What is ABI (Ankle-Brachial Index)?
ABI (Ankle-Brachial Index) is the ratio of systolic blood pressure measured at the ankle to the systolic blood pressure measured at the brachial artery, used as the first-line noninvasive test for diagnosing peripheral arterial disease (PAD) and as a powerful predictor of cardiovascular mortality. 1
Clinical Definition and Calculation
The ABI is calculated by dividing the ankle systolic pressure by the brachial systolic pressure. The measurement involves obtaining blood pressures from both arms and both ankles (at the posterior tibial and dorsalis pedis arteries) using a Doppler ultrasound device 1.
Diagnostic Thresholds
An ABI <0.90 confirms the diagnosis of lower-extremity PAD (Class I, Level of Evidence A) 1. The interpretation framework includes:
- ABI <0.90: Diagnostic for PAD
- ABI 0.91-1.00: "Borderline" cardiovascular risk, warrants further evaluation
- ABI 1.11-1.40: Normal reference range
- ABI >1.40: Suggests arterial calcification (common in diabetes); requires toe-brachial index or other imaging 1
Prognostic Significance Beyond PAD Diagnosis
The ABI is not merely a diagnostic test but a powerful predictor of mortality. For patients with ABI ≤0.90 compared to the reference range (1.11-1.40), the pooled hazard ratio for cardiovascular mortality is 4.2 in men and 3.5 in women 1. This association holds true regardless of PAD symptoms 1.
The test predicts all-cause mortality with an odds ratio of 2.74, cardiovascular mortality with OR 3.23, and even non-cardiovascular mortality with OR 2.23 2.
Measurement Technique
The standardized sequence is: first arm → first posterior tibial artery → first dorsalis pedis artery → other posterior tibial → other dorsalis pedis → other arm (Class I, Level of Evidence C) 1. If the first arm pressure exceeds the second by >10 mm Hg, repeat the first arm measurement and disregard the initial reading 1.
Clinical Applications
- For symptomatic patients: ABI is the first-line test when PAD is suspected based on leg pain with walking that resolves with rest 1
- For asymptomatic individuals: ABI provides incremental cardiovascular risk information beyond standard risk scores (Class IIA, Level of Evidence A) 1
- Post-exercise testing: If ABI >0.90 but clinical suspicion persists, perform post-exercise ABI; a decrease >30 mm Hg or >20% suggests PAD 1
Important Caveats
In diabetic patients, ABI sensitivity is limited (only 35.48% in one study) due to arterial calcification causing falsely elevated readings 3. When ABI >1.40, the arteries are too stiff to compress accurately, necessitating toe-brachial index or imaging 1.
The choice between using the higher versus lower ankle pressure affects diagnostic performance: using the lower pressure increases sensitivity (89% vs 66%) but decreases specificity (93% vs 99%) 1. For cardiovascular risk stratification, use the lower ABI of both legs to identify more at-risk individuals 1.