How do I perform an ankle‑brachial index (ABI) to assess peripheral arterial disease?

How to Perform an Ankle-Brachial Index (ABI)

Follow a standardized sequence: measure the first arm, then first posterior tibial (PT) artery, first dorsalis pedis (DP) artery, other PT artery, other DP artery, and finally the other arm, repeating the first arm measurement at the end. 1

Patient Preparation

• Position the patient supine and allow 5-10 minutes of rest before measurement
• The patient should remain lying flat throughout the procedure to avoid hydrostatic pressure errors
• If the patient cannot lie flat, seated measurements can be corrected using the formula: Corrected ankle pressure = Measured ankle pressure - D × 0.078, where D is the vertical distance in millimeters between arm and ankle cuffs 2

Equipment and Technique

Doppler Probe Positioning

• Apply Doppler gel over the arterial pulse location
• Place the probe at a 45° to 60° angle to the skin surface
• Move the probe until the clearest arterial signal is heard 1

Blood Pressure Measurement Protocol

• Inflate the cuff progressively to 20 mm Hg above the point where the flow signal disappears
• Maximum inflation is 300 mm Hg; if flow is still detected, deflate rapidly to avoid pain
• Deflate slowly to detect the pressure level where the flow signal reappears 1

Measurement Sequence

Use this specific counterclockwise sequence:

1. Right arm (brachial artery)
2. Right PT artery
3. Right DP artery
4. Left PT artery
5. Left DP artery
6. Left arm (brachial artery)
7. Repeat right arm measurement 1

Handling the Repeated Arm Measurement

• Average both right arm measurements unless they differ by >10 mm Hg
• If the difference exceeds 10 mm Hg, discard the first measurement and use only the second 1

Calculating the ABI

For Diagnostic Purposes (Confirming PAD)

Use the higher of the PT or DP pressure at each ankle divided by the higher brachial pressure. This approach maximizes specificity (99%) to avoid overdiagnosis and unnecessary testing, though it reduces sensitivity 1

For Cardiovascular Risk Assessment

Use the lower of the PT or DP pressure at each ankle divided by the higher brachial pressure. This identifies more at-risk individuals, as PAD may affect only one ankle artery initially 1

Critical Rule for Both Legs

Always use the lower ABI of the two legs for final interpretation. Using the higher ABI of both legs will miss individuals with unilateral or asymmetric disease who remain at high cardiovascular risk 1

Interpretation Thresholds

• ABI <0.90: Confirms PAD diagnosis (Class I, Level A) 1
• ABI 0.91-1.00: Borderline cardiovascular risk; further evaluation appropriate 1
• ABI >1.40: Suggests arterial calcification; use toe-brachial index or imaging instead (Class I, Level A) 1

When to Pursue Additional Testing

• ABI >0.90 with clinical suspicion: Perform post-exercise ABI or imaging 1
• Post-exercise criteria for PAD: Ankle pressure decrease >30 mm Hg OR ABI decrease >20% (Class IIa, Level A) 1
• Absent ankle artery signal with normal contralateral ABI: Perform duplex ultrasound to distinguish arterial occlusion from technical difficulty 1

Common Pitfalls to Avoid

• Don't use only one arm measurement: Bilateral arm pressures are essential, as subclavian stenosis can falsely lower the denominator
• Don't skip the repeated arm measurement: The "white coat effect" can artificially elevate the first reading 1
• Don't use ABI in patients with arterial calcification (diabetes, chronic kidney disease): Values >1.40 are unreliable; proceed directly to toe-brachial index 1
• Don't rely on physical examination alone: Femoral bruits and absent pulses indicate moderate-to-severe disease but have low sensitivity for mild PAD 3

Reproducibility Considerations

Reproducibility matters most when the ABI is near threshold values. With a 95% confidence interval of ±0.10 for repeat measurements, an ABI ≤0.80 is definitively abnormal and doesn't require confirmation 1