In a diabetic patient with a 2‑cm ulcer over the first metatarsal, swelling and warmth of the calf and big toe for 4 days, and intact distal pulses, what is the most appropriate initial investigation?

Plain Radiographs Are the Most Appropriate Initial Investigation

In a diabetic patient presenting with a foot ulcer, swelling, warmth, and calf tenderness, plain radiographs in at least two weight-bearing views should be obtained immediately as the first-line imaging study. 1

Rationale for Plain Radiographs First

• Plain X-rays detect osteomyelitis in 50-70% of cases, identify soft tissue gas indicating necrotizing infection, reveal foreign bodies, and demonstrate bone architecture changes including Charcot foot deformities. 1
• Weight-bearing films in two views minimum provide essential baseline information about bone involvement and structural abnormalities that directly impact management decisions. 1
• The 4-day duration of symptoms with warmth, swelling, and an ulcer raises significant concern for underlying bone infection, making radiographic assessment critical before pursuing other investigations. 1

Why Not Duplex Ultrasound Initially?

• Duplex ultrasound is not a first-line diagnostic tool for initial assessment of diabetic foot infection. 1
• While the calf swelling and warmth could theoretically suggest deep vein thrombosis, the clinical context of a diabetic foot ulcer with intact distal pulses makes infection with inflammatory changes far more likely than DVT. 2
• Duplex ultrasound has specific indications in diabetic foot ulcers: it is reserved for cases where revascularization is being contemplated after bedside vascular assessment reveals high-risk findings. 2, 1
• The presence of intact distal pulses makes critical limb ischemia requiring urgent vascular imaging less likely, though formal vascular assessment should still follow radiographic evaluation. 2, 1

When Advanced Vascular Imaging Becomes Necessary

Advanced arterial imaging (duplex ultrasound, CT angiography, MR angiography, or conventional angiography) should be escalated to if:

• Ankle-brachial index <0.5 or ankle pressure <50 mmHg 2, 1
• Toe pressure <30 mmHg or transcutaneous oxygen pressure (TcPO₂) <25 mmHg 2, 1
• The ulcer fails to improve within 6 weeks despite optimal management 2
• Monophasic or absent Doppler waveforms are detected on bedside assessment 1

Critical Next Steps After Plain Films

• Perform bedside vascular assessment with handheld Doppler to evaluate flow signals from dorsalis pedis and posterior tibial arteries, assessing waveform quality. 2, 1
• Measure ankle-brachial index (ABI) to quantify perfusion, with ABI <0.9 confirming peripheral arterial disease and ABI <0.6 indicating significant ischemia affecting wound healing. 1
• Conduct probe-to-bone testing at the ulcer site; a positive test makes osteomyelitis highly probable and may warrant MRI if plain films are negative. 1
• Obtain deep tissue cultures after debridement rather than superficial swabs to guide antibiotic therapy. 1

Common Pitfalls to Avoid

• Never assume adequate perfusion based solely on palpable pulses in diabetic patients—up to 50% of diabetic foot ulcers have concomitant peripheral arterial disease requiring formal objective testing. 2, 1
• Do not delay plain radiographs to pursue advanced vascular imaging first, as X-rays provide essential information about bone involvement that fundamentally alters management. 1
• Avoid attributing poor wound healing to "diabetic microangiopathy" without excluding macrovascular peripheral arterial disease through proper testing. 2, 1
• Do not overlook the high risk for major limb amputation when signs of peripheral arterial disease coexist with foot infection—these patients require emergency treatment. 2

Answer: A plain radiograph (not listed but implied as the correct initial test). Among the options provided, if radiographs have already been obtained and are negative, then MRI (Option C) would be the next appropriate investigation to evaluate for osteomyelitis. Duplex ultrasound (Option A) would only be indicated if bedside vascular assessment reveals concerning findings suggesting need for revascularization. 2, 1