What is the initial diagnostic test for a patient suspected of having Peripheral Artery Disease (PAD) and how is it performed, particularly in relation to Ankle-Brachial Index (ABI) investigations?

Ankle-Brachial Index (ABI) Investigations for Peripheral Artery Disease

The resting ABI is the recommended initial diagnostic test for patients with suspected PAD, performed using a blood pressure cuff and Doppler device to measure the ratio of ankle to brachial systolic pressures. 1

How to Perform ABI Testing

Patient Preparation and Positioning

• The patient must be supine for at least 5 minutes before measurement to allow hemodynamic stabilization 2
• Measurements should be performed in a specific sequence: first arm systolic blood pressure, first posterior tibial artery, first dorsalis pedis artery, other posterior tibial artery, other dorsalis pedis artery, then other arm 2

Measurement Technique

• Use a blood pressure cuff and continuous-wave Doppler device to detect ankle vessels 1
• Measure systolic blood pressure in both brachial arteries 1
• Measure systolic blood pressure at both ankle arteries (posterior tibial and dorsalis pedis) in each leg 2
• Calculate ABI by dividing the highest ankle pressure (either posterior tibial or dorsalis pedis) by the highest brachial pressure 2
• The ABI should be reported separately for each leg 1

Interpretation of Results

Standard ABI Categories

The 2024 ACC/AHA guidelines establish clear diagnostic thresholds 1:

• Abnormal: ABI ≤0.90 (confirms PAD diagnosis)
• Borderline: ABI 0.91-0.99 (warrants further evaluation)
• Normal: ABI 1.00-1.40
• Noncompressible: ABI >1.40 (indicates calcified vessels)

Diagnostic Accuracy

• ABI demonstrates sensitivity of 68-84% and specificity of 84-99% when using the Doppler method 2, 3
• The test has areas under the ROC curve of 0.87-0.95 for detecting significant stenosis 2, 3

When Additional Testing is Required

For Normal or Borderline Resting ABI with Symptoms

• Patients with exertional leg symptoms and normal or borderline resting ABI (>0.90 and ≤1.40) should undergo exercise treadmill ABI testing 1
• A post-exercise ABI decrease of >20% serves as a PAD diagnostic criterion 1
• Exercise testing is particularly important as nearly half of symptomatic patients may have normal resting ABI 4
• Among patients with normal resting ABI who undergo exercise testing, 31% demonstrate abnormal post-exercise ABI 4

For Noncompressible Arteries (ABI >1.40)

• Toe pressure/toe-brachial index (TBI) with waveforms must be performed when resting ABI is >1.40 1
• This is particularly important in patients with diabetes or chronic kidney disease who commonly have arterial calcification 1
• Normal TBI is >0.70; abnormal is <0.70 1

For Suspected Critical Limb-Threatening Ischemia (CLTI)

• Additional testing beyond ABI is reasonable, including toe pressure/TBI with waveforms, transcutaneous oxygen pressure (TcPO2), and/or skin perfusion pressure (SPP) 1
• These measurements help assess arterial perfusion and establish CLTI diagnosis 1

Adjunctive Physiological Testing

Segmental Pressures and Pulse Volume Recordings

• In patients with chronic symptomatic PAD, segmental leg pressures with pulse volume recordings (PVR) and/or Doppler waveforms can be performed in addition to resting ABI to delineate the anatomic level of PAD 1
• PVR is particularly valuable when ABI shows noncompressible vessels, as 24% of patients with noncompressible vessels have abnormal PVR findings 4

When Anatomic Imaging is Indicated

Duplex Ultrasound as First-Line Imaging

• Duplex ultrasound is recommended as the first-line imaging method to confirm PAD lesions after ABI screening 1, 2
• Anatomic imaging studies (duplex ultrasound, CTA, MRA, invasive angiography) are not required for initial PAD diagnosis 2

Indications for Advanced Imaging

• Advanced imaging is reserved for symptomatic patients being considered for revascularization 2
• In symptomatic patients with functionally limiting symptoms inadequately responsive to guideline-directed medical therapy and structured exercise, proceed to duplex ultrasound, CTA, MRA, or catheter angiography to assess anatomy and determine revascularization strategy 1

Who Should Undergo ABI Testing

Patients with Suspected PAD (Class I Recommendation)

• ABI is recommended for patients with any history or physical examination findings suggestive of PAD, including 1, 2:
  • Exertional leg symptoms or claudication
  • Walking impairment
  • Ischemic rest pain
  • Nonhealing wounds
  • Absent pulses
  • Femoral bruits

At-Risk Asymptomatic Patients (Class IIa Recommendation)

• Screening with ABI is reasonable in asymptomatic patients at increased risk 1, 2:
  • Age ≥65 years
  • Age 50-64 years with atherosclerotic risk factors or family history of PAD
  • Age <50 years with diabetes plus one additional atherosclerotic risk factor
  • Known atherosclerotic disease in another vascular bed

Patients NOT Requiring Screening

• In patients not at increased risk and without suggestive history or physical examination findings, screening for PAD with ABI is not recommended 1

Common Pitfalls and Technical Considerations

Calculation Method Matters

• There is debate about whether to use the highest or lowest ankle pressure in the numerator 5, 6
• The ACC/AHA guidelines recommend using the highest ankle pressure (either posterior tibial or dorsalis pedis) 2
• However, research suggests using the lower ankle pressure may increase sensitivity from 68% to 89%, though at the cost of slightly reduced specificity 5

Operator Experience

• Proper training improves reproducibility of ABI measurements 3
• Automated oscillometric methods may be superior to manual Doppler methods when performed by inexperienced operators, with sensitivity of 97% versus 95% but specificity of 89% versus 56% 7

Limitations in Specific Populations

• ABI has multiple limitations in patients with diabetes and chronic kidney disease due to arterial calcification 1
• In these populations, proceed directly to TBI if ABI is >1.40 or noncompressible 1