A 28-year-old woman with 60% TBSA deep partial and full-thickness burns is admitted to the burn unit. On day 3, she develops fever (38.8°C), tachycardia (HR 120/min), and purulent drainage from the burn wound with surrounding erythema and edema. Which investigation is most appropriate to confirm burn wound infection and guide antimicrobial therapy?

## Diagnosis of Burn Wound Infection **Key Point:** Quantitative wound biopsy culture with a threshold of ≥10⁵ organisms per gram of tissue is the gold standard for diagnosing invasive burn wound infection and differentiating it from colonization. ### Why Quantitative Biopsy Culture? 1. **Defines infection vs. colonization:** - <10⁵ CFU/g = colonization (does not require systemic antibiotics) - ≥10⁵ CFU/g = invasive infection (requires systemic therapy and possible surgical debridement) 2. **Tissue-based diagnosis:** Biopsy samples the burn wound itself, not just the surface, reflecting true bacterial burden and invasion. 3. **Guides therapy:** Identifies the specific organism(s) and allows targeted antibiotic selection based on sensitivity. 4. **Prognostic value:** High bacterial loads (>10⁵) correlate with: - Increased risk of sepsis - Need for excision and grafting - Mortality risk ### Comparison of Diagnostic Methods | Method | Sensitivity | Specificity | Clinical Use | |---|---|---|---| | **Quantitative biopsy (≥10⁵)** | High | High | Gold standard; guides systemic therapy | | **Swab culture** | Low | Low | Identifies colonizing organisms only; surface flora | | **Blood culture** | Low | High | Positive only in septicemia; late finding | | **Procalcitonin** | Moderate | Moderate | Marker of systemic inflammation; not diagnostic | **Clinical Pearl:** Burn wound infection typically presents on **days 3–7** post-injury with fever, tachycardia, and purulent drainage with surrounding cellulitis — exactly as in this case. This is the classic presentation of invasive burn wound infection. **High-Yield:** The **Lund-Browder chart** is used for TBSA assessment, but once infection is suspected, quantitative biopsy is the investigation of choice. Surface swabs are **NOT** reliable because they do not differentiate colonization from invasion. **Mnemonic: BIOPSY-5** — **B**urn wound → **I**nvasion suspected → **O**btain tissue → **P**rocess for culture → **S**et threshold at 10⁵ → **Y**ield organism ID and sensitivities.