From the Guidelines
The ankle-brachial index (ABI) is a non-invasive test that compares blood pressure measured at the ankle to blood pressure measured at the arm, used to assess peripheral artery disease (PAD) in the legs, with a sensitivity of 68%–84% and a specificity of 84%–99% for PAD diagnosis, as reported in the 2024 ESC guidelines for the management of peripheral arterial and aortic diseases 1. To perform an ABI:
- Measure systolic blood pressure in both arms using a blood pressure cuff and Doppler ultrasound probe.
- Measure systolic blood pressure at both ankles (dorsalis pedis and posterior tibial arteries).
- Calculate the ABI for each leg by dividing the higher ankle pressure by the higher arm pressure. Interpretation:
- ABI > 1.4: Suggests non-compressible arteries (common in diabetes)
- ABI 1.0-1.4: Normal
- ABI 0.91-0.99: Borderline
- ABI 0.41-0.90: Mild to moderate PAD
- ABI ≤ 0.40: Severe PAD The ABI test is valuable for diagnosing PAD, assessing cardiovascular risk, and monitoring treatment effectiveness, as stated in the 2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS guideline for the management of lower extremity peripheral artery disease 1. Regular ABI screening is recommended for individuals over 50 with risk factors for PAD or those with symptoms of leg pain during exercise, as the prevalence of PAD among individuals without risk factors for atherosclerosis and who are <50 years of age is low, approximately 1% 1. In patients with non-compressible arteries, the toe-brachial index (TBI) can be used to evaluate for PAD, with a TBI of ≤0.70 considered abnormal, as reported in the 2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS guideline 1.
From the Research
Definition of Ankle Brachial Index (ABI)
- The Ankle Brachial Index (ABI) is a sensitive and cost-effective screening tool for peripheral arterial disease (PAD) 2.
- ABI is calculated as the ratio of ankle and brachial systolic blood pressure 3.
- Normal cut-off values for ABI are between 0.9 and 1.4 2.
ABI as a Diagnostic Tool
- ABI is valuable for screening of peripheral artery disease in patients at risk and for diagnosing the disease in patients who present with lower-extremity symptoms 2.
- ABI can be used to predict the severity of peripheral arterial disease, with lower ABI values indicating more severe disease 4.
- However, ABI may underestimate the severity of PAD in patients with noncompressible vessels 5.
ABI and Cardiovascular Risk
- An abnormal ankle-brachial index (below 0.9) is a powerful independent marker of cardiovascular risk 2.
- There is an inverse correlation between ABI values, non-fatal cardiac events, and mortality, with patients with very low ABI (<0.3) having a significantly higher additional risk 2.
- ABI values over 1.3-1.4 correlate with major adverse cardiovascular events 2.
Limitations and Variations of ABI
- Different calculations of ABI, such as using the higher or lower of the two ankle pressures, can impact cardiovascular risk prediction 3.
- The use of the lower ankle pressure (LABI method) may be more sensitive and accurate in detecting PAD compared to the conventional method (HABI method) 4.
- ABI may be influenced by physiological characteristics such as age, sex, body position, and techniques used for ABI determination 6.