An ABI, or ankle-brachial index, is a simple test that compares blood pressure in your ankle to blood pressure in your arm. The ratio between these two numbers reveals how well blood is flowing to your legs and can detect peripheral artery disease (PAD), a condition where narrowed arteries restrict circulation to the limbs. The test is painless, takes about 15 minutes, and requires no needles or special preparation.
How the Test Works
The idea behind the ABI is straightforward. In a healthy person, blood pressure at the ankle should be roughly equal to or slightly higher than blood pressure in the arm. When arteries in the legs become narrowed by plaque buildup, less blood reaches the ankle, and the pressure there drops. The ABI captures this by dividing the systolic blood pressure at your ankle by the systolic blood pressure at your arm.
For example, if your ankle pressure is 120 mmHg and your arm pressure is 130 mmHg, your ABI is 0.92. That single number tells a clinician a surprising amount about your vascular health.
What Happens During the Test
You’ll lie flat on an exam table with your arms and legs at roughly the same level as your heart. Before any measurements start, you rest for at least 10 minutes so your blood pressure stabilizes. A technician wraps a standard blood pressure cuff around your upper arm, then uses a handheld Doppler probe (a small ultrasound device) to listen for blood flow while inflating and deflating the cuff. The same process is repeated on your other arm, and then on each ankle using arteries on the top of your foot and behind the inner ankle bone.
No sedation or numbing is needed. The cuff squeeze feels the same as a routine blood pressure check. You should wear loose clothing so the cuffs can be placed easily on bare skin.
What Your ABI Number Means
ABI results fall into well-defined ranges:
- 1.00 to 1.40: Normal blood flow to the legs.
- 0.91 to 0.99: Borderline. Blood flow may be slightly reduced, and further monitoring or testing is often appropriate.
- 0.41 to 0.90: Mild to moderate peripheral artery disease. Arteries in the legs are partially blocked.
- 0.00 to 0.40: Severe PAD. Blood flow to the legs is significantly restricted, and symptoms like pain at rest or slow-healing wounds are common.
- Above 1.40: The arteries may be stiffened or calcified, making the reading unreliable. A different test is usually needed (more on that below).
Each leg gets its own ABI value, so it’s possible to have normal flow in one leg and reduced flow in the other.
Why the Test Matters Beyond Your Legs
A low ABI doesn’t just flag leg artery problems. It serves as a warning sign for your entire cardiovascular system. Plaque buildup rarely affects only one set of arteries, so narrowed leg arteries often signal narrowing elsewhere, including the arteries feeding the heart and brain.
A large population study in Hisayama, Japan, found that people with an ABI of 0.90 or below had a 2.4 times greater risk of cardiovascular events and a 4.1 times greater risk of coronary heart disease compared to those with normal values. These elevated risks held even after accounting for traditional factors like high blood pressure, cholesterol, smoking, and diabetes. In other words, the ABI adds predictive information that a standard risk assessment might miss.
Who Should Get Tested
The 2024 guidelines from the American College of Cardiology and American Heart Association identify several groups who benefit most from ABI screening:
- Adults 65 and older, regardless of other risk factors.
- Adults 50 to 64 who have risk factors for atherosclerosis, such as diabetes, a history of smoking, high cholesterol, high blood pressure, chronic kidney disease, or a family history of PAD.
- Adults under 50 with diabetes plus at least one additional atherosclerosis risk factor.
- Anyone with known artery disease in another part of the body, such as coronary artery disease or carotid artery narrowing.
Younger adults without these risk factors generally don’t need routine screening. The chance of finding PAD in that group is very low.
Anyone with symptoms that suggest reduced leg circulation, such as leg pain or cramping during walking that goes away with rest, numbness, or wounds on the feet that heal slowly, should also be tested promptly.
Accuracy and Limitations
The ABI is highly specific, meaning when it flags a problem, the finding is almost always real. Specificity ranges from 83% to 99% across studies. Its sensitivity is more variable, ranging from 15% to 79%, which means it can miss some cases of PAD, particularly mild ones.
The biggest limitation involves calcified arteries. In people with diabetes, chronic kidney disease, or advanced age, calcium deposits can stiffen the artery walls so much that the blood pressure cuff can’t compress them properly. This produces an artificially high reading, sometimes above 1.40 or even beyond what the cuff can measure. The result looks normal or elevated on paper while disease may be present underneath.
The Toe-Brachial Index Alternative
When arterial calcification makes the ABI unreliable, clinicians turn to the toe-brachial index (TBI). This test uses a tiny pressure cuff on the big toe instead of the ankle. The small arteries in the toes are rarely affected by the type of calcification that stiffens ankle arteries, so the TBI provides a more accurate picture in these patients. It’s considered the preferred method for evaluating leg blood flow in people with known arterial wall calcification, particularly those with diabetes or chronic kidney disease.
Exercise ABI Testing
Sometimes your resting ABI comes back normal, but your symptoms strongly suggest PAD. Arteries that are only mildly narrowed may deliver enough blood at rest but fail to keep up when your muscles demand more during activity. An exercise ABI test catches these cases.
You’ll typically walk on a treadmill for a set period, and your ankle pressures are measured again within one minute of stopping. A drop in ABI of 18.5% or more after exercise, or a pressure drop of 30 mmHg or more at the ankle, points toward PAD even when the resting number looked fine. This mirrors what many patients experience: legs that feel perfectly normal sitting down but cramp or ache during a walk.