At a first assessment for hypertension, measure both arms. If the readings differ, repeat the measurement; a persistent difference of 10 mmHg or more is common enough to matter and is associated with peripheral arterial and cerebrovascular disease. Thereafter, every reading should be taken in the arm that gave the higher value.
The practical cost of skipping this is silent. If the lower arm becomes the routine arm — often because it is the side the chair faces — every subsequent reading understates the patient's blood pressure, and the understatement is invisible because it is consistent. Patients are then under-treated for years on numbers that look reassuringly stable. Writing the chosen arm in the notes, not just the number, is what makes the check useful to whoever measures next.
- Measure both arms at the first assessment, and repeat if they differ
- Record which arm you used, in the notes and on the clinic letter
- Treat a persistent difference of 10 mmHg or more as a vascular finding worth examining further
- Use the higher-reading arm for all subsequent measurements, including home monitoring
- Tell the patient which arm to use on their home machine — they will otherwise use whichever is convenient
Why it matters
A blood pressure series measured in the wrong arm is consistently wrong, which is exactly what makes it look trustworthy.
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