A difference between your two arms is normal and expected; what matters is how big it is. A systolic gap under about 10 mmHg is unremarkable, while a gap of 10 mmHg or more has been linked in pooled data from nearly 54,000 people to higher cardiovascular and all-cause mortality. The practical consequence is simple and often missed: once you know which arm reads higher, that is the arm you use for every home reading afterwards.
Is it normal for the two arms to give different readings?
Yes. Blood travels a slightly different path to each arm, and the vessels on the two sides are rarely identical, so most people show some difference. Momentary variation adds to it: your pressure changes minute to minute, so measuring the left arm and then the right will show a gap that is partly real difference and partly just time passing. This is why a single pair of readings should never be treated as a verdict.
How big a difference actually matters?
The INTERPRESS-IPD collaboration, which merged individual data from 24 studies and almost 54,000 participants with ten years of follow-up, found that a systolic difference of 10 mmHg or more between arms was associated with increased cardiovascular events and mortality. Current UK and European guidance still uses a slightly higher bar, treating a difference of 15 mmHg or more as the signal for further assessment. Either way, the difference is information for your clinician, not something to interpret alone.
Which arm should I use from now on?
The one that reads higher. This is the part that changes your daily routine. Guidelines are consistent that both arms should be checked when you start monitoring, and that the higher-reading arm becomes the reference arm for everything afterwards, because using the lower arm systematically understates your pressure. If you have been alternating arms because one is more comfortable, your log has been mixing two different measurements.
How do I check both arms properly?
Sit quietly for five minutes first, then measure one arm, then the other, then go back and measure the first arm again. Averaging the two readings from the first arm cancels out much of the drift that comes simply from time passing. Ideally repeat the whole comparison on a second day before concluding anything, because a one-off gap is often just variation. Anything you find is worth showing to your clinician rather than acting on.
What should my log show about this?
Which arm each reading came from, at minimum on the day you compare them, and then consistently once you have chosen. A diary that silently mixes arms can show a 12-point "improvement" that is nothing more than a change of side. If you photograph your monitor into the Monitor2Log diary, the notes column on each row holds that detail; a printable log has the same column.
Does a big difference mean something is wrong?
Not on its own, and it is not something to diagnose from a home monitor. A persistent large gap is a recognised marker that clinicians follow up, and the follow-up is a clinical assessment, not a home measurement. What a home log contributes is the evidence that the gap is persistent rather than a one-morning artefact, which is genuinely useful to the person assessing it.
Sources
- INTERPRESS-IPD Collaboration, individual participant meta-analysis of inter-arm blood pressure difference and mortality.
- Journal of Human Hypertension, inter-arm blood pressure difference as a risk marker.
- NICE NG136, hypertension in adults: diagnosis and management.
- European Society of Cardiology, 2024 guidelines for elevated blood pressure and hypertension.
General information only, not medical advice. A persistent difference between your arms is something to raise with your clinician.