What is the most important diagnostic test for a diabetic patient with a plantar foot ulcer and nearly diminished sensation?

Most Important Diagnostic Test: Ankle-Brachial Index (ABI)

The most important diagnostic test for this diabetic patient with a plantar foot ulcer and diminished sensation is the ankle-brachial index (ABI) to assess for peripheral artery disease (PAD), as up to 50% of diabetic foot ulcers have coexisting PAD, which dramatically increases amputation risk and mortality. 1, 2

Clinical Reasoning

Why Vascular Assessment Takes Priority

• Formal vascular assessment is mandatory for all diabetic foot ulcers because PAD is present in approximately 50% of cases and fundamentally changes prognosis—patients with neuro-ischemic ulcers face significantly increased risk of amputation and death. 1, 2, 3

• The presence of neuropathy (nearly diminished sensation) does not exclude PAD; in fact, diabetic patients with neuropathy often lack typical PAD symptoms like claudication or rest pain even with severe tissue loss, making objective testing essential rather than optional. 2, 4

• Never assume adequate perfusion based solely on clinical examination or palpable pulses—even skilled examiners can detect pulses despite significant ischemia, and clinical examination sensitivity is too low to rule out PAD. 2, 3

The ABI as First-Line Vascular Test

• ABI should be performed immediately with bilateral ankle systolic pressures; values <0.9 indicate PAD, while values <0.5 or ankle pressure <50 mmHg require urgent vascular imaging and revascularization to prevent amputation. 1, 3

• If ABI is ≥1.3 (indicating noncompressible vessels from arterial calcification), proceed to toe-brachial index (TBI) measurement, as TBI <0.75 suggests significant PAD and digital arteries are rarely affected by medial calcification. 1, 2

• Handheld Doppler evaluation of pedal arterial waveforms should accompany ABI measurement—triphasic waveforms provide strong evidence for absence of PAD. 1, 3

Why Other Options Are Secondary

• Monofilament testing (Option B) confirms loss of protective sensation but does not address the critical question of whether this ulcer will heal—the patient already has "nearly diminished sensation," making this diagnosis clinically apparent. 4

• Assessment for pressure points (Option D) becomes relevant only after confirming adequate perfusion and as part of offloading strategy with therapeutic footwear. 4

• CT scan (Option C) is not indicated for initial assessment unless evaluating for osteomyelitis after plain radiographs, which should be obtained but are not the "most important" test given the question context. 3

Critical Prognostic Thresholds

• Toe pressure <30 mmHg or TcPO₂ <25 mmHg warrants urgent vascular imaging and revascularization (strong recommendation). 1, 3

• Skin perfusion pressure ≥40 mmHg, toe pressure ≥30 mmHg, or TcPO₂ ≥25 mmHg each increase the pre-test probability of ulcer healing by at least 25%. 1

• If the ulcer does not improve within 6 weeks despite optimal management, vascular imaging and revascularization should be considered regardless of bedside test results. 1

Common Pitfalls to Avoid

• Do not rely on ABI alone in diabetic patients—arterial calcification causes falsely elevated readings; always obtain TBI or waveform analysis if ABI is ≥1.3. 1, 2

• Do not attribute poor wound healing to "diabetic microangiopathy" without excluding macrovascular PAD through proper testing—macrovascular disease is typically the cause. 1, 2, 3

• Do not delay vascular assessment—objective testing is mandatory in all diabetic foot ulcers, as clinical examination alone cannot rule out PAD. 2, 3