An ABI test, or ankle-brachial index test, is a quick, painless screening that compares blood pressure readings in your ankle and your arm to check for narrowed arteries in your legs. The result is a simple ratio: if the blood pressure at your ankle is significantly lower than in your arm, it suggests that blood isn’t flowing freely to your lower limbs, a condition called peripheral artery disease (PAD). A normal ABI falls between 1.0 and 1.3, while a reading of 0.90 or below points to PAD.
How the Test Works
Blood pressure is normally the same or slightly higher in your legs compared to your arms. When plaque builds up inside the arteries supplying your legs, it narrows the pathway and reduces the pressure downstream, much like pinching a garden hose lowers the water pressure at the nozzle. The ABI captures this difference as a number by dividing the systolic (top number) blood pressure at your ankle by the systolic pressure in your arm.
A healthy person with no blockages will have a ratio close to 1.0, meaning the pressures are roughly equal. The further that ratio drops below 1.0, the more significant the narrowing. Because the test relies on a straightforward pressure comparison, it can flag circulation problems before symptoms ever appear.
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 begin, you rest for at least 10 minutes so your blood pressure stabilizes. The technician or clinician wraps a standard blood pressure cuff around your upper arm, then places another cuff just above your ankle. The cuff needs to be wide enough to fit your limb properly, at least 40% of the limb’s circumference, to get an accurate reading.
Instead of using a stethoscope, the clinician uses a small handheld Doppler ultrasound probe. A dab of gel goes on the skin over the artery, and the probe picks up the sound of blood flow. The cuff inflates until that sound disappears, then slowly deflates until the pulse returns. That point is your systolic pressure. This is repeated on both arms and both ankles, targeting the arteries behind the inner ankle bone and on the top of the foot. The whole process typically takes 15 to 30 minutes, and there are no needles, dyes, or discomfort involved.
Understanding Your Results
Your ABI is calculated for each leg separately. The clinician takes the higher of the two ankle pressures in that leg and divides it by the higher of the two arm pressures. The resulting number falls into one of several ranges:
- 1.0 to 1.3: Normal. No significant blockage.
- 0.9 to 1.0: Borderline. Mild narrowing may be developing.
- 0.7 to 0.9: Mild PAD. Noticeable reduction in blood flow.
- 0.4 to 0.7: Moderate PAD. Symptoms like leg pain during walking are common at this stage.
- Below 0.4: Severe PAD. Blood flow is seriously compromised, and tissue damage becomes a concern.
A reading above 1.3 can also be a red flag. It usually means the artery walls have become stiff and calcified, which prevents the cuff from compressing them normally. This artificially inflates the reading and can mask underlying disease. Diabetes and chronic kidney disease are the most common reasons for this kind of false result.
When Doctors Recommend an ABI
You’re most likely to be offered an ABI if you already have symptoms of poor leg circulation, like cramping or aching in your calves when you walk that goes away with rest (called claudication), wounds on your feet that heal slowly, or noticeably cooler skin on one leg. But the test is also used as a screening tool in people without symptoms who carry significant risk factors.
The American College of Cardiology and the American Heart Association recommend screening with an ABI for adults 65 and older, adults 50 and older who have risk factors for artery disease or a family history of PAD, and adults under 50 who have diabetes plus at least one additional risk factor. The major risk factors include smoking, high blood pressure, high cholesterol, obesity, diabetes, and physical inactivity. Those four conventional cardiovascular risk factors (smoking, high blood pressure, high cholesterol, and diabetes) account for roughly 75% of all PAD cases. At least one of them is present at diagnosis in 96% of men who develop the disease.
The U.S. Preventive Services Task Force takes a more cautious position, noting that evidence is still insufficient to recommend routine ABI screening in all asymptomatic adults. In practice, this means your doctor will weigh your individual risk profile rather than ordering the test for everyone.
When ABI Results Are Unreliable
The ABI is a reliable first-line screening tool for most people, but it has a known blind spot. In people with heavily calcified arteries, the blood pressure cuff can’t compress the vessel enough to get a true reading. The result looks normal or even elevated when actual blood flow is reduced. This is particularly common in people with diabetes and those with end-stage kidney disease.
When a clinician suspects the ABI may be falsely reassuring, a toe-brachial index (TBI) test can fill the gap. This variation works the same way but measures blood pressure in the big toe (or second toe) instead of at the ankle. The smaller arteries in the toes are far less prone to calcification, so the reading tends to be more accurate in these patients. The two tests are often used together to get the most complete picture of leg circulation.
What a Low ABI Means for You
A low ABI does more than diagnose leg artery disease. Because PAD and heart disease share the same underlying process (plaque buildup in arteries), a low reading is also a strong signal that your arteries elsewhere, including those supplying the heart and brain, may be narrowing too. People with a low ABI face a higher risk of heart attack and stroke, even if their legs feel fine.
If your ABI comes back low, your doctor will likely discuss lifestyle changes that slow or reverse plaque progression: quitting smoking, managing blood pressure and cholesterol, staying physically active, and controlling blood sugar if you have diabetes. For people with symptoms like leg pain during walking, supervised exercise programs have a strong track record of improving walking distance and quality of life. In more advanced cases, procedures to restore blood flow, such as placing a stent or surgically rerouting blood around the blockage, become part of the conversation.
A borderline result (0.9 to 1.0) often prompts a repeat test or an exercise ABI, where the measurements are taken again after you walk on a treadmill. Some blockages only become apparent when your muscles demand more blood flow, so resting numbers can look normal while post-exercise numbers drop. This helps catch early-stage disease that might otherwise be missed.