How is the Ankle/Brachial Index (ABI) calculated?

The Ankle/Brachial Index (ABI) is calculated as the ratio of the highest ankle pressure to the highest brachial pressure. Specifically, it involves dividing the ankle pressure by the higher of the two brachial pressures. This methodology is significant because it provides a measure of peripheral artery disease (PAD) by comparing blood flow in the lower extremities relative to the upper extremities.

Using the higher brachial pressure serves to ensure that the calculation reflects the best possible vascular status of the upper extremities, giving a more accurate representation of the difference in blood flow and potential occlusions present in the lower leg compared to the upper body. A ratio of less than 0.90 typically indicates some degree of peripheral artery disease, guiding further clinical intervention.

In contrast, averaging pressures or using the lower brachial pressure would not provide an accurate assessment, as these methods might underestimate the severity of vascular issues or mislead clinicians regarding a patient's peripheral vascular health.