Cellulitis

Overview

An acute bacterial infection of the deep dermis and subcutaneous tissue, usually Streptococcus pyogenes or Staphylococcus aureus. Classically a unilateral, hot, tender, spreading erythema of the lower leg. Erysipelas is the more superficial, sharply-demarcated variant. The key job is to spot the spreading infection that needs urgent surgery — necrotising fasciitis.

Recognise

• Unilateral, hot, tender, poorly-demarcated spreading erythema with swelling; often a lower limb
• A portal of entry (tinea pedis, leg ulcer, wound, eczema); systemic features (fever, malaise) in more severe disease
• Erysipelas: well-demarcated, raised, bright-red, often the face — more superficial

Differentials & how to tell them apart

Clinical image

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Investigations

Clinical diagnosis + severity by the Eron classification. Bloods (FBC, CRP, U&E) and blood cultures if systemically unwell; mark the erythema margin to track progression; look for and treat the portal of entry.

Management

Oral flucloxacillin first-line (IV if Eron III–IV / systemically unwell)

1. 1Mark the margin, elevate the limb, treat the portal of entry. Mild (Eron I) → oral flucloxacillin (clarithromycin/doxycycline if penicillin-allergic).Gate: Pain out of proportion to the appearance, rapid spread, crepitus or systemic toxicity = necrotising fasciitis → urgent SURGICAL referral and broad-spectrum IV antibiotics, NOT oral therapy and wait
2. 2Severe (Eron III–IV), systemically unwell, facial, or comorbid → admit for IV antibiotics; facial cellulitis → co-amoxiclav.
3. 3Recurrent cellulitis → treat tinea pedis/lymphoedema; consider antibiotic prophylaxis.

Monitor & prognosis

Margin progression, observations, CRP, response within 48h.

Good with prompt antibiotics; recurrence if the portal/lymphoedema is untreated.