The ankle-brachial index (ABI) is calculated by dividing the systolic blood pressure measured at the ankle by the systolic pressure measured at the brachial artery. A normal ABI is 1.0–1.4. Values below 0.9 indicate peripheral arterial disease (PAD); values below 0.4 indicate critical limb ischemia. ABI is commonly performed by vascular technologists in conjunction with arterial duplex ultrasound as part of a complete peripheral arterial evaluation.