Initial Investigation: Doppler Ultrasound with ABI Measurement
The initial investigation for this patient should be Doppler ultrasound with ankle-brachial index (ABI) measurement (Option D), as this is the mandatory first-line objective vascular assessment for any diabetic patient presenting with a foot ulcer, particularly when accompanied by unilateral leg swelling, pain, and warmth that raises concern for both peripheral arterial disease and possible deep venous thrombosis. 1, 2, 3
Why Doppler/ABI is the Correct Initial Test
All patients with diabetes and a foot ulcer must have peripheral arterial disease (PAD) excluded immediately, and this requires objective vascular testing—not just clinical examination. 1, 2 The American Diabetes Association explicitly states that ankle or pedal Doppler arterial waveforms, ankle systolic pressure, and ABI calculation are mandatory components of the initial evaluation for any diabetic foot ulcer. 1, 3
Critical Clinical Context
This patient's presentation is particularly concerning because:
- Up to 50% of diabetic foot ulcers have coexisting PAD, and these patients face dramatically increased amputation and mortality risk 2, 3
- The unilateral leg swelling, pain, tenderness, and warmth could represent either arterial insufficiency, venous thrombosis, or infection—all of which require immediate assessment 1
- Palpable pulses cannot exclude significant PAD—even skilled examiners can detect pulses despite critical ischemia, making objective testing mandatory 2, 3
The Doppler/ABI Assessment Protocol
The bedside Doppler examination should include: 1, 2, 3
- Hand-held Doppler evaluation of flow signals from both dorsalis pedis and posterior tibial arteries
- ABI calculation: ankle systolic pressure divided by brachial systolic pressure
- Waveform analysis: absent or monophasic Doppler signals suggest PAD 1, 2
Important Caveat for Diabetic Patients
If ABI is >1.3-1.4, the test is unreliable due to arterial calcification (Mönckeberg sclerosis), which is common in diabetes. 1, 2, 3 In this scenario, you must proceed to:
- Toe-brachial index (TBI): values <0.7 suggest significant PAD 1
- Transcutaneous oxygen pressure (TcPO2): values <50 mmHg suggest impaired healing potential 1
Why the Other Options Are Incorrect
CT and CT Angiogram (Options A & C) are not initial investigations—they are reserved for detailed anatomical mapping when revascularization is being planned after PAD has been confirmed by non-invasive testing. 1 The European Society of Cardiology states that CT angiography or MR angiography is indicated "when revascularization is considered," not as a screening tool. 1
Plain radiography would be appropriate to assess for osteomyelitis (given the ulcer), but vascular assessment takes priority because ischemia must be identified immediately to guide all subsequent management decisions. 1
Next Steps After Doppler/ABI
If the Doppler/ABI reveals: 1, 2
- Normal perfusion (ABI 0.9-1.3, triphasic waveforms): Focus on infection control, offloading, and wound care with reassessment if healing is poor after 6 weeks
- Mild-moderate PAD (ABI 0.6-0.9): Optimize wound care, consider toe pressures, and reassess with duplex ultrasound if healing response is inadequate
- Severe PAD (ABI <0.6, toe pressure <30 mmHg, or ankle pressure <50 mmHg): Proceed urgently to duplex ultrasound or angiography for revascularization planning
Critical Pitfall to Avoid
Never delay objective vascular testing based on clinical examination alone. 2, 3 The unilateral leg symptoms in this case could easily be misattributed to cellulitis or DVT, but without confirming adequate arterial perfusion first, any wound will fail to heal and the limb remains at high risk for amputation. 1, 2