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

How to Compute the Ankle-Brachial Index (ABI)

The ABI is calculated by dividing the ankle systolic pressure by the highest brachial systolic pressure, with the choice of which ankle pressure to use (higher vs. lower) depending on your clinical purpose: use the higher ankle pressure for diagnostic confirmation of PAD to maximize specificity, or use the lower ankle pressure for cardiovascular risk assessment to maximize sensitivity. 1, 2

Basic Formula

ABI = Ankle Systolic Pressure / Highest Brachial Systolic Pressure 2

Step-by-Step Measurement Protocol

Patient Preparation

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

Equipment Requirements

• Use an 8-10 MHz Doppler ultrasound probe with Doppler gel 2, 3
• Apply blood pressure cuffs with width covering at least 40% of limb circumference 2, 3
• Cover any open wounds with impermeable dressing 3

Measurement Sequence

Follow this standardized counterclockwise sequence: 1, 3

1. Right arm
2. Right posterior tibial (PT) artery
3. Right dorsalis pedis (DP) artery
4. Left PT artery
5. Left DP artery
6. Left arm
7. Repeat right arm measurement

Handling the Repeat Arm Measurement

• Average the first and last right arm measurements unless they differ by >10 mm Hg 1
• If the difference exceeds 10 mm Hg, discard the first measurement and use only the second 1, 3
• If the difference between arms exceeds 15 mm Hg, suspect subclavian artery stenosis 2

Determining the Denominator

Always use the highest brachial systolic pressure from either arm as the denominator 1, 2

Determining the Numerator: Critical Clinical Decision

For Diagnostic Purposes (Confirming PAD Diagnosis)

Use the higher of the two ankle pressures (PT or DP) as the numerator 1, 2

• This provides higher specificity (0.99 vs. 0.93) 1
• Minimizes overdiagnosis in healthy subjects 1
• Avoids unnecessary further testing and treatment 1
• Accept that more false-negatives will occur, but clinical suspicion should prompt further investigation 1

For Cardiovascular Risk Assessment

Use the lower of the two ankle pressures (PT or DP) as the numerator 1, 2

• This provides higher sensitivity (0.89 vs. 0.66) 1
• Identifies more individuals at cardiovascular risk 1, 2
• Shows stronger association with cardiovascular risk factors and subclinical atherosclerosis 1

Interpretation of ABI Values

Normal and Abnormal Ranges

• Normal: 0.91-1.40 (optimal range: 1.11-1.40) 2, 3
• Borderline: 0.90-1.00 2, 3
• Abnormal (PAD): ≤0.90 1, 2, 3
• Non-compressible arteries: >1.40 (suggests arterial calcification) 2, 3

When ABI >1.40

This indicates medial arterial calcification making the ABI unreliable—you must obtain toe-brachial index (TBI) instead 4, 3

• This is particularly common in diabetes mellitus, chronic kidney disease, and end-stage renal disease 4, 3
• Toe pressure <30 mm Hg indicates critical limb-threatening ischemia requiring urgent vascular evaluation 4

Critical Pitfalls to Avoid

Calculation Errors

• Never use the average of both arm pressures as the denominator—always use the highest 2
• Never automatically use the higher ankle pressure for all purposes—match the method to your clinical goal 2

Technical Errors

• Avoid placing cuffs over recent bypass grafts due to thrombosis risk 2, 3
• Do not measure over open wounds without impermeable dressing 2, 3

Interpretation Errors

• When ABI is borderline (0.90-1.00), request repeat measurements rather than relying on a single value 3
• When ABI >1.40 with clinical suspicion of PAD, do not dismiss the possibility of disease—perform TBI or duplex ultrasound 2, 4, 3
• If an ankle artery signal is absent but the other ankle artery shows normal ABI, perform duplex ultrasound to determine if PAD is present 1

When One Ankle Artery Signal is Absent

If arterial flow in one ankle artery cannot be detected and the ABI based on the other ankle artery is within normal range, perform additional vascular tests (duplex ultrasound) to determine whether PAD is present 1

• The reason is seldom arterial agenesis but most likely arterial occlusion or technical difficulties in localizing the artery 1