Why Is Blood Pressure Different in Each Arm?

A small difference in blood pressure between your left and right arms is completely normal and shows up in most people. The gap exists because of how arteries branch off the aorta, and in the majority of cases it amounts to only a few millimeters of mercury. Where the difference becomes clinically meaningful is when it consistently exceeds about 10 mmHg on the systolic (top number) reading, which can be a marker of underlying vascular disease and, research now shows, a predictor of future cardiovascular problems even in otherwise healthy people.

How Common Is the Difference

Almost everyone has some measurable gap between arms. In a large study of more than 3,000 healthy young adults, only about 5% had perfectly identical systolic readings in both arms. Roughly a third had a systolic difference of 1 to 4 mmHg, another 38% fell in the 5 to 10 mmHg range, and about 26% showed a systolic gap greater than 10 mmHg.¹ That last figure is higher than many clinicians expect, especially in young, seemingly healthy people. A systematic review pooling data from multiple studies found that about 20% of patients had a systolic interarm difference of 10 mmHg or more, while roughly 4% hit the 20 mmHg mark.²

The takeaway from those numbers is that having different readings in each arm is the rule, not the exception. What varies is the size of the gap and whether it stays consistent over repeated measurements.

The Anatomy Behind the Gap

The explanation starts where the aorta leaves the heart. The brachiocephalic artery, which supplies the right arm, branches off the aortic arch first and is oriented more directly in line with the flow of blood coming up from the heart. The left subclavian artery, which feeds the left arm, branches off separately and at a slightly different angle. Because the right-side branch sits closer to the pressure source and is more aligned with the direction of flow, blood pressure in the right arm tends to run slightly higher.³ This anatomical asymmetry is built in from birth and has nothing to do with disease. It is also why most clinical guidelines suggest, when in doubt, using the arm with the higher reading as your reference.

Factors like height, resting heart rate, and even which arm the measurement is taken on first can nudge the numbers. In the study of young adults mentioned earlier, height, residential area, and a resting heart rate above 90 beats per minute all significantly affected the size of the interarm gap.¹ The mechanisms behind these associations are not fully worked out, but they reinforce the point that interarm differences reflect a mix of anatomy, physiology, and measurement conditions rather than a single clean cause.

When a Large Difference Signals Vascular Disease

A persistent systolic gap of 15 to 20 mmHg or more shifts the picture from normal variation to possible blockage. The most common culprit is subclavian artery stenosis, a narrowing of the artery that supplies one arm. In a study of patients already being treated for carotid artery disease, about 18% had significant subclavian stenosis, and nearly 70% of those patients showed an interarm systolic gap of 20 mmHg or more.⁴ Looking at it from the other direction, among people who had a 20 mmHg or larger gap, about two-thirds turned out to have significant subclavian stenosis.

If the narrowing is severe enough, it can cause subclavian steal syndrome. This happens when the blockage forces blood to flow backward through the vertebral artery on the affected side, essentially “stealing” blood from the brain to supply the arm. Symptoms can include episodes of dizziness, sudden loss of balance, and pain or fatigue in the arm during use.⁵ Most people with a moderate interarm difference will never develop steal syndrome, but the blood pressure gap is one of the earliest clues that the narrowing exists.

Aortic Dissection and the Emergency Setting

In an emergency room, a sudden and large interarm difference takes on a different urgency. Acute aortic dissection, a tear in the wall of the aorta, can compress or block one of the branch arteries and produce an abrupt drop in blood pressure on the affected side. Emergency department research found that a systolic gap exceeding 20 mmHg roughly tripled the odds that a patient was having an acute aortic dissection compared to other chest pain presentations.⁶

The pattern of which arm reads lower matters too. A Japanese study of aortic dissection patients found that when the left arm reading was more than 15 mmHg higher than the right, and the right arm systolic was below 130 mmHg, the odds of a dissection involving the ascending aorta were dramatically elevated. Every patient in that study whose left arm reading exceeded the right by more than 20 mmHg had a dissection that extended into the brachiocephalic artery, which feeds the right arm.⁷ None of this means a blood pressure difference by itself confirms a dissection. Patients typically present with severe, tearing chest or back pain, and the interarm gap is one piece of the diagnostic puzzle, not a standalone alarm.

What the Difference Predicts About Your Future Health

Even when no obvious vascular disease is present, a consistently large interarm gap appears to be an independent marker of cardiovascular risk. A primary care study in the UK found that patients with a systolic interarm difference of 10 mmHg or more had roughly three and a half times the risk of dying during follow-up, even after adjusting for other risk factors. Among patients with no pre-existing cardiovascular disease, the risk of death was still about two and a half times higher.⁸

A much larger individual-participant meta-analysis confirmed the trend on a broader scale. For every 5 mmHg increase in systolic interarm difference, all-cause mortality rose by about 5% and cardiovascular mortality by about 6%. The risk increase was detectable from a threshold as low as 5 mmHg and climbed steadily from there.⁹ The same research group also showed that using the higher of the two arm readings, rather than whichever arm happened to be measured, provided better predictions for mortality and cardiovascular events.¹⁰

The implication is practical. If your doctor only checks one arm, and it happens to be the lower one, your actual cardiovascular risk may be underestimated. The interarm difference acts as a kind of vascular stress signal: even in people who do not yet have a diagnosis of artery disease, a wider gap suggests that arteries somewhere in the body are stiffer or more narrowed than expected. Research in patients with treated high blood pressure found that a systolic interarm gap of 5 mmHg or more was associated with stiffer arteries and reduced blood flow to the legs.¹¹

How Measurement Technique Inflates or Hides the Gap

A surprising amount of the interarm difference you see on any given reading has nothing to do with your arteries and everything to do with how the measurement was taken. If a clinician checks your left arm, then wraps the cuff around your right arm a minute or two later, the readings will naturally differ because blood pressure fluctuates from moment to moment. Sequential measurement consistently produces a larger apparent interarm gap than simultaneous measurement. One study found that the average systolic difference dropped from about 8 mmHg with sequential checks to about 6 mmHg when both arms were measured at the same time.¹² That difference matters when you are trying to figure out whether a 10 mmHg gap is real or an artifact.

Cuff size is another common source of error, especially if your arms are different diameters. A randomized trial showed that using a regular-sized cuff on someone who needs a large or extra-large cuff inflated the systolic reading by about 5 mmHg for a large arm and nearly 20 mmHg for an extra-large arm. Going the other direction, putting a regular cuff on a thin arm lowered the reading by about 4 mmHg.¹³ If your arms differ noticeably in circumference and the same cuff is used on both, you can get a misleading interarm difference that is really just a cuff-fit problem.

The gold standard for detecting a true interarm difference is to measure both arms at the same time using properly fitted cuffs, and to repeat the process on at least one additional occasion. Community screening studies have used dual-cuff oscillometric devices for exactly this reason.¹⁴ If your doctor checks only one arm and you are concerned about accuracy, asking them to check the other arm on the same visit is a reasonable request.

Why Checking Both Arms Still Is Not Routine

Despite decades of guideline recommendations to measure blood pressure in both arms at least once, compliance remains low. A review noted that hypertension guidelines have urged dual-arm measurement for at least 25 years, yet most general practitioners do not consistently do it.¹⁵ The reasons are mostly practical: it takes extra time, clinics may not have dual-cuff devices, and the clinical significance of a small gap was, until recently, seen as uncertain. The accumulation of prognostic evidence over the past decade has strengthened the case, but workflow habits in busy primary care settings change slowly.

The consequence for patients is straightforward. If you have always had your blood pressure taken in just one arm, you may not know whether a significant interarm difference exists. It is worth having at least one check with both arms to establish a baseline. If the gap is under 10 mmHg, you can reliably use either arm going forward (preferring the higher-reading arm for treatment decisions). If the gap is 10 mmHg or more, your doctor may want to investigate further or at least use the higher arm consistently.

Pregnancy and Interarm Differences

Pregnant women have their own reasons to pay attention to interarm blood pressure differences. A study of more than 200 pregnant women found that about 8% had a systolic interarm difference of 10 mmHg or more. The more striking finding was what the gap meant for detecting hypertension in pregnancy: roughly 31% of hypertensive cases would have been missed if blood pressure was only measured in the left arm, and about 24% would have been missed if only the right arm was used.¹⁶ Since undiagnosed hypertension in pregnancy can lead to serious complications, using only one arm for all prenatal visits introduces real clinical risk.

Preliminary research has also explored whether the interarm gap in early pregnancy can predict preeclampsia. In a study of more than 7,000 pregnancies, women who later developed preeclampsia had a slightly but significantly higher diastolic interarm difference in the first trimester compared to those who did not. There was no similar signal in systolic readings.¹⁷ This is still early-stage research and not yet used clinically, but it illustrates how the interarm gap may eventually become a screening tool beyond traditional vascular disease.

Peripheral Artery Disease and Diabetes

Given that the interarm difference reflects vascular health, researchers have naturally asked whether it can help diagnose peripheral artery disease affecting the legs. The connection is indirect. While a large interarm gap points to disease in the subclavian or upper-limb arteries, it does not reliably predict blockages in the leg arteries on its own. One study found that the interarm systolic difference had essentially no correlation with the ankle-brachial index, the standard measure for leg artery disease.¹⁸

That said, the relationship becomes more nuanced when diabetes is in the picture. A study of people with diabetes found that among the three major cardiovascular complications (heart disease, stroke, and peripheral artery disease), only peripheral artery disease had a significant independent association with a larger interarm gap. Diabetes on its own did not drive the interarm difference; rather, the difference reflected underlying arterial changes that happened to overlap more with leg artery disease than with heart attack or stroke risk.¹⁹ The age of the patient modified this association, with the link becoming stronger in older adults.

Dialysis Patients and Fistula Access

People on hemodialysis often have a surgically created fistula or graft in one arm that connects an artery directly to a vein for dialysis access. This raises a specific concern: can you still safely measure blood pressure in the arm with the access, and does the access itself create a misleading interarm gap? A study of more than 300 dialysis patients found that the arm with the fistula actually ran about 9 mmHg higher in systolic pressure on average compared to the opposite arm. About 5% of patients had an interarm difference of 20 mmHg or more. No access clotted as a result of the blood pressure measurement, confirming that bilateral measurement is safe in this population.²⁰

This matters because dialysis patients are often told to never use the fistula arm for blood pressure checks, which means they may go years with measurements from only one arm. If the non-fistula arm happens to be the lower-reading arm, their blood pressure could be systematically underestimated. The emerging evidence suggests that checking both arms, at least occasionally, is both safe and diagnostically useful for detecting steal syndrome, a complication where the fistula diverts too much blood away from the hand.