A 58-year-old man undergoes elective open cholecystectomy. On postoperative day 5, he develops fever (38.5°C), erythema, and purulent discharge from the surgical incision. Which investigation is most appropriate to confirm the diagnosis and guide antimicrobial therapy?

Explanation

## Investigation of Choice for Surgical Site Infection ### Clinical Context The patient presents with classic signs of surgical site infection (SSI) on postoperative day 5: fever, incision erythema, and purulent drainage. The diagnosis is clinically apparent, but microbiological confirmation and antibiotic susceptibility are essential for targeted therapy. ### Why Wound Swab Culture & Sensitivity is Correct **Key Point:** Wound swab for culture and sensitivity is the gold standard investigation for SSI because it: - Directly samples the infected site - Identifies the causative organism - Provides antibiotic susceptibility data for targeted therapy - Guides de-escalation from empiric broad-spectrum antibiotics **High-Yield:** SSI microbiology varies by surgery type: - Clean surgery (cholecystectomy): *Staphylococcus aureus*, *Streptococcus pyogenes*, *E. coli* - Clean-contaminated (biliary, GI): Gram-negatives + anaerobes - Contaminated/dirty: Polymicrobial ### Proper Technique for Wound Swab 1. Cleanse surrounding skin with antiseptic 2. Swab purulent material from the wound depth (not superficial skin) 3. Use sterile swab; place in appropriate transport medium 4. Send for aerobic AND anaerobic culture (SSI can be polymicrobial) **Clinical Pearl:** Superficial skin flora contamination is common; deep wound swabs are more representative of true infection. ### Timing and Interpretation - Culture results typically available in 48–72 hours - Interim empiric therapy (e.g., cefazolin or cephalosporin for clean surgery SSI) should be started immediately - De-escalate once susceptibilities are known **Mnemonic:** **SWAB = Site-specific, Wound-depth, Aerobic + anaerobic, Broad-spectrum initial therapy**