skin and soft tissue infections
in review
5 min read
Updated 2026-08-23
Contents
skin and soft tissue infections
Classify as purulent (abscess-forming → S. aureus including MRSA) vs non-purulent (cellulitis/erysipelas → β-haemolytic streptococci). Non-purulent cellulitis: cephalexin or IV cefazolin; adding MRSA cover does not improve outcomes. Purulent: I&D is definitive; add TMP-SMX or doxycycline. Necrotising fasciitis: surgical emergency — broad-spectrum antibiotics + urgent debridement.
cellulitis/erysipelas empiric antibiotics
- mild (outpatient): cephalexin 500 mg PO QID × 5–7 days
- moderate (inpatient): cefazolin 2 g IV q8h
- if penicillin/cephalosporin allergy: clindamycin 300–450 mg PO QID (rising resistance — check local rates) or doxycycline 100 mg PO BID
- severe allergy + inpatient: vancomycin 15–20 mg/kg IV q8–12h
- do NOT add MRSA cover for non-purulent cellulitis — no outcome benefit
- step down to PO cephalexin when improving; total 5–7 days
purulent SSTI / abscess empiric antibiotics
- primary treatment: incision and drainage (antibiotics adjunctive, not substitute)
- mild (post-I&D, outpatient): TMP-SMX DS PO BID × 5–7 days or doxycycline 100 mg PO BID × 5–7 days
- moderate/severe: vancomycin 15–20 mg/kg IV q8–12h (or linezolid 600 mg PO/IV BID if oral option needed)
- if concern for mixed gram-negative flora (perineal, diabetic foot): add piperacillin-tazobactam 4.5 g IV q6h
necrotising fasciitis empiric antibiotics
- SURGICAL EMERGENCY → urgent surgical consultation for debridement — antibiotics do not substitute for source control
- broad-spectrum: vancomycin 25–30 mg/kg IV load + piperacillin-tazobactam 4.5 g IV q6h (or meropenem 1 g IV q8h)
- add clindamycin 900 mg IV q8h (toxin suppression — particularly for group A strep and Clostridium)
- if confirmed monomicrobial group A strep: penicillin G 4 MU IV q4h + clindamycin 900 mg IV q8h
local antibiogram
Community MRSA prevalence varies widely across Canadian regions. Non-purulent cellulitis is predominantly streptococcal — MRSA cover is not routine. Check local resistance patterns for clindamycin and TMP-SMX.
classification
| type | organism | clinical features |
|---|---|---|
| non-purulent (cellulitis, erysipelas) | β-haemolytic streptococci (usually GAS) | spreading erythema, warmth, tenderness; no drainable collection |
| purulent (abscess, furuncle, carbuncle) | S. aureus (MRSA or MSSA) | fluctuant collection, surrounding cellulitis; I&D curative |
| necrotising | polymicrobial (type I) or monomicrobial GAS (type II) | pain out of proportion, crepitus, systemic toxicity, rapidly spreading |
severity grading
| grade | features | management |
|---|---|---|
| mild | no systemic signs, small area | outpatient PO antibiotics |
| moderate | systemic signs (fever, tachycardia) OR failed outpatient therapy OR large area | inpatient IV antibiotics |
| severe | sepsis, rapid progression, crepitus, pain out of proportion | urgent surgical consult + broad-spectrum IV |
when to suspect necrotising fasciitis
- pain disproportionate to exam findings — most reliable early sign
- wooden-hard induration extending beyond visible erythema
- skin colour changes: dusky, grey, or haemorrhagic bullae
- crepitus (gas in tissues — late sign)
- systemic toxicity: high fever, tachycardia, hypotension
- rapid progression despite antibiotics
LRINEC score
Wong (2004): CRP, WBC, haemoglobin, sodium, creatinine, glucose. Score ≥6 raises suspicion; ≥8 strongly predictive. Sensitivity is moderate — do not use to rule out necrotising fasciitis. Clinical suspicion alone warrants surgical exploration.
CT with IV contrast may show fascial thickening, fat stranding, gas — but should not delay surgery if clinical suspicion is high.
special situations
diabetic foot infections
- mild (superficial, <2 cm cellulitis): cephalexin 500 mg PO QID ± TMP-SMX (MRSA cover)
- moderate (deeper tissue, >2 cm, lymphangitis): amoxicillin-clavulanate 875/125 mg PO BID or cefazolin IV + metronidazole (anaerobic cover)
- severe (sepsis, osteomyelitis concern): piperacillin-tazobactam 4.5 g IV q6h or ertapenem 1 g IV daily — always probe to bone (positive probe-to-bone test: ~90% PPV for osteomyelitis)
bite wounds
- prophylaxis (all human bites, deep cat bites, immunocompromised): amoxicillin-clavulanate 875/125 mg PO BID × 3–5 days
- if penicillin allergy: TMP-SMX + metronidazole, or moxifloxacin
- Pasteurella multocida (cat bites) — penicillin-susceptible; Eikenella corrodens (human bites) — resistant to clindamycin, first-gen cephalosporins
injection drug use
- always cover MRSA (TMP-SMX or vancomycin) + consider mixed flora (anaerobes, gram-negatives)
- search for deep collections (ultrasound or CT) — abscess formation common
- consider endocarditis in IVDU with SSTI + bacteraemia
duration
| type | duration |
|---|---|
| uncomplicated cellulitis | 5–7 days (can shorten to 5 if rapid response) |
| abscess (post-I&D) | 5–7 days |
| necrotising fasciitis | until debridement complete + 48 h afebrile — typically 2–4 weeks total |
| diabetic foot (no osteomyelitis) | 1–2 weeks |
| diabetic foot + osteomyelitis | 6 weeks (or shorter if complete resection) |
traps
- bilateral “cellulitis” is almost never infection — consider stasis dermatitis, contact dermatitis, DVT, or lymphoedema
- marking the erythema border helps track progression — reassess at 48–72 h (early worsening within 24 h is common even with appropriate antibiotics as inflammation peaks)
- do not add MRSA cover for non-purulent cellulitis — two large RCTs showed no benefit of TMP-SMX added to cephalexin
- blood cultures in uncomplicated cellulitis are positive in <5% — not routine; obtain only if septic or immunocompromised
- rising clindamycin resistance (>25% in many regions) limits its role — confirm susceptibility before relying on it
- abscess I&D without antibiotics cures most uncomplicated abscesses — antibiotics are adjunctive; TMP-SMX post-I&D reduces recurrence (Talan (2016))