Wound Classification and SSI Risk

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your Saudi GP Board Surgery paper.

Surgical sepsis, wound classification, and SSI prevention form the backbone of this 3% blueprint block. The CDC wound classes dictate SSI risk:

Class	Definition	Approx. SSI risk
I Clean	Uninfected operative wound, no entry into GI/respiratory/genitourinary tracts	<5%
II Clean-contaminated	Controlled entry into GI or respiratory tract without unusual contamination	<10%
III Contaminated	Open, fresh accidental wounds, major breaks in sterile technique	15–20%
IV Dirty-infected	Old traumatic wounds with retained devitalized tissue; perforated viscus	~40%

Exam pearl: Perforated appendicitis = Class IV, not Class II.

Give prophylactic cefazolin within 60 minutes of incision; redose at 3–4 hours. In suspected necrotizing fasciitis, take to OR within hours — never delay for imaging.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Wound Classification and SSI Risk

The CDC/NHSN four-tier system anchors every infection-control question on the SCFHS paper. Class assignment depends on the operative findings, not the preoperative diagnosis. A perforated viscus discovered intraoperatively reclassifies the case upward. Modifiable risk factors include hyperglycemia (target peri-op glucose <180 mg/dL), smoking cessation ≥4 weeks preoperatively, albumin ≥3 g/dL, normothermia (>36°C), and normoxia.

Skin and Soft Tissue Infection Spectrum

Infection	Layer	Border	Typical Organism
Erysipelas	Superficial dermis	Sharply demarcated, raised	Group A Streptococcus
Cellulitis	Deep dermis/subcutis	Indistinct	Staph aureus, Strep
Necrotizing fasciitis	Fascia + subcutis	Pain out of proportion, crepitus, bullae	Polymicrobial or Type II Strep pyogenes

LRINEC score ≥6 flags high-risk necrotizing infection — laboratory criteria include CRP ≥150 mg/L, WBC <15 or >25, Hb <13.5 g/dL, sodium <135 mmol/L, glucose >180 mg/dL, creatinine >1.6 mg/dL.

Tetanus Prophylaxis Decision Tree

Wound type	≥3 prior Td doses	<3 or unknown doses
Clean, minor	Td/Tdap only if last dose >10 years	Td/Tdap, no TIG
All other wounds	Td/Tdap only if last dose >5 years	Td/Tdap plus TIG 250 IU IM

TIG is never given to a fully immunized patient — that is a frequent exam trap.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Source Control Principles

Source control supersedes antibiotic choice in surgical sepsis. Four pillars apply: drainage of abscesses, debridement of necrotic tissue, removal of infected foreign bodies or hardware, and definitive control of ongoing contamination (e.g., resection, diversion, repair). Failure to achieve source control predicts mortality independent of antibiotic appropriateness.

Antibiotic Pharmacology Edge Cases

Aminoglycosides: 1 mg/kg q8h conventional dosing OR 5–7 mg/kg q24h extended-interval dosing in normal renal function; monitor peak (4–10 µg/mL) and trough (<1–2 µg/mL). Once-daily dosing reduces nephrotoxicity.
Vancomycin: 15–20 mg/kg actual body weight q8–12h, targeting trough 15–20 µg/mL for severe MRSA infection; requires renal adjustment and is the slowest prophylactic antibiotic, so the 120-minute pre-incision window applies.
Clindamycin: added to necrotizing fasciitis regimens for exotoxin suppression in streptococcal and staphylococcal toxic shock; monitor for C. difficile colitis.

Common Exam Traps

Confusing erysipelas (superficial, streptococcal) with cellulitis (deeper, staphylococcal) — the answer hinges on the dermal layer involved.
Classifying perforated appendicitis as Class II — pre-existing infection makes it Class IV.
Redosing cefazolin — required when operative time exceeds 2× the antibiotic half-life (cefazolin t½ ≈ 1.8 h, so redose at 3–4 h).
Treating post-op day 1–2 wound erythema as cellulitis — this is normal inflammatory hyperemia without purulence or systemic signs.

Worked Micro-Example

A 55-year-old diabetic presents with a perineal wound 48 hours after drainage of a perianal abscess. He is febrile 39°C, HR 122, BP 88/60, and scrotal skin shows dusky discoloration with subcutaneous crepitus. Diagnosis: Fournier’s gangrene (Class IV wound, necrotizing fasciitis of the perineum). Management: Immediate fluid resuscitation, broad-spectrum coverage (piperacillin-tazobactam + vancomycin + clindamycin), and emergent OR debridement within 1 hour — every hour of delay increases mortality.

Practice Prompts

A patient with 4 prior tetanus doses sustains a contaminated crush injury 7 years ago — does he need TIG?
An elective sigmoid resection without spillage is documented as Class III — what classification error has occurred and what is the corrected SSI risk estimate?

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Wound Classification and SSI Risk