Diagnostic Test for Peripheral Arterial Disease (PAD)
The ankle-brachial index (ABI) is the first-line noninvasive test that should be ordered to diagnose PAD when clinical suspicion exists based on symptoms or examination findings. 1, 2
How to Perform and Interpret the ABI
The ABI measures systolic blood pressure in the ankles compared to the arms using a Doppler device. An ABI <0.90 confirms the diagnosis of lower-extremity PAD with 75% sensitivity and 86% specificity 2, 3. This threshold is supported by Class I, Level A evidence from major cardiovascular societies 1, 2.
Measurement Sequence
Follow this specific order for limb pressure measurement 1:
- First arm
- First posterior tibial (PT) artery
- First dorsalis pedis (DP) artery
- Other PT artery
- Other DP artery
- Other arm
Use the lower ABI of the two legs for diagnosis, as PAD may be unilateral or asymmetric—using the higher value will miss patients with significant disease 1.
When the ABI is Inconclusive
Normal ABI (>0.90) with Persistent Clinical Suspicion
Order a post-exercise ABI or duplex ultrasound (DUS) 1, 2. A post-exercise ankle pressure decrease >30 mmHg or ABI decrease >20% confirms PAD 1.
High ABI (>1.40) Suggesting Medial Calcification
This occurs commonly in diabetes and chronic kidney disease. Order alternative tests: toe-brachial index (TBI), toe pressure, or Doppler waveform analysis 1, 2. The ABI becomes unreliable when vessels are incompressible from calcification.
Borderline ABI (0.91-1.00)
These patients warrant further evaluation as they represent intermediate cardiovascular risk 1. Consider post-exercise testing or DUS.
Additional Imaging When Revascularization is Considered
While ABI diagnoses PAD, duplex ultrasound, computed tomography angiography (CTA), or magnetic resonance angiography (MRA) are needed to map anatomy before revascularization 2. DUS has 85-90% sensitivity and >95% specificity for detecting stenosis >50% 2, but another imaging modality typically provides the roadmap for intervention.
Common Pitfalls to Avoid
- Don't rely on pulse palpation or auscultation alone—these have poor sensitivity 2
- Don't use ABI alone in diabetic patients—they frequently have falsely elevated values from medial calcification; proceed directly to TBI or alternative testing if ABI >1.40 1, 2
- Don't accept a normal resting ABI as definitive if symptoms strongly suggest PAD—exercise testing unmasks moderate stenoses 2
- Don't use the higher ABI of the two legs—this misses unilateral disease 1
The evidence consistently demonstrates that while newer biomarkers and advanced imaging exist 4, 5, the ABI remains the guideline-recommended, validated first-line diagnostic test that should be ordered when PAD is suspected 1, 2.