How do you perform an ankle-brachial index (ABI)?

How to Perform the Ankle-Brachial Index (ABI)

Patient Preparation

Position the patient supine with head and heels fully supported for 5-10 minutes of rest in a room maintained at 19°C–22°C (66°F–72°F) before beginning measurements. 1, 2

• Ensure the patient has not smoked for at least 2 hours prior to measurement, as smoking can decrease ankle pressures by approximately 0.09 and affect ABI accuracy 1
• Cover any open wounds or lesions with impermeable dressing to prevent contamination 1, 2
• Confirm the patient remains still during pressure measurements; if unable to remain motionless (e.g., tremor), consider alternative diagnostic methods 1

Equipment Selection

• Use an 8-10 MHz Doppler ultrasound probe with Doppler gel 1, 2
• Select blood pressure cuffs with width covering at least 40% of the limb circumference 1, 2
• Use the straight (parallel) wrapping method for cuff application, as this provides better interobserver reproducibility compared to spiral wrapping 1

Measurement Sequence

Follow a standardized counterclockwise sequence: right arm → right posterior tibial (PT) → right dorsalis pedis (DP) → left PT → left DP → left arm → repeat right arm. 2

Brachial Pressure Measurement

• Place the cuff on the upper arm using straight wrapping method 1
• Use the Doppler probe to detect brachial artery flow 1
• Inflate the cuff 20 mm Hg above the level where flow signal disappears (maximum 300 mm Hg) 1
• Deflate slowly and record the pressure at which flow signal reappears 1
• Measure both arms; if the difference between arms exceeds 15 mm Hg, suspect subclavian artery stenosis 3

Ankle Pressure Measurement

• Place the cuff around the ankle 2 cm above the superior aspect of the medial malleolus using straight wrapping 1
• Do not place the cuff over a recent bypass graft (risk of thrombosis) or directly over ulcers 1
• Position the Doppler probe at a 45° to 60° angle to the skin surface over the PT or DP artery 1
• Move the probe until the clearest signal is heard 1
• Inflate the cuff 20 mm Hg above flow signal disappearance, then deflate slowly to detect reappearance 1
• Measure both PT and DP arteries in each ankle 1, 2

Repeat Measurement Protocol

• Repeat the first arm measurement at the end of the sequence 1, 2
• Average both right arm measurements unless they differ by >10 mm Hg 1, 2
• If the difference exceeds 10 mm Hg, disregard the first measurement and use only the second 1, 2

ABI Calculation

ABI = Ankle Systolic Pressure / Highest Brachial Systolic Pressure 2, 3

Selecting the Numerator

• For diagnostic purposes (identifying PAD): Use the higher of the two ankle pressures (PT or DP), which provides specificity of 0.99 versus 0.93 1, 2, 3
• For cardiovascular risk assessment: Use the lower of the two ankle pressures, which provides sensitivity of 0.89 versus 0.66 1, 2, 3

Selecting the Denominator

• Always use the highest brachial systolic pressure from either arm as the denominator 2, 3

Interpretation of Results

• Normal ABI: 0.91-1.40 (optimal range: 1.11-1.40) 2, 4
• Abnormal (PAD present): ≤0.90 2, 4
• Borderline: 0.90-1.00 (interpret with clinical context; consider repeat measurement) 2, 4
• Non-compressible arteries: >1.40 (suggests arterial calcification; consider toe-brachial index) 2, 4

Critical Pitfalls to Avoid

• Failing to use Doppler for brachial measurements: The detection of brachial blood flow should be done by Doppler, not by auscultation alone 1
• Incorrect cuff placement: Placing cuffs over recent bypass grafts risks thrombosis; avoid this completely 1
• Using average of both arm pressures as denominator: Always use the highest single brachial pressure 3
• Relying on a single borderline measurement (0.91-1.00): Request repeat measurements for confirmation 2
• Not considering alternative tests when ABI >1.40 with clinical suspicion of PAD: Perform toe-brachial index or duplex ultrasound in these cases 2, 4
• Applying compression therapy without checking ABI: ABI <0.50 is an absolute contraindication for compression; ABI 0.50-0.80 requires modified compression 4

Special Considerations

• If arterial flow at the ankle is not detected and the ABI based on the other ankle artery is normal, perform additional vascular testing 1
• In patients with diabetes or end-stage renal disease, ABI may be falsely elevated due to arterial calcification; consider toe-brachial index 2, 4
• When clinical suspicion of PAD persists despite normal resting ABI, consider post-exercise ABI measurement 2, 4