Ankle-Brachial Index (ABI) Calculator

The Ankle-Brachial Index is a non-invasive test that compares blood pressure at the ankle to blood pressure at the arm. It is calculated by dividing the systolic blood pressure at the ankle by the systolic blood pressure at the brachial (upper arm) artery. An ABI below 0.90 indicates peripheral artery disease (PAD). It is used by clinicians to screen for and monitor PAD.

According to the AHA and ACC: ABI 1.00-1.40 = normal; ABI 0.91-0.99 = borderline; ABI 0.41-0.90 = mild to moderate PAD; ABI 0.40 or below = severe PAD. ABI above 1.40 suggests arterial calcification (non-compressible vessels), which is common in diabetes and chronic kidney disease and may itself indicate cardiovascular risk. Both very low and very high ABI values warrant clinical evaluation.

PAD is a narrowing of the arteries that carry blood to the legs and feet, usually due to atherosclerosis. Symptoms include claudication (leg pain or cramping when walking), numbness, weakness, and in severe cases non-healing wounds or gangrene. PAD affects approximately 6.5 million Americans over 40. It is a strong marker of systemic cardiovascular disease, including coronary artery disease and stroke risk.

A clinician uses a Doppler ultrasound device and blood pressure cuffs to measure systolic pressure at the posterior tibial or dorsalis pedis arteries of both ankles, and at the brachial arteries of both arms. The patient lies flat for 5-10 minutes before measurement. The highest ankle pressure on each side is divided by the highest of the two arm pressures. This calculator uses these same inputs to produce the ABI.

The ACC/AHA recommends ABI testing for: adults aged 65 or older; adults aged 50-64 with risk factors (diabetes, smoking, hypertension, or hypercholesterolaemia); adults under 50 with diabetes plus one additional PAD risk factor; anyone with exertional leg symptoms, non-healing wounds, or abnormal lower extremity pulse examination. PAD prevalence is about 12-20% in those over 70.