Ankle–Brachial Index (ABI)
Screens for peripheral arterial disease by comparing the higher ankle systolic pressure with the higher brachial systolic pressure.
The ankle–brachial index is the ratio of the higher ankle systolic pressure (dorsalis pedis or posterior tibial) to the higher of the two brachial systolic pressures. It is a validated, non-invasive first-line test for lower-limb peripheral arterial disease and an independent marker of cardiovascular risk.
📐 Formula
ABI = higher ankle systolic pressure / higher brachial systolic pressure
🎯 When to Use
- Symptoms of claudication, rest pain, or non-healing lower-limb wounds.
- Cardiovascular risk stratification in patients with diabetes, smoking history or age > 65.
- Before compression therapy for venous disease to exclude significant arterial insufficiency.
⚙️ How It Works
- Measure brachial systolic pressure in both arms and record the higher value.
- Measure ankle systolic pressure at the dorsalis pedis and posterior tibial arteries for each leg; use the higher of the two for that leg.
- Divide the ankle pressure by the higher brachial pressure to obtain the ABI for that leg.
- Report each leg separately.
📊 Interpreting the Result
- > 1.40: non-compressible, calcified vessels (common in diabetes/renal disease) — ABI unreliable, use toe-brachial index.
- 1.00–1.40: normal.
- 0.90–1.00: borderline.
- 0.40–0.90: peripheral arterial disease, typically with intermittent claudication.
- < 0.40: severe disease / critical limb ischaemia.
⚠️ Limitations & Pitfalls
- Medial arterial calcification produces falsely elevated or non-compressible readings, especially in diabetes and chronic kidney disease.
- A resting ABI can be normal in patients with claudication; exercise ABI improves sensitivity.
- Operator dependence in cuff placement and Doppler technique.
📚 References
- Aboyans V, Criqui MH, Abraham P, et al. Measurement and interpretation of the ankle-brachial index: AHA scientific statement. Circulation. 2012;126(24):2890-2909.
- Gerhard-Herman MD, Gornik HL, Barrett C, et al. 2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity PAD. Circulation. 2017;135(12):e726-e779.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.