Ankle-Brachial Index (ABI)

About Ankle-Brachial Index (ABI)

The Ankle-Brachial Index is a simple, non-invasive bedside test that compares systolic blood pressure at the ankle to systolic blood pressure at the brachial artery. First described by Winsor in 1950 and standardized by the American Heart Association in 2012, the ABI is the recommended first-line test for peripheral artery disease (PAD) and is endorsed by the ACC/AHA PAD guideline, the American Diabetes Association, the U.S. Preventive Services Task Force, and the World Health Organization. A low ABI reflects hemodynamically significant stenosis in the lower-extremity arterial tree and is independently associated with elevated cardiovascular morbidity and mortality.

What the calculator measures

The ABI is calculated separately for each leg: the highest of the posterior tibial and dorsalis pedis systolic pressures at the ankle is divided by the highest brachial systolic pressure (measured in both arms). The lower of the two leg values is reported as the patient's ABI. A normal ABI is 0.9 to 1.4. Values below 0.9 indicate PAD; the lower the value, the more severe the disease. Values above 1.4 suggest non-compressible arteries due to medial calcinosis (often seen in diabetes or chronic kidney disease) and require alternative testing such as toe-brachial index.

How the formula works

ABI = (highest ankle systolic pressure) / (highest brachial systolic pressure). For each leg, the higher of the dorsalis pedis and posterior tibial pressures is used because the two arteries have separate embryologic origins and one may be occluded while the other remains patent. Using the higher of the two ensures that the ABI reflects the best-case perfusion of the foot. The lower of the two leg values is reported because PAD is often asymmetric and the more affected leg is the one that drives clinical decision-making. For the brachial pressure, the higher of the two arm pressures is used because subclavian or brachial artery stenosis can cause an abnormally low arm pressure and artificially inflate the ABI. Using the higher arm pressure ensures the denominator reflects systemic blood pressure accurately.

Interpretation

The ABI interpretation follows the 2012 AHA Scientific Statement:

• ABI ≤0.40: Severe PAD. Critical limb ischemia is likely; patients typically have rest pain, non-healing ulcers, or gangrene. Urgent vascular evaluation is warranted.
• ABI 0.41-0.69: Moderate PAD. Patients typically have classic claudication (calf pain with exertion that resolves with rest). Vascular evaluation and aggressive risk factor modification are indicated.
• ABI 0.70-0.89: Mild PAD. Patients may have atypical leg symptoms or be asymptomatic. Risk factor modification and exercise therapy are indicated.
• ABI 0.90-1.40: Normal. PAD is unlikely.
• ABI >1.40: Non-compressible arteries (medial calcinosis). Common in diabetes and chronic kidney disease. The ABI is uninterpretable; toe-brachial index (TBI) should be measured instead.

Clinical use

The ABI is recommended for all patients with symptoms suggestive of PAD (claudication, rest pain, non-healing wounds, or abnormal pulses), all patients with established ASCVD (coronary, cerebrovascular, or aortic disease), and asymptomatic adults at elevated risk (age ≥65, age ≥50 with diabetes or smoking history, age ≥40 with diabetes and one additional risk factor). The ABI also serves as a cardiovascular risk marker: an ABI <0.9 roughly doubles 10-year cardiovascular mortality and is incorporated into some risk scores (e.g. the 2018 ACC/AHA guideline lists ABI <0.9 as a 'risk-enhancing factor' that can tip intermediate-risk patients toward statin initiation).

Limitations

The ABI has several important limitations. First, it can be falsely elevated in patients with medial calcinosis (Monckeberg sclerosis), which is common in diabetes and chronic kidney disease and renders the ankle arteries non-compressible. In these patients, the toe-brachial index (TBI) is preferred because the digital arteries are less affected by calcinosis. Second, the ABI may be normal in patients with mild aortic or iliac stenosis that is not yet hemodynamically significant at rest; exercise ABI (measured after treadmill walking) can unmask these lesions. Third, the ABI is operator-dependent and requires careful technique; inter-observer variability can be significant. Fourth, the ABI does not localize the site of stenosis; for that, duplex ultrasound, CT angiography, or MR angiography is needed.

Comparison with other PAD tests

Compared with duplex ultrasound, the ABI is faster and cheaper but does not localize the lesion. Compared with CT or MR angiography, the ABI is noninvasive and radiation-free but is less sensitive for mild stenosis. Compared with pulse volume recording, the ABI is simpler and more reproducible but is less useful in patients with heavily calcified arteries. The ABI is the recommended first-line test; further testing is reserved for patients with abnormal ABI, atypical symptoms, or symptoms that warrant anatomic delineation for revascularization planning.

How to use the result

An ABI of 0.7 indicates moderate PAD with approximately 30% reduction in ankle pressure relative to arm pressure. The clinical conversation should address smoking cessation (the most important modifiable risk factor for PAD progression), statin therapy (LDL-C goal <70 mg/dL in PAD), antiplatelet therapy (aspirin 81-162 mg daily or clopidogrel 75 mg daily), supervised exercise therapy (the most effective non-pharmacologic treatment for claudication), and glycemic control in diabetes. Patients with severe PAD (ABI ≤0.4) or rest pain require urgent vascular surgery evaluation.