Study shows the position the arm must be in to get an accurate blood pressure reading(Image: Getty Images)

People may be wrongly told they have high blood pressure over simple mistake they make

US experts have highlighted a problem with blood pressure readings taken when the arm is by someone's side or on their lap - flagging how it should be measured

by · The Mirror

People may have been wrongly diagnosed with high blood pressure because their arm was in the wrong position when the reading was taken, researchers have warned.

A review by US experts has revealed some arm positions - such as resting the arm on the lap - can substantially overestimate blood pressure readings.

The NHS advises that correct positioning is to sit on an upright chair with a back, placing both feet flat on the floor and resting the arm on a table or desk. People should also rest for five minutes before taking a reading and then take another reading several minutes later to check it is accurate.

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How not to do it - experts warn about the readings produced when the arm rests on the lap( Image: Getty Images)

Dr Tammy Brady, from the Johns Hopkins University School of Medicine in the US, said arm position makes a "huge difference" when it comes to an accurate blood pressure measurement. Her report said: “Few studies have rigorously evaluated the effects of arm position on blood pressure.

“This crossover randomised clinical trial demonstrates the considerable effect of arm position on blood pressure readings. Specifically when measurements are obtained with arm positions frequently used in clinical practice - ie on the lap or at the side - the readings obtained are markedly higher than those obtained with the arm positioned according to published guidelines.”

The NHS advises that normal blood pressure is between 90/60mmHg and 120/80mmHg. High blood pressure is considered to be 135/85 or higher if the reading was taken at home, or 140/90mmHg or higher if the reading was taken at a pharmacy, GP surgery or clinic.

The study, published in the journal JAMA Internal Medicine, looked at blood pressure readings when arms were supported on a desk, arms were supported on a lap, and when arms were unsupported and hung at the patient's side. Researchers recruited 133 adults who ranged from age 18 to 80, whose blood pressure was taken during a single visit.

Experts say when taking a blood pressure reading people should always have their arm on firm support such as a desk or table( Image: Getty Images)

Putting an arm on the lap overestimated systolic pressure - the top number in a blood pressure reading - by 3.9mmHg and diastolic blood pressure - the bottom number - by 4mmHg. Meanwhile, an unsupported arm hanging at the side overestimated systolic pressure by 6.5mmHg and diastolic by 4.4 mmHg.

In the study participants were asked to go to the toilet before readings were taken. They all walked for two minutes to replicate somebody coming to the clinic, and then sat down and rested for five minutes with their backs and feet supported.

The results indicate people could end up having high blood pressure readings just because of the position of their arm - with an arm hanging by the side offering the highest readings.

Dr Brady said when having their blood pressure reading taken people should always have their arm on firm support such as a desk or table. Explaining the difference, the study concluded: “The vertical distance between the heart and the cuff increases when the arm is positioned in the lap or at the side. This increase in distance when the arm is positioned at levels below the heart leads to an increase in hydrostatic pressure (the force exerted due to gravitational pull) in the brachial artery.”