How do you compute the Ankle-Brachial Index (ABI)?

How to Compute the Ankle-Brachial Index (ABI)

The Ankle-Brachial Index (ABI) should be calculated as the ratio of the higher ankle systolic pressure (either posterior tibial or dorsalis pedis artery) to the higher brachial systolic pressure from either arm using a Doppler ultrasound device. 1, 2

Patient Preparation

• Position the patient supine 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, 2
• Ensure the patient has not smoked for at least 2 hours before the measurement, as smoking can decrease ankle pressures and affect ABI results 2
• Cover any open wounds with impermeable dressing to prevent contamination 1
• The patient should remain still during pressure measurements 1

Required Equipment

• 8-10 MHz Doppler ultrasound probe 1, 2
• Doppler gel 1
• Blood pressure cuffs of appropriate size (width should be at least 40% of limb circumference) 1, 2
• Avoid placing cuff over recent bypass grafts due to risk of thrombosis 2

Measurement Procedure

1. Arm Measurements:

   • Measure systolic blood pressure in both arms 2
   • If the difference between arms exceeds 10 mmHg, repeat the measurement of the first arm at the end of the sequence 1
   • If the difference between the first and second measurement of the same arm exceeds 10 mmHg, use only the second measurement 1

2. Ankle Measurements:

   • Place the cuff around the ankle 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
   • Measure systolic pressure at both PT and DP arteries in each ankle 2

3. Recommended Sequence:

   • Right arm, right PT, right DP, left PT, left DP, left arm 1
   • For repeat measurements, use the reverse order 1

ABI Calculation

• Standard Formula: ABI = Higher ankle pressure (PT or DP) / Higher brachial pressure 2
• Calculate ABI separately for each leg 2
• Use the higher of the two ankle pressures (PT or DP) for diagnostic purposes to minimize overdiagnosis (higher specificity) 1

Alternative Calculation Methods

• Using the lower ankle pressure provides better sensitivity (0.89 vs 0.66) but lower specificity (0.93 vs 0.99) 1, 3
• For cardiovascular risk assessment, using the lower ankle pressure identifies more individuals at risk 4, 5
• Using the average of PT and DP pressures would likely result in intermediate values for sensitivity and specificity 1

Interpretation of Results

• Normal ABI: 0.91-1.40 (optimal range: 1.11-1.40) 4
• Borderline: 0.91-1.00 (interpret with clinical context) 4
• Abnormal (indicating PAD): ≤0.90 4
• Non-compressible arteries: >1.40 (suggests arterial calcification) 4

Special Considerations

• When an ankle artery signal is absent and the ABI based on the other ankle artery is normal, consider additional vascular tests (e.g., duplex ultrasound) 1
• In patients with clinical suspicion of PAD but normal resting ABI, consider post-exercise ABI measurement 4, 2
• Serial measurements provide more accurate assessment than a single measurement 4, 2
• ABI has limitations in patients with diabetes or end-stage renal disease due to arterial calcification 4
• The Doppler method shows better reproducibility compared to oscillometric methods, with lower coefficient of variation 1

Common Pitfalls

• Failing to use the higher brachial pressure as the denominator when there is significant difference between arms 1
• Not considering arterial calcification when ABI is >1.40 4
• Relying solely on a single measurement when serial measurements would provide more accurate results 1
• Using inappropriate cuff size, which can lead to inaccurate pressure readings 1
• Not allowing adequate rest time before measurement, which can affect pressure readings 1, 2