How is the ankle-brachial index calculated?

• A. Ankle systolic pressure divided by the highest brachial systolic pressure
• B. Ankle systolic pressure divided by the lowest brachial systolic pressure
• C. Ankle systolic pressure divided by the average brachial systolic pressure
• D. Ankle diastolic pressure divided by the brachial systolic pressure

Answer: (A)

The ankle-brachial index measures arterial perfusion by comparing systolic pressures in the leg to those in the arm. To be reliable, you take the higher of the two ankle systolic pressures (dorsalis pedis or posterior tibial) and divide it by the higher of the two brachial systolic pressures. This approach accounts for possible asymmetry or undervaluation from a single cuff reading and prevents misclassification of disease due to a low arm pressure. Diastolic pressures aren’t used in this index, and using an average or the lowest arm pressure would distort the ratio. Therefore, ankle systolic pressure divided by the highest brachial systolic pressure is the correct calculation.