Ankle-Brachial Index Testing for Peripheral Artery Disease
The ABI is the first-line noninvasive diagnostic test for PAD, with an ABI ≤0.90 confirming the diagnosis and demonstrating robust diagnostic performance (sensitivity 68-84%, specificity 84-99%). 1, 2
When to Perform ABI Testing
Symptomatic Patients (Always Test)
- Perform ABI in any patient with exertional leg symptoms, claudication, walking impairment, ischemic rest pain, nonhealing wounds, absent pulses, or femoral bruits 1
- These symptomatic patients require testing regardless of age or risk factors 1
Asymptomatic Screening (Consider Testing)
- Screen patients age ≥65 years 1
- Screen patients age 50-64 years with atherosclerotic risk factors (smoking, hypertension, hyperlipidemia) or family history of PAD 1
- Screen patients age <50 years with diabetes plus one additional atherosclerotic risk factor 1
- Screen patients with known atherosclerotic disease in another vascular bed (coronary, carotid, renal arteries) 1
Proper ABI Measurement Technique
Patient Preparation and Sequence
- Position patient supine for at least 5 minutes before measurement 1, 3
- Follow this exact sequence: first arm systolic BP → first posterior tibial artery → first dorsalis pedis artery → other posterior tibial artery → other dorsalis pedis artery → other arm 1, 3
- If the first arm systolic BP exceeds the other arm by >10 mmHg, repeat the first arm measurement and disregard the initial reading 3
Calculation Method
- Divide the highest ankle pressure (either posterior tibial OR dorsalis pedis) by the highest brachial pressure 1, 3
- Use the lower ABI of both legs for clinical decision-making, as PAD may be unilateral or asymmetric 3
Interpreting ABI Results
Diagnostic Thresholds
- ABI ≤0.90: Abnormal—confirms PAD diagnosis 1, 3, 2
- ABI 0.91-0.99: Borderline—warrants further cardiovascular risk evaluation 1, 3, 2
- ABI 1.00-1.40: Normal range 3
- ABI >1.40: Non-compressible vessels—requires alternative testing with toe-brachial index or imaging 3, 2
Diagnostic Performance
- The Doppler-based ABI demonstrates areas under the ROC curve of 0.87-0.95 for detecting significant stenosis 1, 2
- Specificity is consistently high (84-99%), making a positive test highly reliable for confirming PAD 1, 2, 4
- Sensitivity is more variable (68-84%), particularly lower in elderly patients and those with diabetes 4
Critical Pitfalls and When Additional Testing is Needed
Common Measurement Errors
- Using only one ankle artery (either PT or DP alone) will miss significant disease—always measure both 3
- Using the higher ABI between legs rather than the lower will underdiagnose PAD 3
- Inadequate rest period before measurement affects accuracy 3
- Lack of proper training reduces reproducibility, especially near the 0.90 threshold 3, 2
When Resting ABI is Insufficient
- Normal ABI with exertional symptoms: Perform exercise treadmill ABI testing—31% of symptomatic patients with normal resting ABI will have abnormal post-exercise ABI 2, 5
- Non-compressible vessels (ABI >1.40): Measure toe-brachial index or obtain pulse volume recordings 3, 2
- Borderline values (0.91-0.99) with symptoms: Consider exercise testing or Doppler waveform analysis 2
Role of Advanced Imaging
- Anatomic imaging (duplex ultrasound, CTA, MRA, invasive angiography) is not required for initial PAD diagnosis 1
- Reserve duplex ultrasound for symptomatic patients being considered for revascularization to confirm lesion location and guide intervention strategy 1, 2
- Duplex ultrasound serves as the first-line imaging method after positive ABI screening when revascularization is contemplated 1, 2
Clinical Significance Beyond PAD Diagnosis
- An ABI <0.90 or >1.40 indicates 2-4 fold increased risk for cardiovascular events and all-cause mortality, independent of PAD symptoms 3, 6
- ABI provides incremental cardiovascular risk stratification beyond standard risk scores 3
- This makes ABI valuable not just for PAD diagnosis but for global cardiovascular risk assessment 3