ABI Calculator - Ankle-Brachial Index
Enter the highest systolic blood pressure from both arms and the highest systolic reading from each ankle (dorsalis pedis or posterior tibial artery). The calculator divides each ankle pressure by the brachial pressure to give a right ABI and a left ABI, then classifies each result into a standard clinical category from normal through severe peripheral artery disease. Results update as you type.
Formula
Worked example
Brachial pressure 130 mmHg, right ankle 135 mmHg, left ankle 128 mmHg: Right ABI = 135/130 = 1.04 (Normal); Left ABI = 128/130 = 0.98 (Borderline). Overall risk is based on the lower value of 0.98.
What is the Ankle-Brachial Index?
The Ankle-Brachial Index (ABI) is the ratio of the systolic blood pressure measured at the ankle to the systolic blood pressure measured at the brachial artery (upper arm). In a healthy artery, blood pressure at the ankle should be equal to or slightly higher than arm pressure, so a normal ABI is between 1.0 and 1.4. When arteries in the legs are narrowed by atherosclerosis, pressure drops distal to the obstruction, producing a lower ankle reading and a reduced ABI. Because ABI requires only a blood pressure cuff and a handheld Doppler probe, it is one of the most cost-effective screening tools available for peripheral artery disease (PAD) and overall cardiovascular risk.
How to measure blood pressure for ABI testing
ABI measurement follows a standardised protocol. The patient rests supine for at least 5 to 10 minutes before measurement. Blood pressure is recorded from both brachial arteries using a Doppler probe; the higher of the two values is used as the denominator for both legs. At each ankle, the systolic pressure is recorded from two sites: the dorsalis pedis artery on the top of the foot and the posterior tibial artery behind the medial malleolus. The higher reading from the two sites on each foot is used as the numerator for that leg. Dividing each ankle value by the brachial pressure gives the right and left ABI. Patients who smoke should abstain for at least two hours before the test, as nicotine raises peripheral vascular resistance acutely.
Clinical significance and who should be tested
An ABI below 0.9 has approximately 90% sensitivity and 98% specificity for angiographically confirmed peripheral artery disease. Beyond diagnosing PAD, a low ABI is an independent predictor of major adverse cardiovascular events including myocardial infarction and stroke, even in people without symptoms. Current guidelines from the American Heart Association recommend ABI screening for: all adults aged 70 or older; adults aged 50 to 69 with a history of smoking or diabetes; and younger patients with known cardiovascular risk factors or unexplained leg symptoms such as claudication. An ABI above 1.4 indicates non-compressible, calcified vessels and is also associated with elevated cardiovascular risk; the toe-brachial index (TBI) is the preferred test in this situation.
Limitations and when to seek further assessment
ABI is a screening tool, not a complete vascular workup. It can underestimate disease severity when vessels are heavily calcified (common in diabetes and chronic kidney disease), because incompressible arteries give falsely elevated readings. Conversely, ABI is less sensitive for disease isolated to small vessels. Exercise ABI testing can unmask mild disease that resting ABI misses: a drop of 0.15 or more after a standardised treadmill protocol is considered significant. If both resting and exercise ABI are normal but symptoms persist, imaging with duplex ultrasound, CT angiography or MR angiography provides a more detailed anatomical picture. An inter-limb ABI difference greater than 0.15 warrants further investigation even if both absolute values appear acceptable.
ABI interpretation ranges
| ABI range | Classification | Clinical significance |
|---|---|---|
| Above 1.4 | Non-compressible vessels | Calcification; use toe-brachial index |
| 1.0 to 1.4 | Normal | No significant arterial disease |
| 0.9 to 0.99 | Borderline | Monitor; address risk factors |
| 0.8 to 0.89 | Mild PAD | Arterial narrowing present |
| 0.5 to 0.79 | Moderate PAD | Vascular specialist referral |
| Below 0.5 | Severe PAD | Urgent vascular specialist referral |
Standard clinical classification from the American Heart Association and the American College of Cardiology.
Frequently asked questions
What is a normal ABI value?
An ABI between 1.0 and 1.4 is considered normal, indicating healthy arterial blood flow to the legs. Values from 0.9 to 0.99 are borderline and warrant monitoring and cardiovascular risk factor management. An ABI of 0.9 or below indicates peripheral artery disease of varying severity.
Why do I need to use the highest arm pressure in the ABI formula?
Using the higher of the two brachial readings avoids falsely lowering the denominator, which would artificially inflate the ABI and potentially miss disease. A difference of more than 15 mmHg between the arms is itself a clinical finding that may indicate subclavian artery stenosis.
What does an ABI above 1.4 mean?
An ABI greater than 1.4 suggests that the ankle arteries are calcified and non-compressible, so the cuff cannot fully occlude them. This gives a falsely high reading. It is not a sign of good blood flow; instead it is associated with diabetes, chronic kidney disease, and elevated cardiovascular risk. A toe-brachial index (TBI) using a smaller cuff at the great toe is the preferred next test.
How do I interpret the difference between my right and left ABI?
A difference of 0.15 or more between the right and left ABI is clinically significant and may indicate asymmetric arterial disease on the lower-scoring side. Both values should be reported separately to capture this asymmetry, and the lower value is used when classifying overall disease severity.
Can I calculate ABI without a Doppler probe?
The standard protocol uses a handheld Doppler probe to detect the return of blood flow when the cuff is deflated, because Korotkoff sounds are often inaudible at the ankle. Oscillometric automated cuffs validated for ABI are available and can be acceptable in some settings, but manual Doppler remains the reference standard. Using a standard stethoscope alone is not recommended because it routinely underestimates ankle systolic pressure.
What symptoms suggest I might have a low ABI?
The most common symptom of PAD is intermittent claudication, a reproducible cramping pain, ache, or fatigue in the calf, thigh, or buttock that comes on with walking and is relieved within minutes of rest. More severe disease can cause rest pain in the foot, non-healing leg ulcers, and tissue loss. Up to half of people with a low ABI have no leg symptoms at all, which is why population screening in high-risk groups is recommended.
Sources
- Gerhard-Herman MD et al. 2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity Peripheral Artery Disease. Circulation. 2017.
- Aboyans V et al. Measurement and Interpretation of the Ankle-Brachial Index: A Scientific Statement From the American Heart Association. Circulation. 2012.