What testing should be ordered if an Ankle-Brachial Index (ABI) impression states it is unable to be computed?

Testing Options When ABI Cannot Be Computed

When an Ankle-Brachial Index (ABI) impression states it is unable to be computed, a toe-brachial index (TBI) should be measured as the first alternative diagnostic test. 1

Understanding Why ABI May Be Uncomputable

• The most common reason for an uncomputable ABI is noncompressible arteries (ABI >1.40), typically due to medial arterial calcification, particularly in patients with diabetes mellitus or advanced chronic kidney disease 1
• Other reasons may include severe pain preventing cuff inflation, extensive wounds, or recent surgery in the ankle area 2

Recommended Alternative Testing Algorithm

First-Line Alternative Tests:

1. Toe-Brachial Index (TBI):

   • TBI is the preferred first alternative when ABI cannot be computed 1
   • TBI is specifically recommended for patients with suspected PAD when the ABI is >1.40 1, 2
   • Digital arteries are usually less affected by calcification, making TBI more reliable in patients with noncompressible vessels 3
   • A TBI <0.70 is considered abnormal and diagnostic of PAD 1, 4

2. Doppler Waveform Analysis:

   • Analysis of arterial waveforms can be performed even when pressure measurements are unreliable 1, 5
   • Absence of triphasic waveforms may indicate arterial disease even when ABI cannot be computed 5
   • This method can detect PAD even when ABI values are falsely normal due to arterial calcification 2, 5

3. Pulse Volume Recording (PVR):

   • PVR is useful in patients with noncompressible vessels as it relies on limb volume change rather than the pressure required to impede flow 2
   • Can provide functional assessment of arterial flow when pressure measurements are unreliable 1

Second-Line Tests for Perfusion Assessment:

• Transcutaneous Oxygen Pressure (TcPO2):

  • Useful for evaluating local perfusion in patients with noncompressible arteries 1
  • TcPO2 >30 mm Hg has been used to predict ulcer healing 1
  • Particularly valuable in patients with nonhealing wounds or gangrene 1

• Skin Perfusion Pressure (SPP):

  • Can evaluate local perfusion when ABI is uncomputable 1
  • SPP ≥30 to 50 mm Hg is associated with increased likelihood of wound healing 1
  • Can be used in angiosome-targeted assessment for revascularization 1

When to Consider Anatomic Imaging

If the patient is symptomatic and revascularization is being considered, anatomic imaging should be performed after physiological testing confirms PAD:

• Duplex Ultrasound:

  • First-line imaging method to confirm PAD lesions 1
  • Provides information on arterial anatomy and hemodynamics 1
  • 85-90% sensitivity and >95% specificity for detecting stenosis >50% 1

• Computed Tomography Angiography (CTA) or Magnetic Resonance Angiography (MRA):

  • Useful for anatomic characterization of PAD lesions and guidance for revascularization strategy 1
  • Should not be performed for anatomic assessment in asymptomatic patients 1

• Invasive Angiography:

  • Reserved for patients with critical limb ischemia in whom revascularization is considered 1
  • Reasonable for patients with lifestyle-limiting claudication with inadequate response to medical therapy 1

Clinical Pitfalls to Avoid

• Do not rely solely on clinical examination to rule out PAD when ABI cannot be computed, as sensitivity of pulse palpation is poor 1, 6
• Do not assume normal perfusion in a patient with diabetes or renal disease when ABI cannot be computed, as these populations have high rates of falsely normal or uncomputable ABIs 6
• Avoid proceeding directly to invasive imaging without first attempting alternative physiological tests like TBI 1
• Remember that of symptomatic patients with PAD with ≥50% stenosis on duplex ultrasound, 43% may have normal/inconclusive resting ABIs (49% in diabetics and 57% in chronic kidney disease) 6