When an Infection is Classified as a Surgical Site Infection
An infection is classified as a surgical site infection (SSI) when it occurs within 30 days after surgery (or up to 1 year if an implant or prosthesis was placed) and involves the surgical incision or any anatomical space opened during the procedure, meeting specific depth-based diagnostic criteria. 1
Time-Based Criteria
- Standard procedures: SSI must occur within 30 days of the operation to qualify as a surgical site infection. 2, 1
- Procedures with implants or prosthetics: The timeframe extends to 1 year (or 90 days in some definitions) when any prosthetic material or implant is placed during surgery. 1, 3, 4
Anatomical Classification and Diagnostic Requirements
Superficial Incisional SSI
- Involves only the skin and subcutaneous tissue above the fascia. 1
- Must have at least one of the following: purulent drainage from the incision, organisms cultured from aseptically obtained fluid or tissue, clinical signs of local infection (pain, tenderness, localized swelling, redness, or heat), or diagnosis by the attending surgeon. 2, 1
Deep Incisional SSI
- Involves the fascia and muscle layers beneath the superficial incision. 1
- Requires at least one of: purulent drainage from the deep incision, spontaneous wound dehiscence (separation of deep layers) or deliberate opening by surgeon when patient has fever or localized pain, abscess identified during reoperation or by imaging (ultrasound, CT), or clinical diagnosis by the attending surgeon. 2, 1, 5
Organ/Space SSI
- Involves any anatomical space or organ opened or manipulated during the procedure, excluding the incision itself. 1
- Must demonstrate purulent drainage from a drain placed in the organ/space, organisms cultured from aseptically obtained fluid, abscess identified on reoperation or imaging, or clinical diagnosis by the attending surgeon. 2, 1
Critical Diagnostic Pitfalls to Avoid
- Early postoperative fever (first 48 hours) is rarely SSI unless caused by highly virulent organisms like β-hemolytic streptococci or Clostridium species; most SSIs present between postoperative days 4-6. 2, 1
- Purulent drainage is diagnostic of SSI and does not require culture confirmation to initiate treatment. 1
- Simple wound colonization without clinical signs of infection does not meet SSI criteria; there must be evidence of tissue invasion with inflammatory response. 5
- Wound dehiscence with infection automatically qualifies as deep incisional SSI when it involves fascial separation, regardless of other criteria. 1, 5
Relationship to Wound Classification
The baseline risk of developing SSI correlates directly with the degree of intraoperative contamination: 2
- Clean wounds: 1.5% infection rate
- Clean-contaminated: 7.7% infection rate
- Contaminated: 15.2% infection rate
- Dirty/infected: 40% infection rate
This classification helps predict SSI likelihood but does not define when an infection qualifies as SSI—the time and anatomical criteria above remain the definitive standards. 2, 6