From the Guidelines
A normal Ankle-Brachial Index (ABI) is between 0.9 and 1.4, indicating healthy arterial blood flow in the lower extremities. This range is crucial for assessing the risk of peripheral artery disease (PAD) and guiding further management. To measure ABI, the highest systolic blood pressure at the ankle is divided by the highest systolic pressure in the arm 1. Values below 0.9 suggest PAD, with severity increasing as the number decreases, while values above 1.4 typically indicate calcified, non-compressible arteries.
Key Points to Consider
- ABI is a valuable non-invasive screening tool for PAD, especially in patients with risk factors like smoking, diabetes, hypertension, or advanced age 1.
- The test requires a blood pressure cuff and Doppler ultrasound device, and should be performed after the patient has been resting for 5-10 minutes in a supine position for accurate results.
- Regular ABI screening is recommended for at-risk populations as early PAD detection can lead to interventions that prevent complications like claudication, tissue loss, or amputation.
Clinical Implications
- In patients with a normal ABI (0.9-1.4), reassessment every 2-3 years is recommended, or more frequently if risk factors such as smoking are present 1.
- If symptoms suggest PAD but the ABI result is normal, sensitivity can be improved by post-exercise ABI or the toe-brachial index (TBI) at rest 1.
- An ABI >1.40 is mostly related to medial calcinosis but is associated with PAD in 50% of cases, and other tests like Doppler waveform analysis or TBI may be helpful in these cases 1.
From the Research
Definition of Normal ABI
- A normal Ankle-Brachial Index (ABI) is typically defined as a value between 0.9 and 1.4 2.
- This range indicates that the blood pressure in the ankles is similar to the blood pressure in the arms, suggesting that there is no significant narrowing or blockage of the arteries in the legs.
Diagnostic Performance of ABI
- The diagnostic performance of ABI in detecting lower extremity arterial disease (LEAD) has been evaluated in several studies 2, 3, 4.
- One study found that ABI had a sensitivity of 72.3% and a specificity of 69.3% in detecting LEAD, using duplex ultrasonography as the reference standard 2.
- Another study found that the correlations between ABI and walking capacity and quality of life questionnaires were weak, suggesting that ABI should not be used as the sole basis for decision-making in clinical practice 3.
Clinical Utility of ABI
- ABI is a widely used non-invasive test for detecting peripheral arterial disease (PAD) and predicting cardiovascular risk 4, 5.
- However, the US Preventive Services Task Force (USPSTF) has concluded that the current evidence is insufficient to assess the balance of benefits and harms of screening for PAD with ABI in asymptomatic adults 5.
- A systematic review of the diagnostic accuracy of ABI in people presenting with leg pain suggestive of PAD found that the evidence is sparse and that well-designed cross-sectional studies are required to evaluate the accuracy of ABI in this patient group 6.