Infectious Disease

Cellulitis

An acute bacterial infection of the dermis and subcutaneous tissue causing spreading inflammation; the essential clinical task is to distinguish uncomplicated cellulitis from sepsis, orbital disease, abscess and necrotising fasciitis.

In a nutshell

Recognise unilateral spreading infection, exclude dangerous mimics, treat uncomplicated adult disease with oral flucloxacillin, and escalate for sepsis, eye or nose involvement, failure to improve, unusual exposure or possible necrotising fasciitis.

Classic presentation

A unilateral, warm, swollen and tender area with spreading erythema, often on the lower leg, with a possible skin break and variable fever or malaise.

Key points

Cellulitis is a clinical diagnosis: redness alone is not enough, and bilateral chronic leg redness should prompt consideration of venous or inflammatory mimics.
For uncomplicated adult disease, NICE first choice is oral flucloxacillin; check the current BNF and local policy for patient-specific prescribing.
Infection near the eyes or nose needs specialist or hospital assessment and a different antimicrobial pathway.
If IV antibiotics are used, review by 48 hours and switch to oral treatment when possible.
Initial worsening can occur during the first 48 hours, but improvement should start within 2 to 3 days; persisting or worsening disease needs reassessment.
Pain out of proportion, rapid progression, bullae, sensory loss, dusky or black skin, shock or severe toxicity means possible necrotising fasciitis: seek urgent surgery.
Do not routinely use prophylactic antibiotics; specialist prophylaxis may be considered after at least 2 hospital or specialist-treated episodes in 12 months.

First-line investigation

Clinical assessment of the skin, portal of entry, pain, spread and physiology; use targeted bloods, swabs and alternative-diagnosis tests only when they can change management.

Management

Recognise the dangerous branch first

Use ABCDE, observations and NEWS2 when indicated; escalate immediately for sepsis physiology, eye or nose involvement, severe pain out of proportion, rapid progression, bullae, sensory loss or dusky or black skin.^1,5,4
Mark the edge, document the baseline, elevate the limb where possible and inspect for a portal of entry, abscess or deeper complication.^1,2

Confirm the pattern and choose targeted tests

Treat cellulitis as a clinical diagnosis, while actively excluding venous, inflammatory, thrombotic, joint, orbital and necrotising alternatives; use Eron or Dundee only as an adjunct.^1,3
Use bloods, lactate, cultures, swabs, ultrasound or other imaging only when severity, unusual exposure, a focal complication or an alternative diagnosis makes the result useful.^1,5,4

Treat the appropriate severity and site

For uncomplicated adult disease, give oral flucloxacillin for 5 to 7 days and check the BNF for patient-specific prescribing; use clarithromycin, doxycycline or pregnancy-appropriate advice when flucloxacillin is unsuitable.^1,6,7,8
For infection near the eyes or nose, seek specialist advice and use the NICE co-amoxiclav or clarithromycin-plus-metronidazole pathway, with BNF and local guidance checks.^1,9,10

Escalate, source-control and reassess

Admit or use an appropriate ambulatory IV pathway for severe illness, inability to take oral treatment, spreading disease despite oral therapy, lymphangitis, unusual pathogen risk or serious complication; review IV therapy by 48 hours.^1,6
If necrotising fasciitis is possible, give broad-spectrum IV antibiotics according to the local emergency protocol and obtain urgent surgical assessment for debridement; do not wait for routine cellulitis reassessment.^1,4

Prevent recurrence and make safety-netting explicit

Treat tinea, wounds, eczema, diabetes, venous disease and oedema; explain that the skin may take time to normalise and return for significant worsening or failure to improve within 2 to 3 days.^1,2
Do not routinely offer prophylaxis; a specialist may consider it after at least 2 hospital or specialist-treated episodes in 12 months, with shared decision-making and at least 6-monthly review.¹

Exam traps

Bilateral lower-leg erythema is not automatically bilateral cellulitis; reassess for venous stasis, lipodermatosclerosis, eczema or contact dermatitis.
A normal or modest inflammatory marker does not make disproportionate pain and rapid spread safe.
Do not routinely swab intact skin; sample broken skin selectively when exposure, wound or treatment-response factors make microbiology useful.
Eye or nose involvement is not routine cellulitis: use the specialist pathway because of orbital and intracranial risk.
Clarithromycin alone is not the NICE alternative for cellulitis near the eyes or nose in penicillin allergy; the listed pathway combines it with metronidazole.
Do not label IV therapy a failure before the 48-hour review, and do not expect erythema to disappear immediately after effective treatment.
Eron or Dundee scores can structure severity assessment but are not standalone rules for admission or antibiotic choice.
Necrotising fasciitis is a surgical emergency; antibiotics without urgent source-control assessment are inadequate.

Illustrations

Cellulitis with lymphangitic spreadClinical photograph showing erythema around a hand abrasion with a linear red streak extending proximally along the forearm, indicating lymphangitic spread of cellulitis.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Portal of entryDiagram of a breach in the skin barrier allowing skin flora to invade the dermis and subcutaneous tissue.PassFinals · original

Necrotising fasciitis with haemorrhagic bullaePreoperative photograph showing extensive 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

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.