Margen
Letting the arm hang at the side raised systolic pressure by an average of 6.5 mmHg, compared with resting it on a table at heart height, in the ARMS trial. Placing it on the lap produced an average difference of 3.9 mmHg. These are results obtained under controlled conditions with 133 adults and several measurements in each position; they do not guarantee that any given home monitor will drift by exactly the same amount.
The study was a randomised crossover trial: each participant went through the different positions and served as the reference for their own readings. A team from Johns Hopkins University published it in JAMA Internal Medicine in 2024.
A single stray reading is worth almost nothing. Standard home monitoring asks for measurements morning and evening for seven consecutive days, two each time, discarding the first day: that leaves twenty-four figures, and what you take to the appointment is their average.
These six steps concentrate the factors most likely to move the number:
1. Avoid coffee, tobacco and exercise for half an hour.
Wait thirty minutes after your last coffee, cigarette or physical effort. Coming in in a rush, climbing stairs or carrying weight sets off responses that may still be present when the cuff inflates. Let that interval pass before you start.
2. Rest and settle your body properly.
Sit for five minutes in a chair with a back. Keep your back supported, your feet flat on the floor and your legs uncrossed. The chair should let you hold that position without effort. While you rest, do not take a call or hold a conversation that winds you up.
3. Support the arm at heart height.
Lay the whole forearm on a table and relax your hand. The middle of the cuff should end up roughly level with your heart. If the surface is too low, add a cushion or a firm book under the arm. Do not hold the weight up yourself and do not leave the elbow dangling. If your device is a wrist monitor this matters even more: the wrist drifts off heart level with the slightest change of posture, which is why the guidelines prefer upper-arm cuffs.
The reference is the heart because the vertical difference between it and the cuff changes the pressure reaching the point of measurement. An arm in the lap sits lower; a hanging arm also adds the effort of keeping the joint steady while the cuff inflates.
4. Use the right cuff and put it against the skin.
Check that your arm circumference falls inside the range printed on the cuff. In another trial published in 2023, using a regular cuff on people who needed an extra-large one overestimated systolic pressure by about 19.5 mmHg. An excessively large cuff can also shift the result in the opposite direction.
Do not force the accessory that came in the box if the markings say it does not match your arm. Cuffs come in other sizes, and some devices only work with particular models.
Put it directly on the bare arm. A sleeve underneath adds material between the skin and the inflatable bladder; a rolled-up sleeve can compress the upper arm. Line up the edge and the tube following the diagram on the device.
5. Do not talk during the measurement.
Press the button and stay still until the cuff has finished deflating. Talking changes your breathing, activates muscles and can make you move your arm. Leave the phone out of reach too, so you are not turning, typing or reacting while the machine works.
6. Repeat and work out the average.
Wait a minute and take a second reading in the same position. Record both and work out the mean. Do it at comparable times on the days your healthcare professional has indicated, noting the date and the time of each reading. Take the complete record to your appointment, including any result that does not fit with the rest. Note which arm you used, too: the first time, measure both and from then on always use the one that gives the higher figure, and a consistent difference of more than ten millimetres between one arm and the other is in itself a reason to get it looked at.
If the figure comes out very high or appears alongside symptoms, seek urgent medical attention. That situation does not wait for the next well-taken measurement.