How is the Ankle-Brachial Index (ABI) calculated?

How to Calculate the Ankle-Brachial Index (ABI)

The ABI is calculated by dividing the systolic blood pressure at the ankle by the systolic blood pressure in the arm, using the highest brachial pressure as the denominator and selecting the appropriate ankle pressure based on your clinical purpose. 1

Standard Calculation Formula

ABI = Ankle Systolic Pressure / Highest Brachial Systolic Pressure 1

Determining the Denominator (Arm Pressure)

• Use the highest systolic blood pressure measured from either arm as the denominator 1
• Measure both arms and select the higher value 2
• If the difference between arms exceeds 15 mm Hg, suspect subclavian artery stenosis 1
• When measuring sequentially, repeat the first arm measurement at the end and average the two values, unless they differ by >10 mm Hg (in which case discard the first measurement) 1

Determining the Numerator (Ankle Pressure)

The choice of ankle pressure depends on your clinical purpose: 1

For Diagnostic Purposes (Confirming PAD):

• Use the higher of the two ankle pressures (posterior tibial [PT] or dorsalis pedis [DP]) 1, 2
• This method provides higher specificity (0.99 vs 0.93) and minimizes overdiagnosis 1, 2
• Reduces unnecessary testing and treatment in healthy individuals 1

For Cardiovascular Risk Assessment:

• Use the lower of the two ankle pressures (PT or DP) 1, 2
• This method provides higher sensitivity (0.89 vs 0.66) and identifies more at-risk individuals 1, 2
• Better captures patients with disease affecting only one ankle artery 1

Measurement Technique

Patient Preparation

• Position patient supine with head and heels fully supported 1, 2
• Allow 5-10 minutes of rest before measurement 1, 2
• Room temperature should be 19°C–22°C (66°F–72°F) 1, 2
• Patient must not smoke for at least 2 hours prior 1, 2

Equipment and Procedure

• Use an 8-10 MHz Doppler ultrasound probe (most reliable method) 1
• Select cuff width that covers at least 40% of limb circumference 1, 2
• Place ankle cuff 2 cm above the medial malleolus 1
• Measure systolic pressures in both arms and both ankle arteries (PT and DP) bilaterally 2

Interpreting ABI Values

• Normal: 0.91-1.40 (optimal range: 1.11-1.40) 2, 3
• Borderline: 0.90-1.00 (interpret with clinical context) 3
• Abnormal (PAD present): ≤0.90 2, 3
• Non-compressible arteries: >1.40 (suggests arterial calcification) 2, 3

Critical Pitfalls to Avoid

Calculation Errors

• Do not use the average of both arm pressures as the denominator—always use the highest 1
• Do not automatically use the higher ankle pressure for all purposes—match the method to your clinical goal 1, 2
• Avoid relying on a single measurement when values are borderline (0.91-1.00); repeat measurements improve accuracy 2

Technical Errors

• Do not place cuffs over recent bypass grafts (risk of thrombosis) 1
• Do not measure over open wounds without impermeable dressing 1, 2
• Ensure adequate examiner training—reproducibility is significantly better with skilled operators 1

Interpretation Errors

• When ABI >1.40 with clinical suspicion of PAD, perform toe-brachial index or duplex ultrasound—the elevated ABI masks underlying disease due to arterial calcification 2, 3
• When ankle artery signal is absent but the other ankle artery shows normal ABI, perform duplex ultrasound to rule out PAD 1
• In symptomatic patients with normal resting ABI, consider post-exercise ABI 2, 3

Evidence Considerations

The American Heart Association guideline provides the most authoritative framework for ABI calculation 1. While research studies demonstrate that using the lower ankle pressure increases sensitivity for PAD detection (sensitivity 0.89 vs 0.68), the guideline explicitly recommends using the higher ankle pressure for diagnostic purposes to maximize specificity and avoid overdiagnosis 1, 4, 5. This apparent contradiction is resolved by understanding that diagnostic accuracy (avoiding false positives) takes priority over screening sensitivity when confirming suspected PAD 1.

The Doppler method remains superior to oscillometric devices, with better reproducibility (coefficient of variation 5.4-24% vs 5.1-20.2%) and reliability, particularly in patients with advanced PAD where oscillometric devices frequently fail 1.