Why is blood pressure measured in the lower leg instead of the arm?

Why Blood Pressure is Measured in the Lower Leg

Blood pressure is measured in the lower leg (ankle) specifically to calculate the ankle-brachial index (ABI), which is the primary diagnostic test for peripheral artery disease (PAD) and identifies patients at significantly increased risk of cardiovascular events and death. 1

Primary Purpose: Diagnosing Peripheral Artery Disease

• The ABI is calculated by dividing the higher ankle systolic pressure (from either the dorsalis pedis or posterior tibial artery) by the higher arm systolic pressure, providing a ratio that indicates whether adequate blood flow reaches the lower extremities 1
• An ABI ≤0.90 confirms the diagnosis of PAD with 68-84% sensitivity and 84-99% specificity, making it the first-line noninvasive diagnostic test 1
• The test requires measuring both arms and both ankles because PAD may be unilateral or asymmetric, and using only one leg could miss significant disease 1, 2

Cardiovascular Risk Stratification Beyond PAD Diagnosis

• Patients with an abnormal ABI have dramatically elevated mortality risk: the hazard ratio for cardiovascular death is 4.2 in men and 3.5 in women compared to those with normal ABI, even in asymptomatic individuals 1
• The ABI provides incremental cardiovascular risk information beyond standard risk scores, identifying high-risk patients who require aggressive medical management regardless of leg symptoms 3
• Taking the lower ABI of both legs identifies more individuals at cardiovascular risk, though using the higher ankle pressure provides better diagnostic specificity for PAD 1, 3

Detecting Subclavian Artery Disease

• Measuring blood pressure in both arms identifies the arm with the highest systolic pressure, which is required for accurate ABI calculation 1
• An inter-arm blood pressure difference >15-20 mm Hg indicates subclavian or innominate artery stenosis, which occurs more frequently in patients with PAD 1
• This also ensures accurate blood pressure measurement for hypertension management, as the arm with higher readings should be used for ongoing monitoring 1

Proper Measurement Protocol

The American Heart Association recommends a specific sequence 1, 2, 3:

1. First arm (brachial artery)
2. First leg posterior tibial artery
3. First leg dorsalis pedis artery
4. Second leg posterior tibial artery
5. Second leg dorsalis pedis artery
6. Second arm (brachial artery)

• If the first arm systolic pressure exceeds the second arm by >10 mm Hg, remeasure the first arm and disregard the initial reading 1, 2
• Patients should be supine for 5-10 minutes before measurement at constant room temperature 3

Interpretation and Clinical Action

ABI values are categorized as 1, 3:

• Normal: 1.00-1.40 (optimal range 1.11-1.40)
• Borderline: 0.91-0.99 (may require exercise ABI testing if clinical suspicion exists)
• Abnormal: ≤0.90 (confirms PAD diagnosis)
• Non-compressible: >1.40 (indicates arterial calcification, requires toe-brachial index instead)

Special Considerations

• In patients with diabetes or chronic kidney disease, arteries may be calcified and non-compressible (ABI >1.40), making the standard ABI unreliable; these patients require toe-brachial index measurement instead 1, 3
• When ABI is normal (>0.90) but clinical suspicion for PAD remains high, post-exercise ABI testing should be performed, with a decrease >30 mm Hg in ankle pressure or >20% decrease in ABI confirming PAD 1, 3
• Absolute ankle pressure correlates better with symptom severity than ABI alone, particularly for critical limb ischemia symptoms like rest pain and tissue loss 4

Common Pitfalls to Avoid

• Failing to measure both legs misses unilateral disease and underestimates cardiovascular risk 1, 2
• Not measuring both posterior tibial and dorsalis pedis arteries in each leg can miss single-vessel disease 1, 2
• Overlooking the need for toe-brachial index in patients with ABI >1.40 results in missed PAD diagnoses in diabetic and renal patients 2, 3
• Using only one arm for brachial pressure measurement may result in inaccurate ABI calculation if subclavian stenosis is present 1