What are the possible reasons for a normal arterial duplex scan in a patient with known peripheral arterial occlusive disease?

Reasons for Normal Arterial Duplex Scan in Known PAOD Patient

A normal arterial duplex scan in a patient with known peripheral arterial occlusive disease most commonly occurs due to technical limitations of the examination, including operator dependence, incomplete vessel visualization (particularly in distal/infrapopliteal vessels), or disease localization outside the scanned arterial segments.

Technical and Operator-Dependent Limitations

Duplex ultrasound is inherently technologist-dependent and requires dedicated, trained technical and medical personnel to perform and interpret accurately 1. The examination demands dedicated time to image all arteries in the lower limbs, and inadequate technique can miss significant lesions.

Key technical pitfalls include:

• Incomplete arterial survey: The examination may not have extended to all relevant arterial segments where disease exists
• Operator skill variability: Studies show sensitivity ranges from 77-86% for hemodynamically significant lesions, meaning 14-23% of significant stenoses can be missed 2, 3
• Poor acoustic windows: Calcified vessels, obesity, or edema can obscure visualization
• Inadequate Doppler angle correction: Leading to velocity measurement errors

Disease Location Beyond Scan Coverage

The most critical reason for false-negative results relates to anatomic coverage limitations:

• Infragenicular/tibial vessel disease: Duplex scanning has notably lower accuracy for infragenicular arteries with sensitivity of only 83% and specificity of 84% 2
• Pedal vessel involvement: Small distal vessels are often inadequately visualized by standard duplex protocols
• Aortoiliac disease: If the scan focused only on femoropopliteal segments and missed proximal disease
• Collateral-dependent circulation: Well-developed collaterals may normalize distal flow patterns despite proximal occlusive disease

Hemodynamic Considerations

A duplex scan may appear "normal" when:

• Non-flow-limiting stenoses (< 50% diameter reduction) are present but not causing hemodynamic compromise detectable by velocity criteria
• Multilevel disease where compensatory mechanisms maintain relatively normal flow patterns in individual segments
• Microvascular disease (particularly in diabetics) causing symptoms but no large-vessel stenosis

Clinical Mismatch Scenarios

When duplex appears normal but PAOD is clinically evident:

1. Proceed to alternative imaging: CTA or MRA provide superior spatial resolution and are less operator-dependent 1
2. Verify physiologic testing: Ensure ABI, toe pressures, TBI, or TcPO2 measurements confirm hemodynamic impairment 1
3. Consider functional assessment: Exercise ABI may unmask disease not apparent at rest
4. Evaluate for non-atherosclerotic causes: Popliteal entrapment, cystic adventitial disease, or vasculitis

Common Pitfalls to Avoid

• Do not dismiss clinical suspicion based solely on a normal duplex when physiologic tests (ABI, toe pressures) indicate disease
• Ensure complete arterial mapping from aortoiliac to tibial vessels was performed
• Recognize that duplex has limited positive predictive value (as low as 19% in screening populations) 4
• Request repeat imaging with different modality (CTA/MRA) when clinical-imaging discordance exists 1

The 2024 ACC/AHA guidelines explicitly state that when clinical suspicion persists with inconclusive duplex results, CTA or MRA should be considered to establish the diagnosis 1. These cross-sectional modalities provide greater anatomic detail and are less personnel-dependent, though they carry risks of contrast exposure or radiation.