Ankle-brachial index (ABI) calculator

How the ABI is worked out

The resting ABI is measured in each leg with a blood pressure cuff and a Doppler device that picks up blood flow in the arteries of the foot and the arm. For each leg, the ABI is the higher of the two ankle pressures on that side (the dorsalis pedis artery on the top of the foot and the posterior tibial artery behind the inner ankle) divided by the higher of the right and left arm pressures (ACC/AHA 2024).

An example with made-up numbers: arm pressures of 130 and 140 mm Hg, and right ankle pressures of 120 and 126 mm Hg. The right leg ABI is 126 ÷ 140 = 0.90.

ABI normal range and categories

The 2024 guideline recommends reporting a resting ABI in four categories (ACC/AHA 2024):

The guideline notes that these reporting criteria have been broadly adopted internationally, which lets results be compared between clinics and over time in the same person (ACC/AHA 2024).

Why an ABI over 1.40 is not good news

In people with diabetes or chronic kidney disease, the ankle arteries can become calcified and too stiff for the cuff to squeeze shut. The ankle pressure then reads falsely high, giving an ABI over 1.4 even when imaging shows significant PAD (ACC/AHA 2024). The small arteries of the toes are rarely affected this way, so the toe-brachial index (toe pressure divided by the higher arm pressure) is used instead; a value of 0.70 or less is considered abnormal (ACC/AHA 2024).

How accurate is the ABI?

In people with symptoms of PAD, the resting ABI picks up 69% to 79% of people with significant narrowing seen on imaging (sensitivity), and 83% to 99% of people without it test normal (specificity). It is less sensitive in people with diabetes (ACC/AHA 2024). People with an abnormal ABI and no symptoms still have a higher rate of heart attack, stroke and death than people with a normal ABI (ACC/AHA 2024).

Who should have an ABI test?

The guideline recommends the resting ABI to diagnose PAD in people whose history or exam suggests it, such as leg pain or cramping when walking that eases with rest, weak foot pulses, or wounds on the legs or feet that heal slowly (ACC/AHA 2024). It says screening with the ABI is reasonable in people at increased risk of PAD (ACC/AHA 2024):

• Age 65 or older
• Age 50 to 64 with risk factors for atherosclerosis (such as diabetes, a history of smoking, high cholesterol or high blood pressure), chronic kidney disease, or a family history of PAD
• Under 50 with diabetes and one more risk factor for atherosclerosis
• Known atherosclerosis in another artery, such as the heart, neck (carotid) or an abdominal aortic aneurysm

It recommends against screening people who are not at increased risk and have no symptoms or signs (ACC/AHA 2024). The US Preventive Services Task Force, looking at adults without symptoms in primary care, found the evidence insufficient to weigh the benefits and harms of ABI screening (an I statement) (USPSTF 2018). See the PAD screening check for what that means for you.

If your ABI is low

PAD is atherosclerosis, the same process that narrows arteries to the heart and brain, so it is also a warning about heart attack and stroke risk (USPSTF 2018). Ask your clinician what your result means for your treatment. The heart risk calculator estimates 10- and 30-year cardiovascular risk, but it was built for people without known cardiovascular disease, so ask your clinician whether it applies to you once PAD is diagnosed. For help with the strongest risk factor you can change, see the quit smoking check.