A 32-year-old man presents with a surgical wound that has failed to heal 6 weeks after appendicectomy. Clinical examination reveals erythema, induration, and purulent discharge. The wound edges are not approximated. Which investigation is most appropriate to identify the causative organism and guide antimicrobial therapy?

Explanation

## Investigation of Choice for Infected Wound **Key Point:** Gram stain and culture of wound swab is the gold standard for identifying causative organisms in wound infections and guiding targeted antimicrobial therapy. ### Why Gram Stain and Culture? 1. **Direct pathogen identification** — Gram stain provides rapid morphological classification (Gram-positive cocci, Gram-negative rods, etc.) 2. **Culture and sensitivity** — Definitive organism identification with antibiotic susceptibility profile 3. **Guides targeted therapy** — Allows de-escalation from empirical broad-spectrum antibiotics 4. **Cost-effective** — Simple, inexpensive, and widely available ### Phases of Wound Healing Context | Phase | Timeline | Key Features | Infection Risk | |-------|----------|--------------|----------------| | Inflammatory | 0–3 days | Hemostasis, neutrophil infiltration | High (first 48 hrs critical) | | Proliferative | 3–21 days | Fibroblast activity, angiogenesis, collagen deposition | Moderate | | Remodeling | 21 days–2 years | Collagen cross-linking, scar maturation | Low | This patient is at 6 weeks (late proliferative/early remodeling phase) with signs of infection — culture is essential. **High-Yield:** In clinical practice, wound swab culture has >90% sensitivity for identifying aerobic pathogens (Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Pseudomonas aeruginosa) and anaerobes if collected properly. **Clinical Pearl:** Swabs should be taken from the wound base or purulent material, not from the surrounding skin, to avoid contamination and false positives. ### Why Not the Other Options? - **Histopathology** — Useful for assessing granulation tissue quality and fibroblast activity, but does NOT identify organisms; requires tissue biopsy (invasive, delays treatment) - **Blood culture** — Indicated only if systemic signs of sepsis (fever, hypotension, elevated WBC); local wound infection does not routinely bacteremia - **Procalcitonin** — Inflammatory marker; useful for severity assessment but does NOT identify the organism and cannot guide targeted therapy