What is the most important diagnostic test for a diabetic patient with a well-demarcated plantar foot ulcer, nearly diminished sensation, and signs of healing with central erythema?

Most Important Diagnostic Test for Diabetic Plantar Foot Ulcer

The most important diagnostic test is probe-to-bone (PTB) testing to assess for osteomyelitis, followed immediately by plain radiography of the foot if PTB is positive or if clinical suspicion for bone involvement exists. 1

Primary Concern: Ruling Out Osteomyelitis

In a diabetic patient with diminished sensation and a plantar foot ulcer showing central erythema, the critical priority is determining whether osteomyelitis is present, as this fundamentally changes management and prognosis. 1

• Perform probe-to-bone testing at the bedside by inserting a sterile metal probe into the ulcer base; if bone is palpable, this has high sensitivity and specificity for underlying osteomyelitis and mandates further investigation. 1

• Order weight-bearing plain radiographs of the affected foot to evaluate for bone destruction, periosteal reaction, or architectural changes consistent with osteomyelitis or Charcot arthropathy. 1

• The presence of infection (indicated by erythema) combined with a deep ulcer in a neuropathic foot creates high risk for bone involvement, which occurs in approximately 20% of infected diabetic foot ulcers. 1

Secondary Priority: Vascular Assessment

After excluding or confirming osteomyelitis, vascular perfusion assessment is essential because inadequate blood flow prevents healing and increases amputation risk. 1

• Measure toe pressure or transcutaneous oxygen pressure (TcPO2) as the most reliable bedside tests for predicting healing potential in diabetic patients with neuropathy. 1

• Toe pressure ≥30 mmHg or TcPO2 ≥25 mmHg increases the probability of healing by at least 25%, while values below these thresholds require urgent vascular imaging and revascularization consideration. 1

• Ankle-brachial index (ABI) is unreliable in diabetic patients with neuropathy due to medial arterial calcification (Mönckeberg sclerosis) causing falsely elevated readings (≥1.3), making toe pressure or TcPO2 the preferred tests. 1, 2

• Pedal Doppler waveform analysis showing triphasic flow provides strong evidence against significant peripheral artery disease, while monophasic or absent waveforms suggest ischemia requiring further evaluation. 1, 2

Clinical Context and Pitfalls

The "central erythema with signs of healing" description raises concern for infection masquerading as healing tissue. 1

• Do not assume healing is progressing based on visual appearance alone—erythema extending beyond the wound margin (>2 cm) indicates infection, not healing. 1

• The "well-demarcated" appearance with surrounding erythema may represent cellulitis or deeper soft tissue infection requiring wound cultures and possible surgical debridement. 1

• Never delay objective vascular testing based on palpable pulses alone—even experienced clinicians can detect pulses despite significant ischemia, and up to 50% of diabetic foot ulcers have coexisting peripheral artery disease. 2

Algorithmic Approach

1. Immediate bedside probe-to-bone test → if positive, proceed to plain radiography. 1

2. Plain radiographs of the foot → evaluate for osteomyelitis, Charcot changes, or foreign bodies. 1

3. Toe pressure measurement or TcPO2 → if <30 mmHg or <25 mmHg respectively, urgent vascular imaging is required. 1

4. If ulcer fails to improve within 6 weeks despite optimal management, proceed to vascular imaging and revascularization regardless of initial bedside test results. 1