PassFinals
Dermatology

Necrotising Fasciitis

Bacteria spreading along fascial planes thrombose the vessels supplying the overlying tissue, so fat and fascia die while the skin above can still look nearly normal.

In a nutshell

Necrotising fasciitis is infection destroying deep fascia and subcutaneous fat, with pain and systemic illness far worse than the skin suggests. Call the on-call surgeon before anything else and debride as soon as possible; never wait for a numeric deadline.

Classic presentation

A 58-year-old with diabetes has a small leg wound, 12 hours of agonising pain, fever and tense shiny swelling, with only patchy erythema to see.

Key points

  • Type I is polymicrobial, 70 to 80% of cases, usually trunk or perineum. Type II is monomicrobial group A streptococcus and usually hits a limb.
  • Fournier gangrene is necrotising fasciitis of the perineum and genitals: same disease, same clock, but call urology.
  • UK incidence is roughly 500 cases a year and mortality is 20 to 40% even when treated.
  • The superficial fascia separating without resistance to blunt dissection, not bleeding, and yielding grey 'dishwater' pus is the operative diagnosis.
  • Review the intravenous antibiotic at 48 to 72 hours against tissue culture, narrow it and consider an oral switch.
  • Clindamycin causes antibiotic-associated colitis more often than most antibacterials; stop it and seek advice if severe diarrhoea develops.

First-line investigation

None. This is a clinical diagnosis confirmed at surgical exploration; bloods and imaging grade the sepsis and map the extent, and neither can exclude it.

Management

Call the surgeon first

  • Suspected necrotising fasciitis is a surgical emergency. Telephone the on-call general surgeon, critical care and microbiology now. Perineal or genital disease is Fournier gangrene: call urology.6,1
  • Do not wait for imaging, a LRINEC score or a swab result. Nothing on a blood test or scan excludes the diagnosis.9,3

Resuscitate on the NG253 sepsis pathway

  • A NEWS2 (National Early Warning Score 2) of 7 or more is high risk. Balanced crystalloid such as Hartmann's: 250 mL over 10 to 15 minutes, reassess, repeat to 1000 mL maximum.8
  • No improvement after 1000 mL: senior clinical decision maker. Oxygen to 94 to 98%, or 88 to 92% if at risk of hypercapnic respiratory failure.8,12
  • Send FBC, U&E, LFT, CRP, glucose, clotting and lactate, and blood cultures before the first antibiotic dose if that costs no time.11,1

Antibiotics within 1 hour, with the antitoxin

  • NHS Fife: piperacillin with tazobactam 4.5 g IV every 6 hours plus clindamycin 1.2 g IV every 6 hours, within 1 hour of calculating NEWS2.6,8
  • True penicillin allergy, NHS Fife: meropenem 1 g IV every 8 hours plus clindamycin 1.2 g IV every 6 hours.6
  • Clindamycin suppresses the streptococcal superantigen driving the shock. It is added to the beta-lactam, never substituted for it.1,4
  • MRSA risk: add vancomycin IV 15 to 20 mg/kg every 8 to 12 hours (maximum 2 g per dose), adjusted to levels, after 25 to 30 mg/kg loading.13
  • No national regimen exists. NHS Borders instead uses flucloxacillin 2 g IV 6-hourly, ciprofloxacin 400 mg IV 12-hourly and clindamycin 600 mg to 1.2 g IV 6-hourly.7,5
  • Section rule: follow your own trust's antimicrobial guideline, phone microbiology, and check allergy, renal function and previous cultures before prescribing.11,12

Immediate theatre

  • Debride as soon as possible. A US guideline uses 12 hours as an evidence benchmark, not a UK target; do not wait for it.3
  • Send deep tissue and fluid from theatre for Gram stain and culture. A superficial swab reflects skin flora, not the deep organisms.1,11
  • Plan a return to theatre at 24 hours to reassess, and expect serial debridements over days until no necrotic tissue remains.1

Critical care, and the adjuncts in their place

  • Most patients need HDU or ICU: vasopressors, renal replacement, ventilation. Isolate with contact precautions and involve the surgical team early for repeated source control.6,8
  • NHS England does not routinely commission hyperbaric oxygen for necrotising soft tissue infection. Intravenous immunoglobulin has no evidence for routine use and no BNF dose here.14,3

Narrow, reconstruct, rehabilitate

  • Review the intravenous antibiotic at 48 to 72 hours against tissue culture, narrow the spectrum, and agree duration and any oral switch with microbiology.11,7
  • Then reconstruction: negative-pressure dressings, grafting and plastic surgery once infection is controlled. Add analgesia, physiotherapy and psychological support for a long rehabilitation.1

Exam traps

  • A LRINEC score below 6 does not exclude necrotising fasciitis. In one prospective study its sensitivity at that cut-off was 43%.
  • Do not send the patient for CT before referring. Imaging is not specific and the delay costs tissue.
  • Antibiotics cannot sterilise dead avascular tissue. They buy time; only the knife is treatment.
  • Dark blotches and blisters are harder to see on black or brown skin, so weight the pain and the observations more.
  • Clindamycin is added for its antitoxin effect, not for extra cover, and goes alongside a beta-lactam, never instead of one.
  • NHS England does not routinely commission hyperbaric oxygen for this, and no evidence supports routine immunoglobulin.

Illustrations

Advanced skin changes in necrotising fasciitisA sensitive pre-operative clinical photograph showing dusky erythema, haemorrhagic bullae, epidermal sloughing and black skin necrosis in advanced necrotising fasciitis.Piotr Smuszkiewicz, Iwona Trojanowska and Hanna Tomczak, Wikimedia Commons · CC-BY-2.0

Key sources

  1. Royal College of Emergency Medicine (RCEM Learning), Necrotising Fasciitis reference topic (UK emergency medicine reference: types I to IV, early versus late signs, the LRINEC table and its cut-offs, the clindamycin antitoxin rationale, operative findings, return to theatre at 24 hours, mortality 20 to 40% and roughly 500 UK cases a year.)Published 14 Apr 2023 | Updated 13 Mar 2026
  2. NHS 111 Wales, Necrotising fasciitis (UK NHS public information: symptoms develop within hours or days, pain seems much worse than the wound justifies, and black, purple or grey blotches and blisters may be less obvious on black or brown skin.)Updated 14 Oct 2025
  3. Eastern Association for the Surgery of Trauma, Optimal timing of initial debridement for necrotising soft tissue infection (Practice management guideline, J Trauma Acute Care Surg 2018;85(1):208-214. Recommends debridement within 12 hours of suspected diagnosis; pooled mortality 14% early versus 25.8% late; states imaging lacks specificity and that no data support routine intravenous immunoglobulin. Used because no UK national guideline exists.)Published 1 Jul 2018
  4. BNF, Clindamycin monograph (Intravenous adult dose 0.6 to 2.7 g daily in 2 to 4 divided doses, up to 4.8 g daily in 4 divided doses in life-threatening infection, single intravenous doses not to exceed 1.2 g. Active against Gram-positive cocci and anaerobes. Antibiotic-associated colitis occurs more frequently with clindamycin than with most antibacterials.)
  5. UK government guidance (Public Health England), Necrotising fasciitis (NF) (Standing GOV.UK guidance. States there are no public health guidelines specific to the prevention and control of necrotising fasciitis, and refers readers to the group A streptococcal infection guidance instead. Content dates from 2013.)Published 1 Apr 2013
  6. NHS Fife, Necrotising Fasciitis / Fournier's Gangrene antimicrobial pathway (NHS health-board pathway. Contact the on-call general surgical team immediately, then give antibiotics: piperacillin with tazobactam IV 4.5 g 6-hourly plus clindamycin IV 1.2 g 6-hourly; in true penicillin allergy meropenem IV 1 g 8-hourly plus clindamycin IV 1.2 g 6-hourly. Fournier's gangrene: contact the on-call urologist. Isolate with contact precautions.)Updated 21 Apr 2026
  7. NHS Borders, Necrotising Fasciitis antimicrobial pathway (NHS health-board pathway, version 3.0. Urgent surgical referral, surgery is the definitive treatment and antibiotics are an adjunct. Flucloxacillin IV 2 g 6-hourly plus ciprofloxacin IV 400 mg 12-hourly plus clindamycin IV 600 mg to 1.2 g 6-hourly; vancomycin replaces flucloxacillin in penicillin allergy.)Updated 20 Jul 2025
  8. NICE NG253, Suspected sepsis in people aged 16 or over: recognition, assessment and early management (guidance PDF) (NG253 recommendations 1.8.3 (broad-spectrum intravenous antibiotics within 1 hour of calculating NEWS2), 1.8.5 to 1.8.9 (balanced crystalloid, initial 250 mL bolus over 10 to 15 minutes, further 250 mL boluses to 1000 mL total, reassess after each, senior review beyond 1000 mL) and 1.11.4 (early surgical involvement for source control). High risk is a NEWS2 of 7 or above.)Published 19 Nov 2025
  9. Royal College of Emergency Medicine (RCEM Learning), Cellulitis and Other Skin Infections: Clinical Decision Rules (States the LRINEC score is not validated, that multiple studies show poor sensitivity and specificity so it should be used with caution, and that although a score of 6 or greater suggests necrotising fasciitis the rule should not override clinical judgement.)Updated 12 Jan 2022
  10. Fann WC and colleagues, Prospective Validation of the LRINEC Score for Necrotising Fasciitis of the Extremities, PLOS ONE 2020;15(1):e0227748 (Prospective cohort of 106 patients with necrotising fasciitis and 825 with cellulitis. At a cut-off of 6 or more, sensitivity 43% (95% CI 34 to 53) and specificity 83%; at 8 or more, sensitivity 27% and specificity 93%. Concludes the score may not be accurate for risk stratification.)Published 24 Jan 2020
  11. NICE NG15, Antimicrobial stewardship: systems and processes for effective antimicrobial medicine use (NG15 recommendations 1.1.24 (shortest effective course, most appropriate dose and route), 1.1.27 (take microbiological samples before prescribing in hospital and review when results are available), 1.1.38 (empirical intravenous antimicrobial from the agreed local formulary) and 1.1.39 (review intravenous prescriptions at 48 to 72 hours and consider oral switch).)
  12. NICE NG253, Antibiotic therapy, intravenous fluid and oxygen (guideline chapter) (NG253 recommendations 1.9.2 and 1.9.7 (use existing local antimicrobial guidance, and for adults an intravenous antimicrobial from the agreed local formulary), and 1.10.1 (oxygen to a target saturation of 94% to 98%, or 88% to 92% if at risk of hypercapnic respiratory failure).)Published 19 Nov 2025
  13. BNF, Vancomycin monograph (Complicated skin and soft tissue infections, adult, by intravenous infusion: 15 to 20 mg/kg every 8 to 12 hours (maximum 2 g per dose), adjusted according to serum-concentration monitoring; in seriously ill patients a loading dose of 25 to 30 mg/kg (usual maximum 2 g) may be used.)
  14. NHS England, Clinical Commissioning Policy: Hyperbaric Oxygen Therapy for necrotising soft tissue infections (all ages), reference 170070P (NHS England will not routinely commission hyperbaric oxygen therapy for necrotising soft tissue infections. Also states UK incidence is estimated at 500 new cases a year with no ongoing surveillance, mortality about 30%, standard treatment is surgical debridement with intensive support and antibiotics, and Fournier's gangrene is necrotising fasciitis of the perineum.)Published 20 Jul 2018

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.