What is the difference between Ankle-Brachial Index (ABI) and arterial duplex ultrasound in diagnosing peripheral artery disease?

Difference Between ABI and Arterial Duplex Ultrasound

ABI is the initial diagnostic test for PAD that measures hemodynamic significance through blood pressure ratios, while arterial duplex ultrasound is an anatomic imaging study reserved for revascularization planning that visualizes vessel morphology and stenosis location. 1

ABI: First-Line Physiological Test

The resting ABI must be performed first in all patients with suspected PAD before any imaging studies. 1, 2

What ABI Measures

• Hemodynamic significance of arterial disease through systolic blood pressure ratios between ankle and arm 1
• Functional impairment from stenotic lesions, not just anatomic presence 3
• Calculated by dividing the higher dorsalis pedis or posterior tibial pressure by the higher brachial pressure 1

ABI Interpretation

• ≤0.90 = abnormal (confirms PAD diagnosis) 1
• 0.91-0.99 = borderline 1
• 1.00-1.40 = normal 1
• >1.40 = noncompressible arteries (requires TBI instead) 1

ABI Advantages

• Simple, noninvasive, low cost, widely available 2
• Can establish diagnosis and guide initial treatment without imaging 1
• Sensitivity 68-84%, specificity 84-99% for PAD detection 2
• Predicts cardiovascular morbidity and mortality risk 4

Arterial Duplex Ultrasound: Anatomic Imaging for Revascularization Planning

Duplex ultrasound should NOT be used as the initial diagnostic test—it is reserved for patients already diagnosed with PAD who are being considered for revascularization. 1, 2

What Duplex Measures

• Vessel morphology and anatomic stenosis location (aortoiliac, femoropopliteal, infrapopliteal) 1
• Blood flow velocity changes at specific stenotic areas 4
• Plaque characteristics and vessel wall assessment 5, 1
• Sensitivity 88%, specificity 95% for detecting >50% stenosis 1

Duplex Advantages Over Other Imaging

• No radiation exposure, no contrast required 1
• Dynamic real-time assessment 1
• Can detect patent distal vessels sometimes missed on angiography 4
• Superior to CTA for infrapopliteal vessel visualization 6

Duplex Limitations

• Highly operator-dependent requiring dedicated trained personnel 1
• Time-intensive and more expensive than ABI 4
• Cannot assess hemodynamic significance alone—must be combined with ABI/TBI 1

Critical Clinical Algorithm

Step 1: Perform ABI First

• Measure resting ABI bilaterally in all patients with suspected PAD 1, 2
• If ABI >1.40, measure toe-brachial index instead 1
• If resting ABI normal but symptoms persist, perform exercise treadmill ABI 1, 3

Step 2: Reserve Duplex for Specific Indications

• Functionally limiting claudication with inadequate response to guideline-directed medical therapy where revascularization is considered 1
• Chronic limb-threatening ischemia requiring revascularization strategy 1
• Inconclusive ABI with persistent clinical suspicion (Class 2b indication) 1

Step 3: Never Perform Duplex Without Clear Purpose

• Do NOT obtain duplex solely for anatomic assessment in confirmed PAD patients not being considered for revascularization (Class 3: Harm recommendation) 1

Common Pitfalls to Avoid

Don't Skip ABI and Go Straight to Imaging

• Duplex does not replace ABI for diagnosis 2, 4
• Anatomic imaging shows stenosis but not hemodynamic significance 3
• A normal duplex does not exclude functionally significant PAD 3

Don't Rely on ABI Alone in High-Risk Populations

• Diabetic and elderly patients may have falsely elevated ABI >1.40 from medial arterial calcification 1, 3, 2
• Measure TBI when ABI >1.40 (Class I recommendation) 1
• ABI may be normal in nearly 25% of patients with chronic limb-threatening ischemia 3

Don't Order Duplex for Asymptomatic Screening

• Duplex is not indicated for PAD screening in asymptomatic patients 1
• Reserve anatomic imaging for revascularization planning only 1, 2

Summary of Key Differences

Feature	ABI	Arterial Duplex
Purpose	Diagnose PAD hemodynamically [1]	Localize stenosis anatomically [1]
When to use	First-line for all suspected PAD [1,2]	After diagnosis, for revascularization planning [1]
What it shows	Functional significance [3]	Vessel morphology and stenosis location [4]
Cost/Time	Low cost, quick [4]	More expensive, time-intensive [4]
Operator dependence	Minimal [1]	Highly dependent [1]