Which arm is more accurate for blood pressure?

Blood pressure measurements are an important tool for assessing cardiovascular health. Blood pressure can vary between the left and right arms, so determining which arm gives a more accurate reading is an important consideration when monitoring blood pressure.

Key Points

• Blood pressure can differ between arms due to anatomical differences in the arteries.
• For most people, the differences between arms are small and not clinically significant.
• For some individuals, however, the difference can be more substantial and impact treatment decisions.
• Guidelines generally recommend using the arm with the higher reading for diagnosing high blood pressure.
• The left arm is preferred for consistency, but using the right arm is acceptable if the difference is minimal.
• When monitoring blood pressure trends, consistently using the same arm will give the most accurate representation.

Anatomical Differences Between Arms

Blood pressure is a measurement of the force blood exerts on artery walls as it is pumped through the circulatory system. Systolic pressure reflects the pressure generated when the heart contracts, while diastolic pressure is the pressure in the arteries when the heart is relaxed between beats.

While we often think of blood pressure as a whole-body parameter, the actual pressure varies between different parts of the circulatory system. The arteries in the arms are close to the heart, so pressures measured at the brachial artery in the upper arm will be higher than pressures farther away from the heart, such as in the legs.

Even between arms, there can be subtle differences:

• The arteries may be slightly narrower or wider in one arm.
• One subclavian artery may be stenotic, or narrowed by fatty plaques.
• One arm may have been weakened by prior injury or surgery.
• One arm may have had lymph nodes removed, affecting drainage.

These anatomical factors can cause real differences in measured blood pressure between the arms. In most healthy individuals, however, these differences are minor.

Typical Difference Between Arms is Clinically Insignificant

Research has shown that a difference in systolic blood pressure of 5 mmHg or less between arms is common. One large study of over 54,000 patients found:

• 68% had a systolic difference ≤ 5 mmHg
• 89% had a difference ≤ 10 mmHg
• 95% had a difference ≤ 15 mmHg

Some key observations from the data:

• Systolic differences were more common than diastolic differences.
• Left arm readings were only slightly higher on average than right arm.
• Differences were more pronounced in older patients with high blood pressure.

For healthy patients, these small differences between arms are not considered clinically significant. Treatment decisions would not change based on such minor differences. However, the story changes for some high-risk patients…

When Inter-Arm Differences Matter

For most people, a small difference in blood pressure between the arms is normal. But for certain high-risk categories of patients, a significant inter-arm difference matters and could impact treatment:

• Older patients – Differences are more likely with increasing age.
• Diabetes – Higher likelihood of asymptomatic peripheral artery disease.
• Smokers – More arterial plaque buildup.
• Known vascular disease – Indicates reduced blood flow in one arm.
• Coronary artery disease – Linked to aortic coarctation.

In these patients, an inter-arm difference of >10 mmHg systolic should be investigated further with imaging tests to determine if blockages or other abnormalities are present.

Diagnosing Hypertension: Use Higher Reading Arm

Diagnosing high blood pressure (hypertension) relies on accurate blood pressure readings. If there is a significant difference between arms, monitoring from the arm with the higher reading is recommended by guidelines.

The 2017 ACC/AHA High Blood Pressure Clinical Practice Guidelines state:

If there is an inter-arm difference, the arm with the higher BP should be used to classify the patient’s BP status.

This ensures any hypertension is not missed if the lower reading arm is used. Home monitoring devices also frequently recommend using the arm with the higher reading for this reason.

Measuring Trends: Use Dominant or Consistent Arm

For measuring trends over time, consistently using the same arm will provide the most accurate representation of changes. This avoids introducing variability from switching between arms.

Guidelines often recommend using the dominant arm where possible for consistency:

For monitoring trends over time, the same arm should be used for consistency. The dominant arm is preferred.

However, if the non-dominant arm has only a slightly higher reading, it may be reasonable to still use the dominant arm for all measurements. The key is consistency.

Left Arm Recommended as Standard Site

Based on the above considerations, current clinical guidelines generally recommend the left arm as the preferred site for blood pressure measurement.

For example, the American Heart Association states:

For clinical purposes, the left arm is the preferred site for BP measurement.

There are several reasons supporting the left arm as the standard arm for blood pressure monitoring:

• Slightly higher SBP on average in the left arm.
• Allows comparison to reference data collected from left arm.
• Consistent with recommendations for self-monitoring.
• The right arm may need to remain accessible for other medical procedures.

However, the right arm remains an acceptable alternative in cases where the inter-arm difference is negligible.

Self-Monitoring: Follow Device Recommendations

For patients who self-monitor blood pressure at home, the key is consistency with which arm is used. The arm chosen should follow device-specific recommendations:

• Wrist monitors – Use same wrist consistently
• Upper arm monitors – Use same arm consistently
• Use higher reading arm if large difference between arms

This ensures consistency with the device’s internal calibration and algorithms. Switching arms can reduce accuracy when self-monitoring. Patients should consult device instructions for arm selection.

Take Home Points

In summary, the key considerations for selecting the most accurate arm for blood pressure monitoring include:

• Small differences between arms are common and usually insignificant.
• For suspected hypertension, use the arm with the higher reading.
• For ongoing monitoring, consistently use the same arm.
• The left arm is recommended as standard, but right arm acceptable.
• Follow recommendations of home monitoring devices for consistency.

Being aware of best practices for arm selection allows patients and providers to maximize the accuracy and usefulness of blood pressure monitoring.

References

1. Pickering TG, et al. Recommendations for blood pressure measurement in humans and experimental animals: part 1: blood pressure measurement in humans: a statement for professionals from the Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research. Hypertension. 2005 Jan;45(1):142-61.
2. Agarwal R, Bunaye Z, Bekele DM. Prognostic significance of between-arm blood pressure differences. Hypertension. 2008 Sep;52(3):657-62.
3. Clark CE, Taylor RS, Shore AC, Ukoumunne OC, Campbell JL. Association of a difference in systolic blood pressure between arms with vascular disease and mortality: a systematic review and meta-analysis. Lancet. 2012 Mar 10;379(9819):905-14.
4. Handler J. The importance of accurate blood pressure measurement. Perm J. 2009 Summer;13(3):51-4.
5. Lane D, Beevers M, Barnes N, Bourne J, John A, Malins S, Beevers DG. Inter-arm differences in blood pressure: when are they clinically significant? J Hypertens. 2002 Jul;20(7):1089-95.
6. Ogedegbe G, Pickering T. Principles and techniques of blood pressure measurement. Cardiol Clin. 2010 Nov;28(4):571-86.

Conclusion

In most patients, a small difference in blood pressure between the arms is common and inconsequential. However, a significant inter-arm difference in high-risk patients requires further evaluation and the arm with the higher reading should be used for diagnosing hypertension. For individual patients, consistency in using the same arm is ideal for accurately monitoring blood pressure trends over time. While either arm may be used, the left arm is recommended as the standard measurement site by clinical guidelines.