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
- Cellulitis: poorly demarcated, deeper. Erysipelas: well-demarcated, raised border, more superficial
- Commonest organisms: group A Streptococcus and Staphylococcus aureus
- First-line: flucloxacillin 500 mg-1 g QDS for 5-7 days. Pen allergy: clarithromycin or doxycycline
- Draw around edge with skin marker to track progression
- Recurrent (>=2 in 12 months): prophylactic phenoxymethylpenicillin 250 mg BD
Overview
Cellulitis is an acute bacterial infection of the dermis and subcutaneous tissue with poorly demarcated borders. Erysipelas is a more superficial form with a well-demarcated, raised border. Both are commonly caused by group A beta-haemolytic streptococcus and S. aureus. Entry points include skin breaks, tinea pedis, ulcers, eczema, and insect bites.
Epidemiology
Accounts for 2-3% of hospital admissions. Lower limbs affected in ~70-80%. Risk factors: lymphoedema, obesity, venous insufficiency, tinea pedis, previous cellulitis, diabetes, immunosuppression, IV drug use. Recurrence rate ~30% at 3 years.
Clinical Features
Symptoms
Painful, warm, erythematous, swollen area — usually unilateral
Systemic symptoms: fever, malaise, rigors
Preceding skin break: wound, bite, tinea pedis
Rapid spread with dusky discolouration and severe pain (necrotising fasciitis)
Pain out of proportion to clinical signs
Crepitus on palpation
Signs
Erythema, warmth, swelling, tenderness — poorly demarcated (cellulitis)
Well-demarcated raised peau d'orange border (erysipelas)
Regional lymphadenopathy
Bullae/haemorrhagic bullae in severe cases
Periorbital involvement (risk of orbital cellulitis)
Investigations
First-line
Clinical diagnosisMark edge with skin marker to monitor. Blood tests if systemically unwell
Blood tests if febrileFBC, CRP, U&E, blood cultures
Second-line
Skin swabOnly if broken skin — MC&S for antibiotic guidance
Specialist
Duplex ultrasoundIf DVT differential — cellulitis and DVT can coexist
CT/MRIIf necrotising fasciitis suspected — should not delay surgical review
1
General measures
- Mark edge with skin marker
- Rest and elevate affected limb
- Analgesia
- Treat predisposing cause (tinea pedis, eczema)
2
Oral antibiotics (mild-moderate)
- Flucloxacillin 500 mg QDS for 5-7 days
- Pen allergy: clarithromycin 500 mg BD or doxycycline 200 mg then 100 mg OD
- Facial: co-amoxiclav 500/125 mg TDS
3
IV antibiotics (severe/Eron III-IV)
- IV flucloxacillin 1-2 g QDS
- MRSA risk: IV vancomycin or teicoplanin
- Step down to oral when improving
4
Preventing recurrence
- Treat predisposing factors
- If >=2 episodes in 12 months: prophylactic phenoxymethylpenicillin 250 mg BD
- Review prophylaxis every 6 months
Complications
- Abscess: May require I&D
- Necrotising fasciitis: Pain out of proportion, rapid spread — emergency surgical debridement
- Sepsis: Manage per Sepsis Six
- Recurrence: ~30% at 3 years
- Lymphoedema: Recurrent episodes damage lymphatics
UKMLA Exam Tips
- 1Cellulitis is almost always UNILATERAL — bilateral red legs = venous eczema
- 2Erysipelas: WELL-DEMARCATED raised border. Cellulitis: POORLY demarcated
- 3Mark the edge with a pen — essential for monitoring
- 4Pain out of proportion + rapid spread = think necrotising fasciitis
- 5Always check between toes for tinea pedis as entry point
practicetest your knowledge on cellulitisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — dermatology and beyond.
open q-bank