What is the ankle-brachial index (ABI)?

What is the Ankle-Brachial Index (ABI)?

The ankle-brachial index (ABI) is a noninvasive diagnostic test that compares the systolic blood pressure in the ankle to the systolic blood pressure in the arm, calculated as a ratio to detect peripheral arterial disease (PAD) and assess cardiovascular risk. 1

How ABI is Measured

The test requires a blood pressure cuff, sphygmomanometer, and handheld Doppler ultrasound device. Follow this specific measurement sequence: first arm → first posterior tibial (PT) artery → first dorsalis pedis (DP) artery → other PT artery → other DP artery → other arm. 1

Key Technical Points:

• Place the Doppler probe at a 45° to 60° angle to the skin surface
• Inflate the cuff 20 mm Hg above the point where the arterial signal disappears
• Deflate slowly to detect when flow returns
• Use Doppler detection for both ankle and brachial pressures (not manual palpation)
• If the first arm pressure differs from the second arm measurement by >10 mm Hg, discard the first measurement and repeat 1

How to Calculate and Interpret ABI

For each leg, divide the higher of the two ankle pressures (PT or DP) by the higher of the two brachial pressures. 1 Using the higher ankle pressure maximizes specificity and prevents overdiagnosis in healthy individuals, though it may miss some cases where clinical suspicion should prompt further testing.

Always use the lower ABI value between the two legs for risk stratification, as this identifies more individuals at cardiovascular risk—PAD may be unilateral or asymmetric. 1

Clinical Interpretation Thresholds

For PAD Diagnosis:

• ABI <0.90 = PAD confirmed (Class I, Level A evidence) 1
• ABI >1.40 = Abnormal (suggests arterial calcification) - requires toe-brachial index or other testing (Class I, Level A) 1
• ABI 0.91-1.00 = "Borderline" - warrants further evaluation 1
• ABI >0.90 with clinical suspicion = Perform post-exercise ABI or imaging 1

Post-Exercise Criteria:

A decrease in ankle pressure >30 mm Hg OR a decrease in ABI >20% after exercise confirms PAD (Class IIa, Level A) 1

For Cardiovascular Risk Assessment:

• ABI <0.90 or >1.40 = High risk for cardiovascular events and mortality, independent of symptoms or other risk factors (Class I, Level A) 1
• ABI can provide incremental prognostic information beyond traditional cardiovascular risk scores (Class IIA, Level A) 1

Primary Clinical Uses

Use ABI as the first-line noninvasive test when PAD is clinically suspected based on symptoms like claudication or physical findings (Class I, Level A). 1

Additional indications include:

• Screening patients with diabetes or known cardiovascular disease 2
• Assessing healing potential of lower extremity wounds before compression therapy 3
• Evaluating limb perfusion after blunt extremity trauma 2
• Risk stratification in asymptomatic high-risk populations 4

Critical Pitfalls to Avoid

Arterial calcification (common in diabetes and chronic kidney disease) can produce falsely elevated ABI values >1.40, making the test unreliable. In these cases, obtain a toe-brachial index instead (Class I, Level A). 1

Missing unilateral disease: Always calculate ABI for both legs separately. Using only the higher leg value will miss patients with significant unilateral PAD who remain at high cardiovascular risk. 1

Intra-leg asymmetry: Recent evidence shows that significant differences (≥0.15) between DP- and PT-derived ABI values within the same leg predict worse limb outcomes, though conventional reporting uses the higher value. 5 If one ankle artery is non-detectable but the other shows normal ABI, perform duplex ultrasound to rule out PAD. 1

Reproducibility concerns: When ABI values fall near diagnostic thresholds (around 0.90), repeat measurement is crucial. The 95% confidence interval for repeat measurements is ±0.10, so an ABI ≤0.80 or ≥1.00 requires no confirmation. 1

Who Should Perform ABI Testing

Qualified physicians, nurses, vascular technicians, and allied health professionals can perform ABI testing after proper training. 1 Training must include both didactic education (vascular anatomy, physiology, PAD presentation, Doppler function) and hands-on practice with demonstration of competence and reproducible results. 1

The test remains significantly underutilized in clinical practice despite its prognostic value, low cost, and ease of performance. 4