AnkleBrachial

Ankle-brachial index (ABI), also called the ankle-brachial pressure index, is a noninvasive test used to screen for and evaluate peripheral artery disease (PAD). It compares the blood pressure in the ankles with the blood pressure in the arms to detect reduced blood flow due to arterial narrowing.

Procedure and calculation: After the patient rests, systolic pressures are measured in both ankles (commonly the dorsalis pedis and posterior tibial arteries) and in both arms using a Doppler probe. For each leg, the ABI is calculated as the higher ankle systolic pressure divided by the higher brachial systolic pressure. The lower of the two leg ABIs is typically reported as the patient’s ABI for diagnostic purposes.

Interpretation: Normal ABI ranges from about 1.0 to 1.4. An ABI of 0.91–0.99 is considered borderline, and ≤0.90 indicates peripheral artery disease. Lower values indicate more severe disease: roughly 0.41–0.90 indicates mild to moderate PAD, and ≤0.40 indicates severe PAD. An ABI greater than 1.40 can reflect noncompressible arteries due to arterial calcification, common in diabetes or chronic kidney disease, which may obscure true blood flow; in such cases, the toe-brachial index (TBI) or imaging studies may be used.

Clinical use and limitations: ABI is a simple, inexpensive test that helps diagnose PAD, assess cardiovascular risk, and guide treatment decisions, including lifestyle changes, antiplatelet therapy, and statin use, with revascularization considered for symptoms or critical limb ischemia. Limitations include impaired accuracy in noncompressible arteries, obesity, edema, and diabetic patients with vascular calcification; test results can be operator-dependent and may require adjunctive tests such as duplex ultrasound or CT angiography.