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: infection of the deeper dermis and subcutaneous tissue. Erysipelas: more superficial, sharply demarcated, raised edges, often on the face
- Most common organisms: Group A Streptococcus (Strep pyogenes) and Staphylococcus aureus
- Risk factors: skin break (wound, tinea pedis, eczema, leg ulcer), lymphoedema, obesity, diabetes, IV drug use
- First-line antibiotic: flucloxacillin 500 mg-1 g QDS for 5-7 days (NICE NG141). Penicillin allergy: clarithromycin 500 mg BD
- Severity classification (Eron): I = outpatient oral. II = systemically unwell or comorbidity. III = significant systemic toxicity. IV = sepsis/necrotising fasciitis → emergency
Overview
Cellulitis is an acute spreading infection of the skin involving the deeper dermis and subcutaneous fat. It is characterised by erythema, warmth, swelling, and pain, typically without a well-defined border. Erysipelas is a more superficial infection involving the upper dermis and lymphatics, producing a well-demarcated, raised, erythematous plaque — classically on the face. Both are most commonly caused by beta-haemolytic streptococci (Group A) and Staphylococcus aureus. The lower limbs are the most common site.
Epidemiology
Cellulitis accounts for approximately 2-3% of acute hospital admissions. It is more common in adults, with incidence increasing with age. The most important risk factor is disruption of the skin barrier — tinea pedis (athlete's foot) is the most commonly identified portal of entry. Other risk factors include lymphoedema (previous surgery, radiotherapy), chronic venous insufficiency, obesity, diabetes, and immunosuppression.
Clinical Features
Symptoms
Erythema — spreading redness (mark the border with a pen to monitor progression)
Pain, tenderness, and swelling of the affected area
Warmth over the affected area
Fever, malaise, rigors (systemic features — suggest more severe infection)
Rapidly spreading erythema with severe pain (consider necrotising fasciitis)
Pain out of proportion to clinical signs (necrotising fasciitis red flag)
Signs
Erythema with poorly defined borders (cellulitis) or well-defined raised edges (erysipelas)
Oedema and induration of skin
Regional lymphadenopathy and lymphangitis (red tracking up the limb)
Bullae formation (suggests more severe infection)
Crepitus (gas in tissues — gas gangrene or necrotising fasciitis)
Skin necrosis or purplish discolouration (necrotising fasciitis)
Investigations
First-line
Clinical diagnosisCellulitis is primarily a CLINICAL diagnosis — investigations are not required for straightforward cases
Mark the borderDraw around the border of erythema with a skin marker pen and date it — to objectively monitor response to treatment
Bloods (FBC, CRP, U&Es)If systemically unwell (Eron II-IV). Raised WCC and CRP support infection
Second-line
Blood culturesIf febrile, systemically unwell, or severe infection — positive in only ~5% of cellulitis cases
Wound swab / aspirateIf there is an open wound, abscess, or post-surgical site — to guide antibiotic therapy
Specialist
Duplex USSIf DVT cannot be excluded clinically (unilateral leg swelling, risk factors) — cellulitis and DVT can coexist or mimic each other
CT or MRIIf necrotising fasciitis suspected — MRI is more sensitive but CT is faster. Surgical exploration should NOT be delayed for imaging
1
Eron Class I — no systemic toxicity, no comorbidity
- Outpatient oral antibiotics
- Flucloxacillin 500 mg QDS for 5-7 days (first-line)
- Penicillin allergy (non-anaphylaxis): clarithromycin 500 mg BD or doxycycline 200 mg then 100 mg OD
- Elevate the affected limb, mark the erythema border, review within 48 hours
2
Eron Class II — systemically unwell or significant comorbidity
- Consider admission for IV antibiotics
- IV flucloxacillin 1-2 g QDS ± IV benzylpenicillin 1.2 g QDS
- Switch to oral when systemically improving, apyrexial for 48 hours, erythema resolving
3
Eron Class III-IV — severe systemic toxicity or life-threatening
- Urgent admission, senior review, IV antibiotics
- If necrotising fasciitis suspected: URGENT surgical referral — debridement is the definitive treatment
- Empirical broad-spectrum antibiotics including anaerobic cover (e.g. meropenem + clindamycin)
4
Treat the portal of entry
- Treat tinea pedis (athlete's foot) with topical antifungal — prevents recurrence
- Manage chronic venous insufficiency, leg ulcers, lymphoedema
- Emollients to maintain skin integrity
5
Recurrent cellulitis
- Prophylactic antibiotics: phenoxymethylpenicillin (penicillin V) 250 mg BD for 1-2 years if ≥2 episodes per year
- Address underlying risk factors: lymphoedema management, compression, tinea treatment, weight management
Complications
- Abscess formation: Localised collection requiring incision and drainage
- Necrotising fasciitis: Life-threatening deep soft tissue infection — pain out of proportion, rapid progression, crepitus, skin necrosis. Surgical emergency
- Sepsis: Cellulitis can progress to bacteraemia and septic shock
- Recurrence: Common, especially with persistent risk factors (lymphoedema, tinea pedis). ~30% recurrence rate
- Lymphoedema: Recurrent cellulitis damages lymphatics, worsening lymphoedema and creating a vicious cycle
UKMLA Exam Tips
- 1Flucloxacillin is first-line for cellulitis — covers both Staph aureus and Strep pyogenes
- 2Mark the erythema border with a pen — this is the standard clinical approach to monitoring response
- 3Erysipelas = well-demarcated, raised edges, superficial. Cellulitis = poorly demarcated, deeper
- 4Pain OUT OF PROPORTION to signs = necrotising fasciitis until proven otherwise — surgical emergency
- 5Tinea pedis is the most common portal of entry for lower limb cellulitis — always examine between the toes
- 6Bilateral lower limb cellulitis is RARE — consider alternative diagnoses (venous eczema, lipodermatosclerosis, DVT)
- 7Periorbital (preseptal) vs orbital cellulitis: orbital = pain on eye movement, proptosis, reduced visual acuity — emergency
practicetest your knowledge on cellulitisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — infectious diseases and beyond.
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