Is there a role for Ankle-Brachial Index (ABI) in diagnosing peripheral arterial disease (PAD) in patients with normal Computed Tomography Angiography (CTA) of the lower extremities?

ABI Testing After Normal CTA: Limited Role

Yes, there is a role for ABI testing even when CTA of the lower extremities is normal, particularly in symptomatic patients with exertional leg symptoms, as exercise treadmill ABI testing can unmask PAD that is not apparent on anatomic imaging or at rest. 1

Diagnostic Hierarchy and Limitations

CTA as Anatomic Assessment

• CTA is reserved for highly symptomatic patients in whom revascularization is being considered, not as a first-line diagnostic tool 1
• Anatomic imaging (CTA, MRA, duplex ultrasound) shows vessel stenosis but does not assess hemodynamic significance or functional impairment 1
• A normal CTA demonstrates no significant anatomic stenosis (typically <50%), but this does not exclude functionally significant PAD 1

ABI Provides Complementary Physiological Data

• ABI is the initial diagnostic test for PAD and measures hemodynamic significance, which is fundamentally different from anatomic visualization 1
• The diagnostic accuracy of resting ABI shows sensitivity of 68-84% and specificity of 84-99% when using Doppler method 2
• However, ABI may be normal in nearly one-quarter of patients with chronic limb-threatening ischemia, making it unreliable as the sole assessment tool 3

Specific Clinical Scenarios Requiring ABI Despite Normal CTA

Exercise Treadmill ABI Testing (Class I Recommendation)

• Patients with exertional non-joint-related leg symptoms and normal or borderline resting ABI (>0.90 and ≤1.40) should undergo exercise treadmill ABI testing to evaluate for PAD 1
• Exercise ABI is critical for differentiating arterial claudication from pseudoclaudication when resting studies are normal 1
• This test objectively measures functional limitations and can diagnose PAD missed by both resting ABI and anatomic imaging 1

Noncompressible Vessels

• When ABI is >1.40 (noncompressible arteries), toe-brachial index (TBI) should be measured (Class I recommendation) 1
• This occurs commonly in patients with long-standing diabetes or advanced age 1
• CTA cannot assess vessel compressibility, making ABI/TBI essential for these patients 1

Wound Assessment and Critical Limb Ischemia

• In patients with normal (1.00-1.40) or borderline (0.91-0.99) ABI in the setting of nonhealing wounds or gangrene, it is reasonable to diagnose CLI using TBI with waveforms, TcPO2, or SPP (Class IIa recommendation) 1
• The concordance between ABI and toe pressure/TBI among patients with CLI is poor, with only 58% of patients meeting criteria for abnormal toe pressures presenting with abnormal ABIs 3
• Perfusion assessment measures such as TcPO2 >30 mm Hg or SPP >40 mm Hg can predict wound healing potential 3

Practical Algorithm

When CTA is Normal but Symptoms Persist:

1. Perform resting ABI bilaterally 1

   • If ABI ≤0.90: PAD confirmed despite normal CTA (possible microvascular disease or functional stenosis not visible anatomically)
   • If ABI 0.91-0.99: Borderline, proceed to exercise testing
   • If ABI 1.00-1.40: Normal, proceed to exercise testing if symptomatic
   • If ABI >1.40: Noncompressible vessels, measure TBI 1

2. Exercise treadmill ABI testing for symptomatic patients with normal/borderline resting ABI 1

   • A decrease in post-exercise ABI of >20% or absolute decrease >0.15 suggests PAD 1
   • Provides objective functional assessment that CTA cannot offer 1

3. Consider alternative diagnoses if all testing remains normal 1

   • Neurogenic claudication (spinal stenosis)
   • Venous claudication
   • Musculoskeletal disorders
   • Chronic compartment syndrome 1

Critical Pitfalls to Avoid

• Do not assume normal CTA excludes PAD: CTA shows anatomy, not hemodynamics; functional stenosis may exist without significant anatomic narrowing 1
• Do not skip exercise ABI testing in symptomatic patients: Resting studies (both ABI and CTA) can miss exercise-induced ischemia 1
• Do not rely on ABI alone in diabetic or elderly patients: Use TBI when ABI >1.40 due to medial arterial calcification 1
• False-negative rates are significant: Studies show ABI sensitivity as low as 65-72% in some populations, with false-negative rates of 27.5% 4, 5

Evidence Quality Note

The 2016 AHA/ACC guidelines provide the most current and comprehensive recommendations (Class I, Level B-NR evidence) 1, superseding the 2011 focused update 1 and 2005 guidelines 1. These consistently emphasize that ABI testing serves a distinct physiological role that anatomic imaging cannot replace, particularly for exercise testing in symptomatic patients with normal resting studies.