A 32-year-old man presents with a 3-week history of fever, localized swelling, and pain over the left tibia. Plain radiographs show periosteal reaction and cortical thickening. Blood cultures are pending. Which investigation is most appropriate to confirm the diagnosis of osteomyelitis and guide antimicrobial therapy?

Explanation

## Investigation of Choice in Osteomyelitis ### Why Bone Biopsy with Culture and Sensitivity? **Key Point:** Bone biopsy is the gold standard for diagnosis of osteomyelitis because it provides: 1. **Microbiological confirmation** — culture identifies the causative organism 2. **Sensitivity testing** — guides targeted antimicrobial therapy 3. **Histopathological confirmation** — differentiates infection from other pathologies (malignancy, inflammatory conditions) **High-Yield:** In acute osteomyelitis, bone biopsy should be performed before starting antibiotics to maximize culture yield. The organism is isolated in 50–90% of cases when done early. ### Comparison of Investigations | Investigation | Sensitivity | Specificity | Diagnostic Yield | Role | |---|---|---|---|---| | **Bone biopsy + culture** | 90% (early) | 100% | Definitive | Gold standard; identifies organism | | **MRI** | 95% | 85% | High | Best for extent/soft tissue involvement; not diagnostic alone | | **CT scan** | 80% | 75% | Moderate | Shows cortical/trabecular changes; less sensitive than MRI | | **Tc-99m bone scan** | 85% | 60% | Moderate | Early detection; low specificity; non-specific uptake | **Clinical Pearl:** Blood cultures are positive in only 50% of acute osteomyelitis cases. Bone culture remains essential for organism identification and guiding therapy, especially if blood cultures are negative. **Warning:** Starting empirical antibiotics before biopsy reduces culture positivity significantly. Biopsy should be prioritized in suspected osteomyelitis. ### Role of Other Investigations - **MRI:** Excellent for defining extent of infection, marrow edema, and soft tissue abscess; used to assess severity and plan surgical intervention, but NOT for organism identification - **CT scan:** Useful for detecting sequestra and cortical destruction; less sensitive than MRI for early detection - **Tc-99m bone scan:** Sensitive but non-specific; useful for detecting multifocal disease but cannot identify organism [cite:Robbins 10e Ch 26]