What is an Ankle-Brachial Index (ABI) ultrasound?

What is an ABI Ultrasound?

The ankle-brachial index (ABI) is a noninvasive diagnostic test that measures the ratio of systolic blood pressure at the ankle to the systolic blood pressure in the arm using a handheld Doppler ultrasound device, serving as the first-line test for diagnosing peripheral artery disease (PAD). 1

How the Test is Performed

The ABI measurement follows a standardized protocol to ensure accuracy:

• Patient positioning: The patient rests supine for 5-10 minutes in a constant room temperature environment before measurement begins 1

• Equipment used: A standard blood pressure cuff and a handheld Doppler ultrasound probe (not a full vascular ultrasound machine) are used to detect arterial flow 1

• Measurement sequence:

  • Systolic blood pressures are measured in both brachial arteries (upper arms) 1
  • Systolic pressures are then measured at both ankle arteries (dorsalis pedis and posterior tibial) in each leg 1
  • The cuff is placed just above the ankle, and the Doppler probe detects blood flow in the arteries 1

• Calculation method: The ABI for each leg is calculated by dividing the higher of the two ankle pressures (dorsalis pedis or posterior tibial) by the higher of the two brachial pressures 1

Interpreting ABI Results

The ABI value provides critical diagnostic and prognostic information:

• Normal ABI: 1.00-1.40 indicates no significant arterial obstruction 1

• Borderline ABI: 0.91-0.99 suggests possible early PAD and warrants further cardiovascular risk evaluation 1, 2

• Abnormal low ABI: ≤0.90 confirms PAD diagnosis with 68-89% sensitivity and 84-99% specificity 1

• Noncompressible arteries: ABI >1.40 indicates arterial calcification (often from diabetes or chronic kidney disease), not absence of disease, and these patients require alternative testing such as toe-brachial index 1, 2

Clinical Significance and Cardiovascular Risk

An abnormal ABI identifies patients at significantly increased risk for cardiovascular events and mortality, independent of symptoms or other risk factors:

• An ABI <0.90 is associated with a 2-4 fold increase in cardiovascular events and all-cause mortality 1, 3

• The relationship between ABI and cardiovascular outcomes follows a reverse J-shaped curve, with lowest risk at ABI 1.11-1.40 and increased risk at both low (<0.90) and high (>1.40) values 2

• ABI serves as a marker of systemic atherosclerosis, not just lower extremity disease 1

When Additional Testing is Needed

The resting ABI may not be sufficient in certain clinical scenarios:

• Exercise ABI testing: Recommended for patients with exertional leg symptoms but normal or borderline resting ABI (0.91-1.40), as moderate arterial stenoses may not be apparent at rest 1, 4

• Toe-brachial index (TBI): Required when ABI >1.40 due to noncompressible vessels, with TBI <0.70 confirming PAD 1, 2

• Segmental pressures and waveforms: Often performed alongside ABI to localize anatomic segments of disease (aortoiliac, femoropopliteal, infrapopliteal) 1

Common Clinical Pitfalls

• Never assume PAD is absent based solely on palpable pulses—even skilled examiners can detect pulses despite significant ischemia 4

• Never rely on ABI alone in diabetic patients—arterial calcification causes falsely elevated readings; always obtain TBI or waveform analysis in this population 1, 4

• Never use a single-leg measurement—always measure bilateral ABI and use the lower value for cardiovascular risk assessment 2, 4

• Never delay vascular assessment in patients with non-healing wounds—objective testing is mandatory, as clinical examination alone has insufficient sensitivity 4