How is the left ankle-brachial index (LABI) calculated?

• A. Highest left ankle pressure divided by highest brachial pressure
• B. Highest brachial pressure divided by highest left ankle pressure
• C. Average of the left and right ankle pressures divided by brachial pressure
• D. Sum of the ankle pressures divided by brachial pressure

Answer: (A)

The left ankle-brachial index is a ratio of systolic pressures, using the left leg as the limb of interest. To get LABI, take the higher of the two left ankle systolic pressures (from the dorsalis pedis and posterior tibial arteries) as the numerator, and divide it by the higher of the two brachial systolic pressures (from either arm) as the denominator. This approach standardizes the measurement and helps avoid underestimating disease by using the strongest comparable pressures from both sides. Using averages, sums, or the opposite leg’s pressure would not reflect the left limb’s perfusion accurately.