Wound Classification

SWC I–IV with clinical interpretation, the interactive Classify This Wound tool, and the contamination → infection biology spectrum.

Based on the 2026 AJVR expert consensus on veterinary SSI definitions.

Section 01

Surgical Wound Classification (SWC I–IV)

Four classes ordered by degree of contamination at the time of surgery. Each entry pairs the consensus definition with clinical interpretation, SSI-risk and antimicrobial implications, veterinary examples, and the most common classification errors.

SWC IClean

From the 2026 AJVR consensus

An uninfected surgical wound in which no inflammation is present and the respiratory, alimentary, genital, or urinary tracts are NOT entered. Clean wounds can be primarily closed and, if necessary, drained with closed drainage.

NoteOperative incisions following nonpenetrating (blunt) trauma belong here if none of those tracts were entered.

VETSSI clinical interpretation

The cleanest category — elective soft-tissue and orthopedic surgery where no contaminated lumen is opened. The bar is genuinely strict: no inflammation, no tract entered.

SSI risk
Lowest baseline SSI risk. Any clean-wound SSI deserves scrutiny — the contamination source is most likely the team, environment, instruments, or technique rather than the patient.
Antimicrobial implication
Routine prophylaxis is often not required for clean procedures; decisions follow patient factors, implant use, and duration.
Benchmarking implication
Clean-wound SSI rate is the most sensitive indicator of surgical-process quality and the classic surgeon-feedback metric.
SWC IIClean-Contaminated

From the 2026 AJVR consensus

An uninfected surgical wound in which the respiratory, alimentary, genital, or urinary tract is entered under controlled conditions and without unusual contamination.

NoteControlled entry without spillage is the defining feature.

VETSSI clinical interpretation

A tract is opened, but deliberately and cleanly. The lumen is a bacterial reservoir, so risk rises above clean — but controlled technique keeps it bounded.

SSI risk
Moderate baseline SSI risk — higher than clean, lower than contaminated.
Antimicrobial implication
Perioperative prophylaxis is commonly indicated because a colonized tract is entered.
Benchmarking implication
Must be benchmarked separately from clean — combining classes makes any comparison invalid.
SWC IIIContaminated

From the 2026 AJVR consensus

Open, fresh traumatic wounds; surgery with major breaks in sterile technique or gross spillage from the gastrointestinal tract; incisions with acute nonpurulent inflammation or necrotic tissue without purulent drainage; and surgical wounds previously left open returning for revision.

NoteAlso includes sites where, due to location, full asepsis cannot be guaranteed because of ongoing contamination — the consensus specifically places perianal and intraoral surgery here.

VETSSI clinical interpretation

Either contamination has actually occurred — spillage, a major sterile-technique break, fresh trauma — or the anatomy makes contamination unavoidable. The consensus deliberately classes oral and anal procedures as contaminated, not clean-contaminated.

SSI risk
High baseline SSI risk.
Antimicrobial implication
Prophylactic — and sometimes therapeutic — antimicrobial use is generally indicated; the clinical context drives the choice.
Benchmarking implication
High expected rate; only meaningful when compared against the same wound class.
SWC IVDirty / Infected

From the 2026 AJVR consensus

Traumatic or surgical wounds presenting with retained devitalized tissue that involve existing clinical signs of infection, or perforated viscera.

NoteEstablished infection or perforation is already present before or at the time of surgery.

VETSSI clinical interpretation

Infection or gross contamination is a pre-existing condition of the case, not a risk. This is also where the relationship to IPATOS — infection present at the time of surgery — is closest.

SSI risk
Highest baseline SSI risk. A subsequent SSI is still recorded — a high-risk wound class never exempts a case from surveillance.
Antimicrobial implication
Therapeutic antimicrobial treatment, not merely prophylaxis, is generally required.
Benchmarking implication
Highest expected rate; comparison is only valid within this class.

Section 02

Classify This Wound

An ordered yes/no decision flow that follows the SWC I–IV criteria. The first qualifying YES determines the class. If you answer NO to all four, the result is SWC I.

Decision support — not a verdict. This tool helps you think through SWC classification using the consensus criteria. Final classification is a clinical judgment by the attending veterinarian, in the context of the full case.

Question 1 of 4

Was established infection present, or a perforated viscus, with retained devitalized tissue?

Existing clinical signs of infection at the site, or a perforated organ — present before or at surgery.

Section 03

Contamination, Colonization & Infection

Four biological states on a single spectrum. The boundary between colonization and infection is not a culture result — it is whether the tissue is reacting.

From the 2026 AJVR consensus

The presence of bacteria within a wound in the absence of any tissue reaction compatible with clinical signs of infection.

In plain terms

Bacteria are present. The tissue is not reacting. This is expected — most surgical and superficial wounds carry bacteria.

Why cultures alone are insufficient

A positive culture from a wound typically reflects colonization — not infection. Most surgical and superficial wounds carry bacteria. Culture cannot distinguish a colonizer from a pathogen on its own.

The consensus draws the line at tissue reaction. Bacteria multiplying alongside a quiet wound is colonization; bacteria multiplying with the wound reacting is infection. That is why every SSI definition pairs an objective finding with a clinical sign — a culture is one piece of evidence, never the whole diagnosis.

Foundational reference

Surgical site infection definitions consensus: a first step toward improving prevention in veterinary medicine.

Verwilghen DR, Pelosi A, Abbas M, et al. — American Journal of Veterinary Research, 2026

DOI: 10.2460/ajvr.25.03.0099 · Open Access — CC BY-NC

Every definition, criterion, surveillance term, and wound class on these pages derives from this expert consensus. Consensus text on each page paraphrases the paper; clinical interpretation, gray zones, and misclassification scenarios are VETSSI editorial.

View on AVMA JournalsDownload PDFdoi.org/10.2460/ajvr.25.03.0099