How do I calculate the Ankle-Brachial Index (ABI)?

How to Calculate the Ankle-Brachial Index (ABI)

Standard Calculation Formula

The ABI is calculated by dividing the systolic blood pressure at the ankle by the highest systolic blood pressure measured in either arm. 1

The basic formula is: ABI = Ankle Systolic Pressure / Highest Brachial Systolic Pressure 1

Step-by-Step Measurement Protocol

Patient Preparation

• Position the patient supine with head and heels fully supported 2, 1
• Allow 5-10 minutes of rest before measurement 2, 1
• Maintain room temperature at 19°C–22°C (66°F–72°F) 2, 1
• Ensure the patient has not smoked for at least 2 hours before measurement 2, 3
• Cover any open wounds with impermeable dressing 3

Equipment Requirements

• Use an 8-10 MHz Doppler ultrasound probe (most reliable method) 2, 1
• Select appropriately sized blood pressure cuffs with width covering at least 40% of limb circumference 2, 1
• Apply Doppler gel over the sensor 2

Measurement Sequence

Follow this standardized counterclockwise sequence: 3

1. Right arm (brachial artery)
2. Right posterior tibial (PT) artery
3. Right dorsalis pedis (DP) artery
4. Left posterior tibial artery
5. Left dorsalis pedis artery
6. Left arm (brachial artery)

Technique for Each Measurement

• Place the cuff 2 cm above the superior aspect of the medial malleolus for ankle measurements 2
• Position the Doppler probe at a 45° to 60° angle to the skin surface 2
• Inflate the cuff progressively to 20 mm Hg above the level where the flow signal disappears 2
• Deflate slowly to detect the pressure level where the flow signal reappears 2
• If the first and last right arm measurements differ by >10 mm Hg, discard the first measurement and repeat 3

Determining the Denominator

Always use the highest systolic blood pressure from either arm as the denominator. 1, 3

Critical caveat: If the difference between arms exceeds 15 mm Hg, suspect subclavian artery stenosis. 2, 1 This finding has 100% sensitivity and specificity for detecting subclavian stenosis and is associated with increased mortality risk. 2

Determining the Numerator: Context Matters

This is where calculation varies based on your clinical purpose:

For Diagnostic Purposes (Confirming PAD)

Use the higher of the two ankle pressures (PT or DP) as the numerator. 1, 3

• This provides higher specificity (0.99 vs 0.93) 1
• Minimizes overdiagnosis in healthy subjects 1
• Reduces false positives 1

For Cardiovascular Risk Assessment

Use the lower of the two ankle pressures as the numerator. 1, 3

• This provides higher sensitivity (0.89 vs 0.66) 1, 4
• Identifies more individuals at cardiovascular risk 1
• Research confirms this modified calculation is far more sensitive in detecting PAD 4

Important nuance: While the American Heart Association traditionally recommends using the higher ankle pressure 2, multiple studies demonstrate that using the lower ankle pressure significantly improves diagnostic yield, particularly in diabetic patients. 5, 4 The lower ankle pressure method had sensitivity of 0.89 vs 0.68 for the higher pressure method, with McNemar's test showing highly significant difference (P < 0.0001). 4

Interpreting ABI Values

• Normal: 0.91-1.40 (optimal range: 1.11-1.40) 1, 6
• Borderline: 0.90-1.00 (interpret with clinical context) 1, 6
• Abnormal (PAD present): ≤0.90 1, 6
• Non-compressible arteries: >1.40 (suggests arterial calcification) 1, 6

Critical Pitfalls to Avoid

Calculation Errors

• Never use the average of both arm pressures as the denominator – always use the highest single reading 1
• Do not automatically use the higher ankle pressure for all purposes without considering clinical context 1
• Avoid relying on a single measurement when values are borderline (0.91-1.00); request repeat measurements 3

Technical Errors

• Never place cuffs over recent bypass grafts (risk of thrombosis) 2, 3
• Do not measure over open wounds without impermeable dressing 3
• Avoid measurements if the patient is unable to remain still (e.g., tremor) 2

Interpretation Errors

• When ABI >1.40 with clinical suspicion of PAD, do not assume absence of disease – perform toe-brachial index or duplex ultrasound 1, 6
• In diabetic patients or those with end-stage renal disease, arterial calcification may falsely elevate ABI, masking true PAD 6, 5
• If ankle artery signal is absent but the other ankle shows normal ABI, perform duplex ultrasound to determine if PAD is present 3

Special Considerations

The Doppler method is the most reliable technique for ABI determination, with better reproducibility than palpation or oscillometric methods. 2 However, automated oscillometric devices can be used as an alternative, showing good correlation with Doppler (r=0.86) and 95% agreement in diagnosing PAD, while being faster (5.8 vs 9.3 minutes). 7

For patients with normal resting ABI but persistent clinical suspicion of PAD, consider post-exercise ABI measurement, as some patients have normal ABI at rest but abnormal values after exercise. 6