Is an Ankle-Brachial Index (ABI) an imaging test?

Ankle-Brachial Index (ABI) Is Not an Imaging Test

The Ankle-Brachial Index (ABI) is a noninvasive hemodynamic test, not an imaging test. It is a simple diagnostic measurement that compares blood pressure at the ankle with blood pressure in the arm to assess arterial blood flow to the lower extremities 1.

What the ABI Actually Is

The ABI is defined as the ratio between:

• The higher of the systolic pressures in the ankle arteries (dorsalis pedis or posterior tibial)
• The higher of the brachial artery systolic pressures 1

This test is performed using:

• A blood pressure cuff
• A Doppler ultrasound device to detect blood flow
• No imaging technology is involved in the basic test

How ABI Testing Is Performed

1. The patient rests in a supine position for 5-10 minutes
2. Systolic blood pressures are measured in a specific sequence:
   • First arm
   • First posterior tibial artery
   • First dorsalis pedis artery
   • Other posterior tibial artery
   • Other dorsalis pedis artery
   • Second arm 1, 2
3. The ratio is calculated using the appropriate pressures

Interpretation of ABI Results

ABI results are categorized as:

• Normal: 1.00-1.40
• Borderline: 0.91-0.99
• Abnormal (indicating PAD): ≤0.90
• Noncompressible vessels: >1.40 1, 2

Distinguishing ABI from Actual Imaging Tests

True vascular imaging tests that are distinct from ABI include:

• Duplex ultrasound
• Computed tomography angiography (CTA)
• Magnetic resonance angiography (MRA)
• Catheter-based angiography 1

These imaging tests are recommended only when anatomical assessment is needed, typically when revascularization is being considered for symptomatic PAD 1.

Clinical Applications of ABI

The ABI is used as:

• First-line noninvasive test for diagnosis of peripheral artery disease (PAD) 1
• A screening tool in high-risk populations 1
• A predictor of cardiovascular events and mortality 1
• A monitoring tool to assess disease progression 3

Special Considerations

• In patients with noncompressible vessels (ABI >1.40), which often occurs in diabetes and chronic kidney disease, alternative tests such as toe-brachial index (TBI) should be used 1
• Exercise ABI testing may be needed when resting ABI is normal but symptoms suggest PAD 1

Common Pitfalls in ABI Testing

• Incorrect patient positioning (not supine for adequate time)
• Using the wrong pressure values in the calculation
• Failing to recognize noncompressible vessels
• Not following standardized measurement protocols 2
• Variability in reliability between different practitioners 3

While the ABI test uses a Doppler device to detect blood flow, it does not produce images of the blood vessels and is therefore not classified as an imaging test but rather as a noninvasive hemodynamic assessment.