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Why can the wrong cuff add almost 20 points to a blood-pressure reading?
A blood-pressure reading can look reassuringly precise while being substantially wrong. In a randomised crossover trial published in 2023, researchers tested 195 adults using an automated device and different cuff sizes. When participants needed an extra-large cuff, using a regular cuff instead their systolic pressure by an average of 19.5 mmHg. Systolic pressure is the upper number in a blood-pressure reading: the pressure when the heart contracts. That difference was not evidence that their cardiovascular health had suddenly deteriorated. It was a measurement error produced by equipment that did not fit.
The cuff does not simply sense pressure at the skin: it squeezes the arm while the device detects pressure oscillations associated with the pulse. Its inflatable bladder must be matched to the arm's circumference. An ill-fitting cuff can therefore distort the result before anyone interprets the numbers. For participants who needed a small cuff, the regular cuff underestimated systolic pressure instead. This was a mismatch, not merely random noise. Repeating a reading with the same unsuitable cuff is no substitute for correcting the fit.
The practical is to check the fit before drawing conclusions. The American Heart Association recommends a cuff suited to the upper arm's circumference and at least five minutes of quiet rest before measurement. The trial's average error is not a correction factor that individuals should subtract from their own readings: the effect varied with the cuff size required. Decisions based on misleading measurements could lead to unnecessary treatment or missed high blood pressure. Reliable monitoring starts with the equipment, not just the number on the screen.