Plain Radiographs of the Left Foot (Option C)
Order plain radiographs of the left foot now as the essential first imaging study for this patient with suspected diabetic foot osteomyelitis. 1, 2
Rationale for Plain Radiographs First
This patient has a high pretest probability for osteomyelitis based on:
- Chronic ulcer (2 months duration) with positive probe-to-bone test 1
- Third-degree ulcer overlying bone 1
- Elevated inflammatory markers (ESR 45, WBC 12,600) 1
- New-onset redness and swelling suggesting progression 1
Plain radiographs must be obtained first in all patients presenting with non-superficial diabetic foot infections before proceeding to advanced imaging. 1, 2 The IDSA guidelines specifically recommend plain radiographs for all patients with new diabetic foot infections to look for bony abnormalities, destruction, soft tissue gas, and foreign bodies (strong recommendation, moderate evidence). 1
Why Not the Other Options Now
Option A (Serum C-reactive protein)
While inflammatory markers like CRP can support the diagnosis of osteomyelitis, they cannot solely rule in or rule out bone infection. 1 The patient already has an elevated ESR (45 mm/hr), which is suggestive of osteomyelitis. 1 Adding CRP would provide redundant information at this stage and should not delay obtaining imaging. 1
Option B (Bone scan)
Nuclear medicine studies are second-line imaging reserved for situations when MRI is unavailable or contraindicated. 1, 2 The IDSA guidelines recommend bone scans combined with labeled white blood cell scans only as alternatives to MRI, not as initial imaging. 1 Starting with a bone scan bypasses the essential baseline plain radiographs. 2
Option D (MRI of the left foot)
MRI is the most sensitive and specific imaging modality for osteomyelitis (strong recommendation, moderate evidence), but it should be performed when the diagnosis remains in doubt AFTER clinical assessment, plain radiographs, and laboratory findings. 1 The IWGDF/IDSA 2023 guidelines explicitly state that MRI should be obtained when diagnostic uncertainty persists despite initial workup. 1, 2 Proceeding directly to MRI without baseline plain radiographs violates guideline-recommended diagnostic algorithms and may result in unnecessary costs. 2
The Diagnostic Algorithm
Step 1: Obtain plain radiographs now to establish baseline bone architecture and identify characteristic osteomyelitis features (cortical erosion, periosteal reaction, bone destruction, sequestrum). 1, 2
Step 2: If plain radiographs show classic osteomyelitis changes, the diagnosis is confirmed and bone culture should be obtained to guide antibiotic therapy. 2 If radiographs are negative or equivocal (which occurs frequently in early osteomyelitis), proceed to MRI for definitive diagnosis. 1, 2
Step 3: MRI provides superior anatomic detail, identifies bone necrosis, and helps plan surgical intervention when needed. 1 A negative MRI effectively rules out osteomyelitis. 3
Critical Pitfalls to Avoid
- Do not skip plain radiographs and proceed directly to advanced imaging. This violates established diagnostic pathways and loses essential baseline information. 2
- Do not rely on soft tissue or wound cultures to guide antibiotic therapy for osteomyelitis—bone cultures are required for accurate pathogen identification. 1, 2
- Plain radiographs have low sensitivity in early osteomyelitis (first 10-14 days), so if initial films are negative but clinical suspicion remains high, repeat radiographs in 2-3 weeks or proceed directly to MRI. 1, 4, 5
The patient is already on appropriate empiric IV antibiotics, so obtaining plain radiographs now will not delay treatment while providing essential diagnostic information to guide the next steps in management. 1, 2