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

How to Measure the Ankle-Brachial Index (ABI)

The Ankle-Brachial Index (ABI) should be measured using a Doppler ultrasound device with the patient in a supine position after 5-10 minutes of rest, following a standardized protocol that includes measuring systolic pressures in both arms and ankles, then calculating the ratio of ankle to arm pressure. 1

Patient Preparation

• Position the patient supine (lying flat) with head and heels fully supported for 5-10 minutes before measurement in a room with comfortable temperature (19°C–22°C/66°F–72°F) 1
• Ensure the patient has not smoked for at least 2 hours before the measurement, as smoking can decrease ankle pressures and affect ABI results 1
• Cover any open wounds with impermeable dressing to prevent contamination 1
• If the patient cannot lie flat, seated measurements can be performed but must be corrected using a validated formula (though this is not the preferred method) 1, 2

Equipment Needed

• 8-10 MHz Doppler ultrasound probe 1
• Doppler gel 1
• Blood pressure cuffs of appropriate size (width should be at least 40% of limb circumference) 1

Measurement Procedure

Step 1: Arm (Brachial) Pressure Measurement

• Apply blood pressure cuffs to both arms 1
• Locate the brachial pulse using the Doppler probe with gel applied 1
• Inflate the cuff 20 mmHg above the level of flow signal disappearance 1
• Slowly deflate and record the pressure at which the flow signal reappears 1
• Measure both arms and use the highest reading as the denominator for ABI calculation 1
• Note: If the difference between arms exceeds 15 mmHg, subclavian artery stenosis should be suspected 1

Step 2: Ankle Pressure Measurement

• Place the cuff around the ankle using parallel wrapping method, with the lower edge 2 cm above the superior aspect of the medial malleolus 1
• Locate the posterior tibial (PT) artery behind the medial malleolus and the dorsalis pedis (DP) artery on the dorsum of the foot using the Doppler probe 1
• For each artery, inflate the cuff 20 mmHg above the level of flow signal disappearance (maximum 300 mmHg) 1
• Slowly deflate and record the pressure at which the flow signal reappears 1
• Measure both PT and DP pressures in both ankles 1

Step 3: ABI Calculation

• For diagnostic purposes (to confirm suspected PAD): ABI = Higher ankle pressure (PT or DP) / Higher brachial pressure 1, 3
• For cardiovascular risk assessment: ABI = Lower ankle pressure (PT or DP) / Higher brachial pressure 3, 4

Standardized Measurement Sequence

• Use a consistent sequence for measurements (e.g., right arm, right PT, right DP, left PT, left DP, left arm) 1
• Repeat the first arm measurement at the end of the sequence and average both results, unless the difference exceeds 10 mmHg (in which case use only the second measurement) 1

Interpretation of Results

• Normal ABI: 0.91-1.40 (optimal range: 1.11-1.40) 3
• Borderline: 0.91-1.00 3
• Abnormal (indicating PAD): ≤0.90 3, 5
• Non-compressible arteries: >1.40 (suggests arterial calcification) 3

Common Pitfalls and Solutions

• Inaccurate cuff size: Ensure cuff width is at least 40% of limb circumference 1
• Improper patient positioning: Always measure with patient supine after adequate rest 1
• Arterial calcification: In diabetic patients or those with end-stage renal disease, high ABI (>1.40) may mask PAD; consider toe-brachial index or other imaging 3
• Missing arterial signals: If no signal is detected in one ankle artery but the other is normal, consider additional vascular tests to rule out PAD 1
• Recent smoking: Ensure patient has not smoked for at least 2 hours before measurement 1
• Improper sequence: Follow a standardized measurement sequence and repeat the first arm measurement at the end 1

Special Considerations

• Avoid placing cuff over recent bypass grafts due to risk of thrombosis 1
• In patients with clinical suspicion of PAD but normal resting ABI, consider post-exercise ABI measurement 3
• Serial measurements provide more accurate assessment than a single measurement 3
• For patients unable to lie flat, seated measurements can be performed with appropriate correction factors, though this is not the preferred method 2