In a diabetic patient with a well‑demarcated plantar foot ulcer, nearly absent sensation, intact peripheral pulses, and a healing ulcer with central erythema, what is the most critical test to perform?

Monofilament Test to Confirm Neuropathic Ulcer

The monofilament test (Option B) is the most critical diagnostic test in this clinical scenario because it objectively confirms the presence and severity of peripheral neuropathy, which is the primary pathophysiologic mechanism underlying this patient's plantar foot ulcer. 1

Clinical Reasoning

This patient presents with the classic triad of a neuropathic diabetic foot ulcer:

• Well-demarcated plantar location (pressure point distribution typical of neuropathic ulcers) 1
• Nearly diminished sensation (indicating severe peripheral neuropathy) 1
• Intact peripheral pulses (excluding significant peripheral artery disease as the primary etiology) 2, 1

The monofilament test using a 10-gram (5.07 Semmes-Weinstein) filament is the standardized bedside assessment that quantifies protective sensation loss and confirms the neuropathic etiology. 1

Why Other Options Are Less Critical

ABI (Option A) – Not the Priority Here

• While vascular assessment is important in diabetic foot ulcers, intact peripheral pulses make significant PAD unlikely as the primary cause. 2, 1
• The International Working Group on the Diabetic Foot states that detection of a triphasic pedal Doppler arterial waveform with intact pulses provides strong evidence for the absence of PAD. 2
• ABI would be the critical test if pulses were absent or diminished, but this patient has intact pulses. 1
• Up to 50% of diabetic foot ulcers have coexisting PAD, but the intact pulses and plantar location strongly suggest a neuropathic rather than ischemic etiology. 1, 3

CT Scan (Option C) – Premature Without Clinical Indication

• CT angiography is reserved for pre-revascularization planning in patients with confirmed PAD and is not a first-line diagnostic test. 4
• Plain radiographs would be the appropriate imaging if osteomyelitis were suspected, not CT. 3
• The clinical presentation (healing ulcer with central erythema, no mention of infection) does not warrant advanced imaging at this stage. 3

Assessment for Pressure Points (Option D) – Management, Not Diagnosis

• While identifying pressure points is essential for offloading and preventing recurrence, it does not establish the diagnosis. 1
• Pressure point assessment is part of the therapeutic plan after neuropathy is confirmed, not the diagnostic test itself. 1

Diagnostic Algorithm for This Patient

1. Perform monofilament testing at multiple plantar sites to confirm loss of protective sensation and document neuropathy severity. 1

2. Obtain handheld Doppler evaluation of dorsalis pedis and posterior tibial arteries to assess waveform quality (triphasic waveforms exclude significant PAD). 2, 1

3. Measure ABI if any concern about perfusion arises (monophasic waveforms, delayed healing despite optimal care), but this is secondary given intact pulses. 2, 1

4. Plain radiographs in two weight-bearing views if any signs of infection develop (increased erythema, purulence, systemic symptoms) to screen for osteomyelitis. 3

Critical Pitfalls to Avoid

• Never assume adequate perfusion based solely on palpable pulses without objective testing if the ulcer fails to heal within 6 weeks despite optimal management. 2, 1
• Never attribute poor wound healing to "diabetic microangiopathy" without excluding macrovascular PAD through proper vascular assessment. 2, 1
• Never delay monofilament testing in a diabetic patient with a plantar ulcer, as confirming neuropathy is essential for appropriate offloading and prevention strategies. 1