A 62-year-old man undergoes elective abdominal aortic aneurysm repair. On postoperative day 5, the surgical wound shows erythema, warmth, and purulent drainage. The patient is hemodynamically stable with a temperature of 38.2°C. To confirm the diagnosis of surgical site infection and guide antibiotic therapy, which investigation is most appropriate?

## Investigation of Choice for Suspected Surgical Site Infection ### Clinical Context The patient presents with classic signs of surgical site infection (SSI) on postoperative day 5: localized erythema, warmth, purulent drainage, and fever. Confirmation and microbiological identification are essential for targeted antimicrobial therapy. ### Why Wound Culture and Sensitivity is Correct **Key Point:** Wound culture and sensitivity is the gold standard investigation for confirming SSI and identifying the causative organism, enabling targeted antibiotic therapy and reducing antimicrobial resistance. **High-Yield:** Culture allows: 1. Definitive identification of the pathogen (aerobic, anaerobic, or polymicrobial) 2. Antibiotic susceptibility testing (AST) to guide empiric therapy 3. Exclusion of colonization vs. true infection (quantitative culture ≥10^5 CFU/mL confirms infection) **Clinical Pearl:** Samples must be obtained from purulent material or tissue at the wound margin, NOT from the wound surface or surrounding skin, to avoid contamination and ensure diagnostic accuracy. ### Role of Other Investigations | Investigation | Role | Limitation | | --- | --- | --- | | **CBC with differential** | Non-specific marker of infection (elevated WBC, left shift) | Does not identify organism; cannot guide targeted therapy | | **Serum procalcitonin** | Biomarker of systemic inflammation | Non-specific; does not identify causative organism | | **High-resolution CT abdomen** | Evaluates for deep/necrotizing infection or abscess | Indicated only if clinical suspicion of deep infection; not first-line for superficial SSI | ### Wound Healing Context During the **inflammatory phase (days 1–5)**, neutrophils and macrophages clear bacteria and debris. Failure of this phase (evidenced by purulent drainage and fever) suggests overwhelming bacterial burden or virulent pathogen — necessitating microbiological identification for appropriate intervention. **Mnemonic: CWIS** — Culture, Wound, Infection, Sensitivity — the diagnostic triad for SSI confirmation and management.