About This Page
This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- SURGICAL EMERGENCY: mortality 20-40% even with treatment. Delay in surgery significantly increases mortality
- Two types: Type I (polymicrobial — mixed aerobes and anaerobes, typically post-surgical or in diabetics/immunocompromised). Type II (monomicrobial — Group A Streptococcus, can occur in healthy individuals)
- KEY RED FLAG: pain OUT OF PROPORTION to clinical signs — the earliest and most important clue. Rapidly spreading erythema, skin necrosis, crepitus, bullae
- Management: (1) Immediate surgical debridement (do NOT delay for imaging), (2) Broad-spectrum IV antibiotics (meropenem + clindamycin ± vancomycin), (3) ITU support
- LRINEC score (Laboratory Risk Indicator for Necrotising Fasciitis): CRP >150, WCC >15, Hb <135, Na+ <135, creatinine raised, glucose >10 — ≥6 = high risk
Overview
Necrotising fasciitis (NF) is a rapidly progressive, life-threatening infection of the deep soft tissues, characterised by widespread necrosis of fascia and subcutaneous tissue. It spreads along fascial planes, often faster than clinically apparent on the skin surface. Type I (polymicrobial) is more common in diabetics, post-surgical patients, and immunocompromised. Type II (Group A Streptococcus — Strep pyogenes) can affect previously healthy individuals, sometimes following minor trauma or varicella. The key to survival is early recognition and immediate surgical debridement.
Epidemiology
NF is rare, with an incidence of approximately 0.4-1 per 100,000 per year. Risk factors include diabetes, obesity, immunosuppression, chronic liver/renal disease, IVDU, recent surgery, and minor skin trauma. NF can occur at any site but most commonly affects the lower limbs, perineum (Fournier gangrene), and abdominal wall. Mortality remains 20-40% despite optimal treatment, rising to >70% with delayed surgery.
Clinical Features
Symptoms
Pain OUT OF PROPORTION to visible skin changes — the most important early symptom
Rapidly worsening erythema and swelling
Fever, rigors (often high and swinging)
Systemic toxicity: confusion, tachycardia, hypotension
Signs
Erythema spreading beyond marked boundaries despite antibiotics for "cellulitis"
Skin colour changes: dusky, purple, or grey-black discolouration (skin necrosis)
Bullae or blistering (haemorrhagic)
Crepitus (gas in tissues — particularly type I/clostridial)
Woody hard induration of subcutaneous tissues
Anaesthesia of overlying skin (cutaneous nerve destruction)
Septic shock
Investigations
First-line
Clinical diagnosis — do NOT delay surgery for investigationsNF is diagnosed clinically and confirmed at surgery (dishwater grey necrotic fascia, tissue planes separate easily). If suspected, GO TO THEATRE
BloodsRaised WCC, CRP, lactate, creatinine. Low Na+. LRINEC score ≥6 supports diagnosis but normal score does NOT exclude NF
Blood culturesBefore antibiotics if possible
Second-line
CT scanMay show gas tracking along fascial planes, fascial thickening, fluid collections — but do NOT delay surgery waiting for CT
Surgical explorationThe definitive diagnostic test — "finger test" (insertion of finger through small incision — if tissue planes separate easily without resistance = NF confirmed)
Specialist
MRIMost sensitive imaging for fascial involvement but TOO SLOW for an emergency — only if diagnosis is uncertain and time permits
Management
Surgical consensus + NICE CKS1
Immediate surgical debridement
- This is the single most important treatment — every hour of delay increases mortality
- Radical debridement of ALL necrotic tissue. Often requires multiple returns to theatre (relook at 24-48 hours)
- May require amputation if limb NF is extensive
2
IV antibiotics
- Broad-spectrum: meropenem (or piperacillin-tazobactam) + clindamycin (anti-toxin effect — inhibits toxin production by Group A Strep) + vancomycin (if MRSA risk)
- Clindamycin is specifically added for its anti-toxin properties (inhibits bacterial protein synthesis including toxin production)
3
ITU support
- Aggressive fluid resuscitation, vasopressors, organ support
- IV immunoglobulin (IVIG) may be considered in streptococcal toxic shock syndrome
Complications
- Septic shock and multi-organ failure: The most common cause of death
- Extensive tissue loss: May require skin grafting, reconstruction, or amputation
- Fournier gangrene: NF of the perineum/scrotum — particularly devastating, high mortality
- Streptococcal toxic shock syndrome: Mediated by superantigens — hypotension, organ failure. IVIG may help
UKMLA Exam Tips
- 1Pain out of proportion = NF until proven otherwise. The single most important clinical clue
- 2Do NOT delay surgery for imaging — NF is a clinical diagnosis confirmed at surgery
- 3Clindamycin is added specifically for its ANTI-TOXIN effect (inhibits protein synthesis → reduces toxin production)
- 4Crepitus suggests gas-forming organisms (Clostridium — gas gangrene). But absence of crepitus does NOT exclude NF
- 5NF vs cellulitis: NF = pain out of proportion, systemic toxicity, skin necrosis, crepitus, failure to respond to antibiotics. Cellulitis = proportionate pain, no necrosis
- 6LRINEC score ≥6 supports diagnosis but a normal score does NOT safely exclude NF — clinical judgement is paramount
practicetest your knowledge on necrotising fasciitisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — infectious diseases and beyond.
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