How can an ankle brachial index test diagnose peripheral arterial disease?

A test called an ankle brachial index or ABI is used to diagnose peripheral arterial disease (PAD). This test uses sound waves (ultrasound) to compare the blood pressure in the patient's ankles to the blood pressure in the patient's arms. If the blood pressure in the ankles is lower than the pressure in the arms, the patient may have PAD. The ABI also tells whether the amount of blood flow in the legs is reduced.