The ankle-brachial index (ABI) is measured by comparing the systolic blood pressure at your ankle to the systolic blood pressure in your arm. A normal result falls between 1.00 and 1.40, and the test takes about 15 to 20 minutes. It’s the primary screening tool for peripheral artery disease (PAD), a condition where narrowed arteries reduce blood flow to the legs.
Equipment You Need
An ABI measurement requires three things: a manual blood pressure cuff, a handheld Doppler ultrasound device, and ultrasound transmission gel. The blood pressure cuff width should be at least 40% of the limb’s circumference. A manual cuff is preferred over an automatic one because automated devices have difficulty reading low blood pressures and tend to detect mean arterial pressure rather than true systolic pressure, which makes the result less accurate.
Patient Positioning and Preparation
The person being tested should lie flat on their back (supine) and rest for at least 5 to 10 minutes before any measurements begin. This rest period matters more than you might think. Sitting upright instead of lying down can artificially increase the ABI value by roughly 0.3, which is enough to push an abnormal reading into the normal range and mask real disease. Both arms and both ankles need to be accessible, with sleeves rolled up and socks removed.
Measuring Brachial Pressure
Start by wrapping the blood pressure cuff around one arm, keeping the limb at heart level. Apply ultrasound gel to the inner elbow crease (the antecubital fossa), directly over where you can feel the brachial pulse. Place the Doppler probe on the gel at a 45 to 60 degree angle, not straight up and down, and adjust until you hear the strongest signal.
Inflate the cuff to about 20 mmHg above the expected systolic blood pressure. The Doppler signal should disappear completely. Then slowly deflate the cuff at roughly 1 mmHg per second. The moment the Doppler signal reappears, the pressure reading on the gauge equals the brachial systolic pressure. Record that number, then repeat on the other arm.
Measuring Ankle Pressure
Place the blood pressure cuff just above the ankle bones (the malleoli). Apply ultrasound gel over two arteries in the foot: the dorsalis pedis artery on the top of the foot and the posterior tibial artery, which sits just behind the inner ankle bone. Using the same Doppler technique, inflate the cuff until the signal disappears, then slowly deflate and record the pressure when the signal returns. Do this for both arteries on both legs, so you end up with four ankle readings total.
Calculating the ABI
You calculate a separate ABI for each leg using this formula:
ABI = highest ankle pressure in that leg ÷ highest brachial pressure from either arm
For the ankle, take whichever is higher between the dorsalis pedis and posterior tibial readings. For the arm, use whichever brachial reading was higher, regardless of which side the leg is on. So if the right ankle’s highest pressure is 140 mmHg and the highest brachial pressure (from either arm) is 150 mmHg, the right leg’s ABI is 140 ÷ 150 = 0.93.
Interpreting Your Results
ABI values break down into clear ranges:
- 1.00 to 1.40: Normal
- 0.91 to 0.99: Borderline, suggesting early or mild arterial narrowing
- 0.41 to 0.90: Mild to moderate peripheral artery disease
- 0.00 to 0.40: Severe peripheral artery disease
- Above 1.40: Non-compressible arteries, meaning the vessels are too stiff for an accurate reading
If the cuff can’t stop blood flow even at 300 mmHg, the arteries are classified as non-measurable. This typically happens when calcium deposits have hardened the artery walls, making them rigid.
When ABI Results Are Unreliable
Several factors can throw off your ABI measurement. Smoking selectively raises ankle systolic pressure without affecting brachial pressure, which produces a falsely elevated (healthier-looking) result. Calcified arteries, common in people with diabetes and kidney disease, resist compression and generate abnormally high readings above 1.40 that don’t reflect actual blood flow.
Automated oscillometric devices introduce their own errors. They struggle with low blood pressures and detect pressure differently than a Doppler probe, making them less reliable for this specific test.
The ABI also has documented limitations for assessing heel wounds. One study found it was not reliable for evaluating blood flow to heel pressure sores specifically.
When a Toe-Brachial Index Is Better
For people with diabetes or known arterial calcification, the standard ABI often reads falsely normal or elevated, even when significant artery narrowing exists. In these cases, a toe-brachial index (TBI) is the preferred alternative. The TBI measures blood pressure in the great toe instead of the ankle. Toe arteries are generally spared from the type of calcification that stiffens larger leg arteries, so their pressure readings remain accurate.
The American Diabetes Association recommends that people with diabetes be evaluated for PAD at least every five years, and that both ABI and TBI be performed when arterial wall calcification is present. Research has confirmed that TBI is the method of choice for evaluating lower limb blood flow in patients with overt calcification.
Situations Where ABI Should Not Be Done
Two situations call for skipping or postponing ABI measurement. The first is a known or suspected deep vein thrombosis (blood clot) in the leg, because inflating the cuff could dislodge the clot and send it to the lungs. The second is severe leg pain from any cause, whether ischemia, a fracture, swelling, or open wounds. The pressure required to occlude an artery is substantial, and applying it to an already painful limb can cause significant distress.