Ankle-Brachial Index (ABI)

What is an ankle-brachial index?

An ankle-brachial index (ABI) is a painless test that measures blood pressure in your ankles and arms. It shows if you have peripheral artery disease (PAD), or plaque buildup in your leg arteries. Your ABI is the blood pressure in your ankle divided by that in your arm.

When do I need an ABI test?

Your healthcare provider may recommend an ABI test if you have PAD symptoms, like leg pain when active. They may also suggest this test if you don’t have symptoms but are: 

• Age 65 or older
• Age 50 to 64 and have certain risk factors for atherosclerosis (like diabetes, high blood pressure, high cholesterol, history of tobacco use) or a history of PAD in your biological family
• Under age 50 and have diabetes along with another risk factor for atherosclerosis

An ABI test can catch PAD before it gets severe. This gives you a chance to get treatment that may reduce your risk of complications like critical limb ischemia and amputation.

How does an ankle-brachial index test work?

The healthcare provider doing your test (usually a nurse) checks the blood pressure of specific arteries in your arms and your ankles. This involves wrapping a blood pressure cuff around each arm and ankle, one at a time. The cuff tightens for several seconds before slowly releasing.

The arteries your provider checks include:

• Your brachial artery in each of your arms
• Your dorsalis pedis artery or posterior tibial artery near each of your ankles.

Your provider also uses a handheld ultrasound device to listen to blood flow in these arteries.

An ABI test is noninvasive. That means it doesn’t involve any needle punctures or cuts in your skin. It’s very similar to your typical blood pressure check, but it involves all your limbs rather than just one.

How do I prepare for an ABI test?

Here’s how to prepare so you’re comfortable during the test and get accurate results:

• Don’t eat or drink anything that contains caffeine on the day of your test.
• Don’t use tobacco products or alcohol during the hour before your test.
• Don’t exercise during the hour before your test.
• Wear loose, comfortable clothes so your provider can easily access your upper arms and ankles.
• Just before the test, go to the restroom and pee until your bladder is empty.

Be sure to tell your provider if you’ve had any procedures to improve circulation in your legs. This may affect how they do the test.

What can I expect during the test?

You’ll lie down and rest for about 10 to 30 minutes before the test starts. You’ll keep lying on your back during the test. Your provider may prop up your arms and ankles so they’re at the same level as your heart. This helps get the most accurate blood pressure readings.

To perform an ABI test, your provider will:

1. Check the blood pressure in one of your arms
2. Check the blood pressure in the ankle that’s on the same side as the arm they just measured
3. Check the blood pressure in your other ankle
4. Check the blood pressure in your other arm

Your provider will tell you if they need to repeat any measurements. The process of taking your blood pressure typically lasts just 10 to 20 minutes. And then you’re done.

What are the risks of this test?

An ABI test is safe and usually doesn’t have any risks. But be sure to tell your provider if:

• You have any open wounds or ulcers. Your provider won’t place the cuff over that area or may need to do another type of test instead.
• You’ve recently had a vascular bypass. Squeezing that area with the cuff could be dangerous and cause blood clots to form.

What can I expect after the test?

After the test, you can go back to doing everything you normally do. The provider who performed the test may be able to share the results with you. Otherwise, you should follow up with the provider who ordered the test.

How do you calculate ABI?

You take the higher systolic (top number) blood pressure from the two ankle readings and divide that by the higher systolic blood pressure from the two arm readings.

For example, let’s say:

• Your blood pressure is 105/65 in your left ankle and 100/60 in your right ankle
• Your blood pressure is 130/85 in your left arm and 125/80 in your right arm

To find the ankle-brachial index, you’d use the higher systolic value from each limb. (In this case, use 105 for your ankle number and 130 for your arm number.) Take the ankle number and divide it by the arm number. So, take 105 and divide it by 130. That makes 0.8. It means you have mild PAD.

What do the results mean?

A normal ankle-brachial index is 1.0 to 1.3. An ABI of 0.9 or lower means you have PAD.

Here’s the ankle-brachial index range that healthcare providers use:

• 1.0 to 1.3: Normal
• 0.9 to 1.0: Borderline
• 0.7 to 0.9: Mild PAD
• 0.4 to 0.7: Moderate PAD
• Lower than 0.4: Severe PAD

A high ankle-brachial index (above 1.4) could mean the blood vessels in your limbs are stiff (calcified) because of advanced age or diabetes. Researchers have found that people with an ankle-brachial index higher than 1.4 have an increased risk of cardiovascular death.

When your arteries are calcified, an ABI test can’t reliably diagnose PAD. So, if your ABI is abnormally high, your provider may use other methods to check blood flow. For example, they might do a toe-brachial index test (which involves measuring blood pressure in your big toe, rather than your ankle). Your provider will explain which tests you need and what the results mean.

When should I call my doctor?

Contact your healthcare provider if you don’t get results right after your ankle-brachial index test. Depending on your results, you may need to schedule other testing or treatments.

You might need to repeat the ankle-brachial index test from time to time, especially if your number is outside of the normal range. Regular testing will help your provider know if your PAD is getting worse. It can also tell your provider how well treatments to open your blood vessels are working.