How is the Ankle-Brachial Index (ABI) calculated for a patient with suspected Peripheral Artery Disease (PAD)?

How to Calculate the Ankle-Brachial Index (ABI)

The ABI is calculated by dividing the ankle systolic pressure by the highest brachial systolic pressure from either arm, with the choice of which ankle pressure to use (higher vs. lower) depending on whether you are diagnosing PAD (use higher ankle pressure for specificity) or assessing cardiovascular risk (use lower ankle pressure for sensitivity). 1

Measurement Sequence Protocol

Follow this standardized counterclockwise sequence for all measurements 1, 2:

1. First arm (right brachial artery)
2. First posterior tibial (PT) artery (right leg)
3. First dorsalis pedis (DP) artery (right leg)
4. Other PT artery (left leg)
5. Other DP artery (left leg)
6. Other arm (left brachial artery)

Critical Rule for Arm Measurements

• If the systolic blood pressure (SBP) difference between arms exceeds 10 mm Hg, repeat the first arm measurement and discard the initial reading 1, 2
• Always use the highest brachial SBP from either arm as your denominator 2

Patient Preparation Requirements

Before measuring, ensure 2:

• Patient supine with head and heels fully supported for 5-10 minutes of rest
• Room temperature maintained at 19°C–22°C (66°F–72°F)
• Patient has not smoked for at least 2 hours prior to measurement
• Use appropriate cuff size (width ≥40% of limb circumference)
• Use 8-10 MHz Doppler ultrasound probe with gel

Calculation Method: Context Determines Which Ankle Pressure to Use

For Diagnostic Confirmation of PAD (Maximizing Specificity)

ABI = Higher ankle pressure (PT or DP) / Highest brachial pressure 1, 2

• This method provides specificity of 0.99 (vs. 0.93 with lower pressure) 1
• Minimizes overdiagnosis in healthy subjects and avoids unnecessary testing 1
• Sensitivity is lower (0.66), so if clinical suspicion remains high despite normal ABI, proceed with additional testing 1

For Cardiovascular Risk Assessment (Maximizing Sensitivity)

ABI = Lower ankle pressure (PT or DP) / Highest brachial pressure 1, 2

• This method provides sensitivity of 0.89 (vs. 0.66 with higher pressure) 1, 3
• Identifies more individuals at cardiovascular risk 1
• Using the lower ABI of both legs identifies more patients at risk of cardiovascular events 1
• Specificity is lower (0.93), but this is acceptable for risk stratification 1, 3

Interpretation Thresholds

• Normal ABI: 0.91–1.40 (optimal range: 1.11–1.40) 1, 2
• Borderline: 0.91–1.00 (warrants further evaluation) 1
• Abnormal (PAD confirmed): ≤0.90 1, 2
• Non-compressible arteries: >1.40 (suggests arterial calcification; requires toe-brachial index) 1, 2

Critical Pitfalls to Avoid

When Ankle Artery Signal is Absent

If one ankle artery has no detectable signal but the other ankle artery shows normal ABI, perform duplex ultrasound to determine if PAD is present rather than assuming the measurement is normal 2

Borderline Values Require Confirmation

When ABI is 0.80–1.00 (near threshold), reproducibility becomes crucial and repeat measurements should be obtained 1

The 95% confidence interval for ABI measurement differences is ±0.10, so an ABI <0.80 is definitively abnormal without need for repeat testing 1

Post-Exercise Testing Indications

When ABI is >0.90 but clinical suspicion of PAD remains high, perform post-exercise ABI 1

• A post-exercise ankle pressure decrease >30 mm Hg OR
• A post-exercise ABI decrease >20% confirms PAD diagnosis 1

Non-Compressible Arteries (ABI >1.40)

When ABI >1.40, the test is unreliable due to arterial calcification and you must obtain toe-brachial index or other noninvasive imaging 1, 4

This is particularly common in patients with diabetes mellitus or chronic kidney disease 4, 5

Equipment Considerations

While Doppler ultrasound remains the gold standard, automated oscillometric devices can be used with awareness of their limitations 6, 7:

• Automated devices show good correlation with Doppler (r=0.85–0.86) but systematically overestimate ankle pressures in patients with reduced values 7, 8
• Sensitivity for detecting ABI ≤0.9 with automated devices is only 71% compared to Doppler 8
• For diagnostic purposes, Doppler technique is preferred; automated devices are more appropriate for screening 7, 8