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Difficulty: MediumDiagnostic Workup and Test Sequencing

During a clinic visit, a physician evaluates a 68-year-old man who complains of severe right foot pain that frequently awakens him from sleep. The patient notes that hanging his foot over the edge of the mattress partially alleviates the discomfort. His medical history is significant for long-standing type 2 diabetes mellitus complicated by peripheral neuropathy, hypertension, and stage 3 chronic kidney disease. Vital signs are within normal limits. Physical examination reveals a 1.5-cm dry, blackened eschar on the distal tip of the right great toe with minimal surrounding erythema. The right foot feels cool to the touch, and both the dorsalis pedis and posterior tibial pulses are nonpalpable. The physician performs an in-office ankle-brachial index (ABI) measurement to evaluate for peripheral arterial disease. The right ABI is calculated to be 1.45. Which of the following is the most appropriate next step in the diagnostic workup?

  1. Measurement of toe-brachial indexAnswer
  2. B
    Computed tomography angiography of the lower extremities
  3. C
    Arterial duplex ultrasonography of the lower extremities
  4. D
    Invasive peripheral catheter angiography
  5. E
    Exercise treadmill testing with repeat ankle-brachial index

Answer

Measurement of toe-brachial index is the required next step when the ABI is non-diagnostic (>1.40) to establish the physiologic presence of peripheral arterial disease.
The correct sequence for evaluating peripheral arterial disease begins with an ankle-brachial index (ABI). Normal values are 0.90 to 1.30. A value >1.40 indicates medial artery calcification (Mönckeberg sclerosis), making the vessels noncompressible. This is a common finding in patients with diabetes mellitus and chronic kidney disease. When the ABI is uninterpretable in a patient with clear signs of critical limb ischemia (rest pain, tissue loss), the most appropriate next step is to obtain a toe-brachial index (TBI). The digital arteries are typically spared from calcification, allowing for accurate assessment of distal perfusion.

Step-by-Step Solution

1
Identify the clinical syndrome presented by the patient.
The patient has rest pain and a non-healing ulcer (eschar) on the great toe, which indicates critical limb ischemia.
Recognizing the severity of the disease establishes the urgent need for accurate perfusion assessment.
2
Interpret the results of the initial diagnostic test (ABI).
The calculated ABI is 1.45.
An ABI greater than 1.40 indicates severely calcified and noncompressible leg arteries, a common finding in long-standing diabetes or end-stage renal disease, rendering the ABI non-diagnostic.
3
Determine the appropriate sequential step according to vascular testing guidelines.
Measurement of the toe-brachial index (TBI) should be performed.
Digital arteries in the toes are rarely affected by medial calcification, allowing for an accurate measurement of perfusion pressures when large vessel compression fails.

Key Concept

In patients with suspected peripheral arterial disease whose initial ankle-brachial index (ABI) is abnormally high (>1.40) due to noncompressible calcified vessels, the next step in the diagnostic sequence is to measure the toe-brachial index (TBI).
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Diagnostic Workup and Test Sequencing

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