Initial Diagnostic Approach for Peripheral Arterial Disease
The ankle-brachial index (ABI) is the required initial diagnostic test for suspected peripheral arterial disease in older adults with risk factors. 1
Who Should Be Tested
Perform resting ABI testing in patients meeting any of these criteria:
- Age ≥65 years 2, 1
- Age 50-64 years with atherosclerotic risk factors (smoking, diabetes, hypertension, hyperlipidemia) 2, 1
- Age <50 years with diabetes plus one additional risk factor 1
- Exertional leg symptoms or nonhealing wounds at any age 2, 1
The presence of three or more risk factors confers a 10-fold increase in PAD risk, making screening particularly important in this population. 3
The ABI Test: Measurement and Interpretation
Measurement Technique
- Obtain systolic blood pressures at both brachial arteries and both ankle arteries (dorsalis pedis and posterior tibial) using a Doppler device 1
- Divide the higher ankle pressure by the higher arm pressure for each leg 1
- Measure blood pressure in both arms initially, as an inter-arm difference >15-20 mmHg suggests subclavian stenosis 1
Diagnostic Thresholds
- ABI ≤0.90: Confirms PAD diagnosis 2, 1
- ABI 0.91-0.99: Borderline 1
- ABI 1.00-1.40: Normal 1
- ABI >1.40: Noncompressible vessels (typically in long-standing diabetes or advanced age) 2, 1
The ABI demonstrates 68-84% sensitivity and 84-99% specificity for angiographic PAD, far superior to history and physical examination alone. 1
When ABI Is Unreliable or Borderline
For Noncompressible Vessels (ABI >1.40)
Use toe-brachial index to establish diagnosis in patients with suspected PAD but noncompressible vessels, commonly seen in long-standing diabetes or advanced age. 2
For Normal Resting ABI with Persistent Suspicion
Perform exercise ABI testing when symptoms suggest PAD but resting ABI is normal (0.91-1.40). 2, 1 This differentiates arterial claudication from pseudoclaudication and provides objective evidence of functional limitation. 2
Role of Ultrasound in PAD Diagnosis
Duplex ultrasound is NOT the initial diagnostic test but serves as a secondary anatomic imaging modality after ABI confirms the diagnosis. 2, 4
When to Use Duplex Ultrasound
- To diagnose anatomic location and degree of stenosis after PAD is confirmed by ABI 2
- To select candidates for endovascular intervention or surgical bypass 2
- For surveillance after revascularization, particularly for venous bypass grafts at 3,6, and 12 months, then yearly 2
Duplex ultrasonography provides information on vessel stenosis/occlusion, vessel wall characteristics, and plaque morphology, but should be reserved for patients requiring anatomic detail for treatment planning. 4
Complete Initial Vascular Assessment
Beyond ABI testing, the initial evaluation should include:
- Pulse palpation: Femoral, popliteal, dorsalis pedis, and posterior tibial pulses 1
- Auscultation: Femoral and carotid bruits 1, 5
- Inspection: Leg and foot appearance for nonhealing wounds, hair loss, or trophic changes 1, 5
- Symptom assessment: Walking impairment, claudication distance, rest pain, or nonhealing wounds 1
Common Pitfalls to Avoid
- Do not use duplex ultrasound as the first-line screening test—ABI is faster, cheaper, and adequate for initial diagnosis 2, 1
- Do not rely on symptoms alone—up to 40-50% of PAD patients are asymptomatic 6, 3
- Do not skip ABI testing in elderly patients without symptoms—PAD indicates systemic atherosclerosis with significantly increased cardiovascular risk even when asymptomatic 7, 6
- Remember to check for noncompressible vessels in diabetic patients, as falsely elevated ABI can miss significant disease 2
When to Proceed to Advanced Imaging
Reserve angiographic imaging (CTA, MRA, or catheter angiography) for determining the optimal revascularization approach in patients requiring intervention, not for initial diagnosis. 2, 4