have hypertension, and have a reading that is above the standard 120/80 BP level.

But the new study throws into doubt the prevalence of hypertension.  The researchers recruited 133 people who had three BP readings while their arm was supported and unsupported.  They discovered that the reading was higher when the arm was in the patient’s lap or unsupported at the patient’s side than when it was resting on a desk.

Placing the arm in the lap produced a reading that overestimated systolic BP by 3.9 mmHg and diastolic pressure (the second number) by 4 mmHg.  An unsupported arm produced a reading that overestimated systolic pressure by 6.5 mmHg and diastolic by 4.4 mmHg.

This overestimate would trigger a wrong diagnosis of type 2 diabetes, the researchers say.

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