Plain Radiographs Are the Most Appropriate Initial Investigation
In a diabetic patient with a foot ulcer, signs of infection, and calf involvement, plain radiographs in three views should be obtained first as the mandatory initial imaging study for all non-superficial diabetic foot infections. 1
Rationale for Plain Radiographs First
The American College of Radiology and International Diabetes Federation recommend plain radiographs as the first-line imaging modality because they are widely available, inexpensive, and can detect bone destruction, gas in soft tissues, and chronic changes suggestive of osteomyelitis 2, 1
Plain films should be obtained with a radio-opaque marker placed at the ulcer site to correlate anatomic location with any bony abnormalities 1
While plain radiographs have limited sensitivity (61.9%) and may not show changes for 2-3 weeks after infection onset, they remain the essential starting point for the diagnostic algorithm 2, 1
Clinical Assessment Components
Perform a probe-to-bone (PTB) test during wound examination, as a positive result (ability to probe directly to bone through the ulcer) is largely diagnostic of osteomyelitis, especially with a 2-cm ulcer 1, 3
Check inflammatory markers including ESR and CRP, as an ESR >70 mm/h substantially increases the likelihood of osteomyelitis 2
The combination of a positive PTB test, elevated ESR, and abnormal plain radiographs has 97% sensitivity and 93% specificity for osteomyelitis, making advanced imaging unnecessary in such cases 2
When to Proceed to Advanced Imaging
If plain radiographs are negative or equivocal but clinical suspicion remains high (positive PTB, elevated inflammatory markers, large ulcer >2 cm²), proceed directly to MRI 2, 3
MRI is the gold standard advanced imaging modality with 96.4% sensitivity and 83.8% specificity for osteomyelitis, and it provides crucial information about deep soft tissue infection extent, abscesses, and involvement of surrounding structures 2, 1, 3
The calf warmth and tenderness in this patient suggests possible deep soft tissue infection or ascending cellulitis, which MRI can accurately delineate to guide surgical decision-making 3
Why Other Options Are Inappropriate
Duplex ultrasound (Option A) is used to evaluate arterial perfusion and would only be indicated if distal pulses were absent or there were signs of ischemia—this patient has intact pulses 1
Conventional angiography (Option B) is for vascular assessment, not infection diagnosis, and is not indicated with intact pulses 1
CT with contrast (Option D) is not recommended as a primary imaging modality for diabetic foot osteomyelitis and has inferior soft tissue resolution compared to MRI 1
MRI (Option C) should be reserved for when the diagnosis remains in doubt after plain radiographs and clinical assessment, not as the initial investigation 2, 1
Critical Management Considerations
Start empirical intravenous antibiotics immediately after obtaining wound cultures, as this patient has signs of moderate infection (calf involvement, warmth, tenderness) 1
Do not delay antibiotic therapy to obtain imaging—treatment should never be postponed in moderate-to-severe infections 1
Obtain deep tissue specimens or bone cultures aseptically rather than superficial wound swabs, as the latter provide unreliable microbiological data 1
Consider hospitalization given the calf involvement and signs suggesting infection beyond the foot 3
Common Pitfalls to Avoid
Do not assume normal early radiographs exclude osteomyelitis—bone changes may not be visible for 2-3 weeks, necessitating follow-up imaging or MRI if clinical suspicion persists 2, 1
Do not order vascular imaging based solely on diabetes diagnosis—clinical assessment of pulses and perfusion guides the need for vascular studies 1
Do not interpret plain radiographs without knowledge of the clinical history, PTB test results, and inflammatory markers, as this increases the risk of misdiagnosis 2