Cellulitis vs necrotising fasciitis: red flags, diagnosis and urgent management
RezzyThis is a classic UKMLA AKT topic because missing necrotising fasciitis (NF) is a "never event" in clinical practice. While cellulitis is common and usually manageable, NF is a surgical emergency that can progress in hours.
I've put together a comparison table to help you spot those critical "red flags" that should make you reach for the surgical bleep immediately.
Cellulitis vs. Necrotising Fasciitis
| Feature | Cellulitis | Necrotising Fasciitis (NF) |
|---|---|---|
| Pain | Tender, but matches the appearance | Pain out of proportion to clinical findings (Early sign) |
| Progression | Gradual (days) | Rapidly spreading (can change in minutes/hours) |
| Skin Color | Bright red/pink | Dusky, purple, or blue/grey discolouration |
| Skin Texture | Warm, slightly swollen | Woody hardness or crepitus (gas under skin) |
| Systemic Signs | Mild fever or none | Severe sepsis/SIRS (hypotension, tachycardia, confusion) |
| Late Signs | Blisters (clear fluid) | Hemorrhagic bullae or skin anaesthesia (nerves are dead) |
Red Flags for NF (The "Must-Knows")
If you see these, stop thinking about antibiotics alone and start thinking about theatre:
- ==Pain out of proportion to the skin appearance== (the most sensitive early sign).
- ==Rapidly spreading erythema== (mark the border; if it crosses it in an hour, it's NF).
- ==Hemorrhagic bullae== (blood-filled blisters).
- ==Crepitus== (feels like Rice Krispies under the skin—indicates gas-forming organisms).
- ==Skin anaesthesia== (the area becomes numb because the infection has destroyed the cutaneous nerves).
Diagnosis & Urgent Management
For Cellulitis:
- Diagnosis: Clinical. Use the Eron Classification to decide if they need admission.
- Management: Mark the border with a pen. First-line in the UK is usually Flucloxacillin (oral or IV depending on severity). If penicillin-allergic, think Clarithromycin or Doxycycline.
For Necrotising Fasciitis:
- Diagnosis: Primarily clinical suspicion. You can use the LRINEC score (Lab Risk Indicator for Necrotising Fasciitis), but a low score does not rule it out. The "Gold Standard" is surgical exploration (finding "dishwater pus" and lack of resistance to finger dissection of the fascia).
- Management:
- Immediate Surgical Debridement: This is the only thing that saves lives. Antibiotics alone won't reach the dead tissue.
- Broad-spectrum IV Antibiotics: Usually a "triple therapy" approach (e.g., Meropenem + Clindamycin + Linezolid). Clindamycin is key because it switches off toxin production in Group A Strep.
- Aggressive Fluid Resuscitation and likely ITU/HDU admission.
Let me find some high-yield lessons and a diagram to help you visualize these differences for your exams.
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