The Ankle-Brachial Index (ABI) is calculated by dividing the highest systolic blood pressure at your ankle by the highest systolic blood pressure in your arms. A normal result falls between 1.00 and 1.40, while anything at or below 0.90 signals peripheral artery disease (PAD), a condition where narrowed arteries reduce blood flow to your legs.
What You Need
An ABI measurement requires a standard blood pressure cuff and a handheld Doppler ultrasound probe, a small device that uses sound waves to detect blood flow. The Doppler probe is placed on the skin with ultrasound gel and angled toward the direction of blood flow. A regular stethoscope won’t work here because the arteries at the ankle are too small and too close to the bone for you to hear reliably with one.
Preparing for the Measurement
You need to lie flat on your back (supine) and rest for 10 to 30 minutes before any readings are taken. This rest period matters more than it might seem. Sitting upright instead of lying down can inflate your ABI value by roughly 0.3, which is enough to mask a diagnosis of peripheral artery disease entirely. Resting also lets your blood pressure stabilize after walking or climbing stairs.
Taking the Blood Pressure Readings
Four blood pressure readings are needed: one from each arm and one from each ankle. At each ankle, you’ll check two arteries, one on the top of the foot (dorsalis pedis) and one behind the inner ankle bone (posterior tibial). The American Heart Association recommends a specific order to minimize error: start with one arm, move to the ankle on the same side, cross to the opposite ankle, then finish with the opposite arm. This sequence keeps the time gap between paired measurements as short as possible.
For each site, wrap the blood pressure cuff just above the area, inflate it until the Doppler signal disappears, then slowly deflate. The point where the Doppler signal returns is the systolic pressure. Record every reading.
The Formula
Each leg gets its own ABI value. The formula for one leg is:
ABI = highest ankle pressure (that leg) ÷ highest brachial pressure (either arm)
At the ankle, you take two readings (dorsalis pedis and posterior tibial) and use whichever is higher. For the arm, you compare the systolic pressure from both arms and use whichever is higher. Then divide.
Worked Example
Say your readings look like this:
- Right arm: 130 mmHg
- Left arm: 126 mmHg
- Right ankle, dorsalis pedis: 125 mmHg
- Right ankle, posterior tibial: 118 mmHg
- Left ankle, dorsalis pedis: 108 mmHg
- Left ankle, posterior tibial: 112 mmHg
The highest brachial pressure is 130 (right arm). For the right leg, the highest ankle pressure is 125 (dorsalis pedis). So the right ABI is 125 ÷ 130 = 0.96. For the left leg, the highest ankle pressure is 112 (posterior tibial). The left ABI is 112 ÷ 130 = 0.86. The right leg falls in the borderline range, while the left leg indicates mild-to-moderate PAD.
Interpreting Your Results
The 2024 ACC/AHA guidelines break ABI values into clear categories:
- 1.00 to 1.40: Normal
- 0.91 to 0.99: Borderline
- 0.41 to 0.90: Mild to moderate PAD
- 0.00 to 0.40: Severe PAD
- Greater than 1.40: Noncompressible arteries (the reading is unreliable)
A result at or below 0.90 is the standard cutoff for diagnosing PAD. The lower the number, the more restricted blood flow has become. People with severe PAD (below 0.40) often have pain at rest or wounds that won’t heal.
A borderline result (0.91 to 0.99) may prompt a treadmill exercise test. Walking on a treadmill increases your legs’ demand for blood flow, which can reveal a problem that doesn’t show up at rest. A drop in ABI after exercise confirms PAD even when the resting number looks nearly normal.
When the ABI Is Unreliable
An ABI above 1.40 doesn’t mean your circulation is excellent. It means the arteries in your lower legs have become stiff and calcified, so the blood pressure cuff can’t compress them properly. This pushes the ankle pressure reading artificially high. Calcified arteries are common in people with diabetes, chronic kidney disease, and advanced age.
This is a significant limitation. In diabetic patients specifically, calcium deposits build up in the walls of the arteries (not the same as cholesterol plaque inside them). The result is a falsely elevated reading that can mask even severe PAD. If your ABI comes back above 1.40, the test hasn’t failed exactly, but it can’t give you a meaningful answer about blood flow.
The alternative in this situation is a Toe-Brachial Index (TBI). Toe arteries are less prone to calcification. The test works the same way conceptually: a tiny cuff goes around your big toe, and the toe pressure is divided by the highest brachial pressure. A TBI of 0.70 or below is considered abnormal. For reference, a TBI of about 0.65 roughly corresponds to a normal ABI of 1.0, and a TBI of 0.50 corresponds to an ABI of about 0.90, the PAD diagnostic threshold.
Who Should Get an ABI Test
The 2024 ACC/AHA guidelines recommend ABI screening for several groups:
- Anyone 65 or older
- Ages 50 to 64 with risk factors like diabetes, smoking history, high cholesterol, high blood pressure, chronic kidney disease, or a family history of PAD
- Under 50 with diabetes plus at least one additional risk factor
- People with known artery disease elsewhere, such as coronary artery disease, carotid artery narrowing, or an abdominal aortic aneurysm
Younger people without any of these risk factors have a very low chance of having PAD, and routine screening in that group isn’t recommended. The test itself is noninvasive and carries essentially no risk, so the question isn’t safety but simply whether the result would be useful.