What are the signs of a surgical site infection?

Signs of Surgical Site Infection

Surgical site infections (SSIs) are diagnosed by the presence of purulent drainage, positive wound cultures, or local signs of infection including pain, tenderness, swelling, and erythema at the incision site. 1

Classification of Surgical Site Infections

SSIs are divided into three distinct categories:

1. Superficial Incisional SSI

• Occurs within 30 days of surgery
• Involves only skin or subcutaneous tissue of the incision
• Diagnosed by at least one of the following:
  • Purulent drainage from the incision
  • Organisms cultured from aseptically obtained fluid/tissue
  • Clinical signs of local infection (pain, tenderness, swelling, erythema)
  • Diagnosis made by attending clinician
  • Stitch abscess 1, 2

2. Deep Incisional SSI

• Occurs within 30 days of surgery (or up to 1 year if implant in place)
• Involves deep soft tissues (fascia and muscle layers)
• Diagnosed by at least one of the following:
  • Purulent drainage from the deep incision
  • Dehiscence of deep incision
  • Abscess found at reoperation, radiological examination, or histopathology
  • Diagnosis made by attending clinician 1, 2

3. Organ/Space SSI

• Occurs within 30 days of surgery (or up to 1 year if implant in place)
• Involves any part of the body opened during surgery other than the incision
• Diagnosed by at least one of the following:
  • Purulent drainage from a drain placed into the organ/space
  • Organisms cultured from aseptically obtained fluid/tissue
  • Abscess found at reoperation, radiological examination, or histopathology
  • Diagnosis made by attending clinician 1, 2

Key Clinical Signs and Symptoms

Local Signs

• Purulent drainage (diagnostic of SSI) 1
• Erythema and induration (swelling)
• Pain or tenderness at the incision site
• Localized warmth
• Spreading inflammation exceeding normal healing
• Wound dehiscence 1, 2, 3

Systemic Signs (indicating more severe infection)

• Temperature >38.5°C
• Heart rate >110 beats/minute
• White blood cell count >12,000/μL 1, 2

Timing of Infection

• Early infection (first 48 hours): Suggests virulent organisms like β-hemolytic streptococci or Clostridium species 1
• Late infection (4-6 days postoperatively): Most common timing, typically polymicrobial 1
• Most SSIs appear within 30 days of surgery (or up to 1 year with implants) 1, 2

Special Considerations

• Deep SSI warning signs: Dusky or macerated wound appearance, persistent purulent drainage despite antibiotics, and suspected deep tissue involvement require immediate surgical consultation 2
• High-risk wounds: "Contaminated" or "dirty" surgical sites have infection rates of 15.2% and 40% respectively, compared to 1.5% for "clean" wounds 1
• Any deep SSI that does not resolve as expected should be investigated as a possible manifestation of a deeper organ/space infection 1
• Flat, erythematous changes around a surgical incision during the first week without swelling or drainage often resolve without treatment 1

Diagnostic Approach

• Visual inspection: The physical appearance of the incision provides the most reliable information 1
• Wound cultures: Should be obtained when purulence or spreading cellulitis is present 1
• Blood cultures: Two sets should be obtained in cases of suspected deep or organ/space SSI 1
• Imaging: Ultrasound is the first-choice modality for suspected deep collections; CT should be considered when ultrasound is not diagnostic 1, 2

Common Pitfalls

• Misdiagnosing necrotizing soft tissue infection as simple SSI
• Overreliance on antibiotics without adequate surgical intervention
• Failure to recognize deep extension of seemingly superficial infections
• Delayed surgical consultation for deep or complex infections 2

Remember that the primary treatment for any SSI is to open the incision, evacuate infected material, and continue appropriate wound care until healing occurs, with antibiotics being an adjunctive therapy rather than the primary treatment 1, 2.