In a diabetic patient with a 2‑cm ulcer on the first metatarsal, big toe swelling and discharge for 4 days, and palpable distal pulses, what is the most appropriate initial investigation to assess vascular status?

Initial Assessment of Diabetic Foot Ulcer with Intact Distal Pulses

In this diabetic patient with a 2-cm ulcer, swelling, discharge, and palpable distal pulses, the most appropriate initial assessment is plain radiographs (weight-bearing films in two views) to evaluate for osteomyelitis and bone architecture, followed immediately by bedside vascular assessment using handheld Doppler to measure ankle-brachial index (ABI), toe-brachial index (TBI), and arterial waveforms—not advanced vascular imaging such as duplex ultrasound, CT angiography, or MRA, which are reserved for specific indications after initial bedside testing.

Why Plain Radiographs Come First

• Plain radiographs are the essential first imaging study for any diabetic foot ulcer with suspected infection, detecting osteomyelitis in 50-70% of cases, identifying soft tissue gas, revealing foreign bodies, and demonstrating bone architecture changes including Charcot foot deformities 1
• Weight-bearing films in two views minimum should be obtained immediately for diabetic foot ulcers with suspected infection 1
• The clinical presentation of 4 days of swelling and discharge strongly suggests infection, making radiographs the priority before any vascular imaging 2

Critical Bedside Vascular Assessment (Not Advanced Imaging)

Palpable pulses do not exclude peripheral artery disease (PAD)—formal objective testing is mandatory:

• Up to 50% of diabetic foot ulcers have concomitant PAD, which dramatically increases amputation risk, even when pulses are palpable 1
• Even skilled examiners can detect pulses despite significant ischemia; pulse palpation alone has poor negative and positive likelihood ratios (0.75,1.38) for detecting PAD 3, 4
• Never assume adequate perfusion based solely on palpable pulses in diabetic patients—formal objective testing is required 1

Specific Bedside Tests to Perform

1. Handheld Doppler evaluation of flow signals from both dorsalis pedis and posterior tibial arteries to assess waveform quality 1

   • Triphasic waveforms largely exclude PAD 4
   • Monophasic or absent waveforms suggest significant disease 5

2. Ankle-Brachial Index (ABI) measurement 1

   • ABI <0.9 confirms PAD 1
   • ABI <0.6 indicates significant ischemia affecting wound healing 1
   • Critical pitfall: ABI ≥1.3 indicates falsely elevated readings from medial arterial calcification (Mönckeberg sclerosis), particularly common in diabetic patients, rendering the test unreliable 4, 1

3. Toe-Brachial Index (TBI) if ABI is >1.3 or unreliable 1

   • TBI <0.75 confirms PAD 4
   • Digital arteries are rarely affected by medial calcification, making TBI more reliable than ABI in diabetic patients 6, 3

When Advanced Vascular Imaging Is Actually Indicated

Duplex ultrasound, MRA, CT angiography, or conventional angiography are NOT first-line diagnostic tools for initial assessment of diabetic foot infection 1. These modalities are reserved for specific indications:

Urgent Imaging Required If:

• Toe pressure <30 mmHg 1
• TcPO₂ <25 mmHg 1
• Ankle pressure <50 mmHg 1
• ABI <0.5 1
• Non-healing ulcer despite optimal wound care with absent or monophasic Doppler waveforms 5

Purpose of Advanced Imaging:

• To obtain anatomical information when revascularization is being considered 2
• The entire lower extremity arterial circulation should be evaluated with detailed visualization of below-the-knee and pedal arteries 2

Additional Critical Assessment Components

• Probe-to-bone test to assess ulcer depth—a positive test makes osteomyelitis highly probable and may warrant MRI if plain films are negative 1
• Measure the extent of erythema surrounding the ulcer and check for systemic signs 1
• Obtain deep tissue cultures after debridement (not superficial swabs) 2

Common Pitfalls to Avoid

1. Never delay plain radiographs to pursue advanced vascular imaging first—X-rays provide essential baseline information about bone involvement and architecture 1
2. Never attribute poor wound healing to "diabetic microangiopathy" without excluding macrovascular PAD through proper testing 1
3. Never rely on ABI alone in diabetic patients—arterial calcification causes falsely elevated readings; always obtain TBI or waveform analysis if ABI is >1.3 5
4. Never assume PAD is absent based solely on palpable pulses—clinical examination sensitivity is too low to rule out PAD 5

Answer to the Multiple Choice Question

None of the options (A-D) represent the most appropriate initial assessment. The correct approach is:

1. Plain radiographs (not listed)
2. Bedside handheld Doppler assessment with ABI/TBI measurement (not listed)

If forced to choose from the given options and assuming radiographs have already been obtained, Option A (Duplex US) would be the answer ONLY if bedside vascular testing revealed concerning findings (ABI <0.5, ankle pressure <50 mmHg, toe pressure <30 mmHg, or monophasic waveforms) 2, 1. However, duplex ultrasound is not the initial assessment—bedside Doppler testing is 4, 1.