In a 47‑year‑old man with type 2 diabetes, neuropathy, hypertension, obesity, a 2‑month plantar ulcer, swelling, malodorous drainage, positive probe‑to‑bone test, leukocytosis and hyperglycemia who has had blood cultures drawn and IV antibiotics started, what is the most appropriate next investigation to confirm or exclude osteomyelitis?

Plain Radiograph of the Left Foot

You should order a plain radiograph of the left foot now as the most appropriate next investigation in this patient with suspected diabetic foot osteomyelitis who has already had blood cultures drawn and IV antibiotics started. 1

Why Plain Radiography Is the Correct Next Step

Guideline-Recommended Initial Imaging

• All patients presenting with a new diabetic foot infection require plain radiographs of the affected foot as the initial imaging study, obtained in three standard views (dorsoplantar, lateral, and medial oblique). 1
• Plain films serve multiple critical purposes: detecting bony abnormalities (destruction, deformity), identifying soft tissue gas, revealing radio-opaque foreign bodies, and establishing a baseline for comparison. 1
• The ulcer location should be marked with a radio-opaque marker to allow accurate assessment of adjacent bone structures. 1

Clinical Context Supporting This Choice

• This patient has a positive probe-to-bone test, which carries a positive likelihood ratio of 6.4–7.2 for osteomyelitis, making bone infection highly likely. 1, 2, 3
• The 2-month duration of the ulcer, malodorous drainage, leukocytosis (WBC 12,600), and elevated ESR (845) all strongly suggest deep infection with possible bone involvement. 1
• Plain radiographs can detect osteomyelitis in approximately 50–70% of cases when present, and while sensitivity is limited (54%), they provide essential baseline information. 1, 3

Why Other Options Are Less Appropriate Now

Serum C-Reactive Protein (Option A)

• While CRP is useful for diagnosis and monitoring, it does not change immediate management when infection is already clinically evident and IV antibiotics have been started. 1
• CRP elevation (along with ESR > 70 mm/h) increases the likelihood of osteomyelitis, but this patient already has markedly elevated ESR and clinical signs of infection. 1, 3
• Laboratory markers are adjunctive; imaging is essential to define the extent of disease. 1

Bone Scan (Option B)

• Bone scans (especially combined leukocyte/bone scans) are second-line imaging reserved for when MRI is unavailable or contraindicated. 1
• Plain radiographs must be obtained first as the standard initial imaging modality per IDSA and IWGDF guidelines. 1
• Nuclear medicine studies are more expensive, time-consuming, and not indicated as the first imaging test. 1

MRI of the Left Foot (Option D)

• MRI is the most sensitive and specific test for osteomyelitis (98% sensitivity, 89% specificity, 100% negative predictive value), but it is indicated when diagnosis remains uncertain after plain radiographs. 1, 2
• The diagnostic algorithm requires plain films first; MRI is reserved for cases where radiographs are inconclusive, surgical planning is needed, or abscess is suspected. 1, 2
• Ordering MRI without baseline plain films violates the stepwise diagnostic approach and may miss other important findings (foreign bodies, gas, Charcot changes). 1

Algorithmic Approach to This Patient

Immediate Actions (Already Completed)

1. Blood cultures obtained ✓
2. IV antibiotics started ✓
3. Clinical assessment showing moderate-to-severe infection ✓

Next Step (Answer to Question)

Order plain radiographs in three views with radio-opaque marker at ulcer site. 1

Subsequent Management Based on Radiograph Results

If radiographs show osteomyelitis:

• Proceed with surgical debridement and obtain intraoperative bone specimens for culture and histology (the gold standard for diagnosis). 1
• Continue IV antibiotics, adjusting based on culture results. 1
• Plan for 4–6 weeks of antimicrobial therapy. 1

If radiographs are normal or equivocal:

• Given the positive probe-to-bone test and high clinical suspicion, order MRI to definitively confirm or exclude osteomyelitis. 1, 2
• MRI will also help detect soft tissue abscess, septic arthritis, or active Charcot neuroarthropathy (which can coexist with infection). 1

If radiographs show Charcot changes:

• Consider MRI to differentiate active Charcot from osteomyelitis, as bone marrow edema occurs in both conditions and expert radiologist interpretation is essential. 1

Critical Pitfalls to Avoid

• Do not skip plain radiographs and proceed directly to MRI; this violates guideline-based stepwise imaging protocols. 1
• Do not assume a negative plain film excludes osteomyelitis; radiographic changes lag infection by 7–10 days to several weeks, and sensitivity is only 54%. 1, 2, 3
• Do not delay imaging while waiting for laboratory results; plain films should be obtained immediately as part of the initial workup. 1
• Do not rely on superficial wound cultures for microbiologic diagnosis; bone biopsy during surgical debridement provides definitive pathogen identification. 1, 2
• Do not forget to assess for Charcot neuroarthropathy, especially in a patient with neuropathy and a "flattened" foot appearance, as this can coexist with infection and requires different management (total contact casting). 1

Additional Concurrent Management

• Optimize glycemic control (current glucose 263 mg/dL); hyperglycemia impairs wound healing and immune function. 1
• Ensure adequate off-loading of the ulcer with a removable knee-high walking device or total contact cast. 1
• Assess vascular status given the construction work history and diabetes; perform ankle-brachial index and consider vascular surgery referral if abnormal. 1, 2
• Plan for surgical debridement if radiographs or subsequent MRI confirm osteomyelitis, as antibiotics alone are insufficient for infected bone. 1