How is the ABI calculated?

• A. Ankle systolic pressure divided by higher brachial systolic pressure
• B. Higher brachial systolic pressure divided by ankle systolic pressure
• C. Mean arterial pressure divided by ankle pressure
• D. Systolic pressure in the thigh divided by arm pressure

Answer: (A)

Ankle-Brachial Index is a simple ratio that compares the blood pressure at the ankle with the pressure in the arm to assess blood flow to the legs. The calculation uses the ankle systolic pressure (taking the higher value from the two ankle arteries) divided by the higher of the two brachial systolic pressures. Using the higher brachial pressure accounts for any arm-side disease or reading differences so the index isn’t falsely elevated or underestimated.

The other ideas don’t fit because they either invert the ratio (ankle divided by arm should be the correct form, not arm divided by ankle), use mean arterial pressure instead of systolic pressure (not how ABI is defined), or involve a different segment (thigh pressure) rather than the ankle.