Interactive tool
Calculate the right and left ABI
Enter the systolic pressures measured with a continuous-wave Doppler probe (8–10 MHz) and an appropriately sized cuff. Empty fields are ignored: to calculate one side only, the higher brachial pressure and the pressures at that ankle are enough.
How to use it
How to measure
Patient preparation
- Patient supine for at least 5–10 minutes, in a comfortably warm room, with head and heels supported.
- No smoking in the previous 2 hours; avoid caffeine and exercise in the half hour before the measurement.
- Cuff width at least 40% of the limb circumference; on very oedematous ankles use a wider cuff.
- CW Doppler probe 8–10 MHz held at 45–60° to the skin, with plenty of gel.
Measurement sequence
- Right brachial → right posterior tibial → right dorsalis pedis → left posterior tibial → left dorsalis pedis → left brachial. If the first brachial reading is 10 mmHg or more higher than the other arm, repeat it at the end of the sequence: the first measurement is often affected by the “white-coat” effect.
- Inflate 20 mmHg above the point where the signal disappears, deflate at 2 mmHg/s and record the pressure at the return of the Doppler signal (not at the Korotkoff sounds).
- If the two brachial pressures differ by 15 mmHg or more, repeat both: a confirmed difference suggests subclavian stenosis and should be stated in the report.
Filling in the calculator
- Enter both brachial pressures: the denominator is always the higher of the two, for both legs.
- For each ankle enter the posterior tibial and the dorsalis pedis pressures: the numerator is the higher of the two (the method recommended by the AHA and ESC for diagnosing PAD).
- If an artery cannot be found, leave the field empty and note it in the report: the calculation uses the other one.
- The result also reports the inter-arm difference and flags values above 1.40 (non-compressible arteries), which make the ABI uninterpretable and require the toe-brachial index (TBI) instead.
Using the mean or the lower ankle pressure artificially lowers the ABI and overestimates the prevalence of PAD. The mean of the two arteries (the “modified” method) is more sensitive for epidemiological studies, but clinical guidelines recommend the higher pressure.
Clinical reference
Guidelines and interpretation
The thresholds below follow the 2024 ESC guidelines on peripheral arterial and aortic diseases and the AHA scientific statement on the measurement and interpretation of the ABI, which are also adopted by the Global Vascular Guidelines on chronic limb-threatening ischaemia.
| Value | Interpretation | Management |
|---|---|---|
| > 1.40 | Non-compressible arteries (medial calcification, typical of diabetes and chronic kidney disease) | ABI uninterpretable: perform the TBI, Doppler waveform analysis or transcutaneous oxygen pressure |
| 1.00 – 1.40 | Normal | If symptoms are typical, consider a post-exercise ABI (treadmill or repeated plantar flexion) |
| 0.91 – 0.99 | Borderline | Exercise ABI: a fall of > 20% in ABI or > 30 mmHg in ankle pressure supports the diagnosis of PAD |
| ≤ 0.90 | Peripheral arterial disease (sensitivity ~75%, specificity ~86% against angiography) | Optimal medical therapy, arterial duplex ultrasound, global cardiovascular risk assessment |
| ≤ 0.40 | Severe PAD | High likelihood of rest pain or tissue loss: chronic limb-threatening ischaemia pathway |
The ABI as a cardiovascular risk marker
An ABI ≤ 0.90 roughly doubles 10-year cardiovascular mortality within each traditional risk-score category, and the same applies to values > 1.40: the relationship with risk is U-shaped. This is why the ABI is recommended as a reclassification test in people at intermediate risk and for screening people with long-standing diabetes, smokers and those over seventy.
Relationship with duplex ultrasound
- The ABI quantifies the overall haemodynamic impairment of the limb; duplex ultrasound locates its cause. The two tests are complementary and should be reported together.
- A normal resting ABI does not exclude a well-compensated isolated iliac stenosis: if clinical suspicion is high, a post-exercise ABI and direct imaging of the iliac axis are needed.
- At follow-up, a fall in ABI of more than 0.15 from the previous examination indicates significant disease progression or restenosis, regardless of the absolute value; after bypass or angioplasty the ABI alone is not enough and should be combined with duplex ultrasound.
- In the diabetic foot the ABI is often falsely normal or raised: ischaemia grading in the WIfI classification favours toe pressure and TcPO2.
Bibliography
Sources
- ESC 2024 (developed with EACTS) — Guidelines for the management of peripheral arterial and aortic diseases, European Heart Journal.
- AHA Scientific Statement (Aboyans V. et al., Circulation 2012) — Measurement and interpretation of the ankle-brachial index.
- Global Vascular Guidelines (Conte MS. et al., J Vasc Surg 2019) — Management of chronic limb-threatening ischemia.
- ABI Collaboration (JAMA 2008) — Ankle brachial index combined with Framingham risk score to predict cardiovascular events and mortality.
A quick-reference tool for healthcare professionals. It does not replace the original guidelines, the summaries of product characteristics or the specialist's clinical judgement. All calculations run in the browser: no patient data are transmitted to or stored on a server.