Key Facts

Cellulitis is one of the most common presentations to primary and emergency care. Correct diagnosis, appropriate antibiotic selection, and identification of complications are essential. Recurrence is common (~30%) and preventive measures including treating tinea pedis and managing lymphoedema are important.

Epidemiology

• ~80,000–100,000 hospital admissions/year for cellulitis in England
• Lower limb accounts for ~70–80% of cases
• Recurrence rate: ~20–30% within 3 years
• More common in older adults, obese patients, and those with lymphoedema

Aetiology

• Group A Streptococcus (S. pyogenes): most common cause overall
• S. aureus: more likely if purulent/abscess component
• Portal of entry: tinea pedis (most common), leg ulcers, wounds, insect bites, eczema
• Special situations: MRSA (IV drug users, healthcare-associated); water exposure (Aeromonas, Vibrio); cat/dog bite (Pasteurella); immunocompromised (Gram-negatives, fungi)

Pathophysiology

• Bacteria enter through break in skin barrier → spread through dermis and subcutaneous tissue
• Bacterial enzymes (hyaluronidase, streptokinase) facilitate tissue spread
• Inflammatory response: vasodilation, oedema, neutrophil infiltration → clinical features of erythema, warmth, swelling, pain
• Lymphatic damage from recurrent cellulitis → lymphoedema → further cellulitis (vicious cycle)

Typical Presentation

• Unilateral erythema with poorly defined margin
• Warmth, swelling, tenderness
• Usually lower limb
• Fever, malaise in moderate-severe cases
• Regional lymphadenopathy

Erysipelas (superficial cellulitis)

• Well-defined, raised erythematous margin
• Typically facial or lower limb
• More sharply demarcated than deep cellulitis
• Streptococcal aetiology (almost always)

Red Flags (suggesting necrotising fasciitis or severe sepsis)

• Pain out of proportion to clinical signs
• Rapidly spreading erythema despite antibiotics
• Crepitus (gas gangrene)
• Skin necrosis, bullae, ecchymosis
• Systemic toxicity: high fever, tachycardia, hypotension
• Altered consciousness

Bedside

• Mark the erythema margin with pen (date and time)
• Observations: temperature, HR, BP, NEWS2
• Blood glucose: undiagnosed diabetes

Bloods

• FBC: leucocytosis
• CRP: elevated; useful for monitoring response
• U&Es: if sepsis or IV antibiotics anticipated
• Blood cultures: only if systemically unwell (Eron III–IV); positive in <5% of cellulitis
• HbA1c: if diabetes suspected

Imaging

• Not routinely required for uncomplicated cellulitis
• Doppler USS: if DVT cannot be excluded clinically
• USS: if abscess suspected (localised fluctuance)
• CT/MRI: if necrotising fasciitis suspected — gas in tissues, fascial thickening

Special Tests

• Wound swab: only if open wound or purulent discharge — not from intact skin
• Skin surface swab: not recommended (low yield)
• Interdigital examination: check for tinea pedis (portal of entry)

Non-pharmacological

• Elevation: affected limb above heart level to reduce oedema
• Mark erythema margins: monitor progression/response
• Rest: especially if lower limb
• Treat portal of entry: tinea pedis (topical antifungal), leg ulcers, eczema
• Emollients: for surrounding dry skin
• Compression: may be needed after acute phase if lymphoedema

Pharmacological

Eron Class I (mild — no comorbidities, systemically well):

• Flucloxacillin 500mg–1g QDS oral for 5–7 days (NICE NG141)
• Penicillin allergy: clarithromycin 500mg BD or doxycycline 200mg then 100mg OD

Eron Class II (systemic illness OR comorbidities):

• Oral flucloxacillin 1g QDS + consider adding phenoxymethylpenicillin 500mg QDS (Strep cover)
• Close review at 24–48 hours

Eron Class III (significant systemic toxicity):

• IV flucloxacillin 1–2g QDS ± IV benzylpenicillin 1.2g QDS
• Switch to oral when improving (OPAT may facilitate earlier discharge)

Eron Class IV (sepsis or necrotising fasciitis):

• IV flucloxacillin 2g QDS + IV clindamycin 600mg QDS (clindamycin inhibits toxin production)
• Urgent surgical review for necrotising fasciitis

MRSA suspected:

• Add doxycycline 200mg then 100mg OD or trimethoprim (guided by local sensitivities)
• IV: vancomycin 15–20mg/kg BD

Recurrence prevention:

• Treat tinea pedis, manage lymphoedema (compression)
• Prophylactic antibiotics: phenoxymethylpenicillin 250mg BD long-term if ≥2 episodes/year (NICE NG141, PATCH trial)

Surgical/Interventional

• Incision and drainage: if concurrent abscess
• Surgical debridement: necrotising fasciitis (emergency)

Referral Criteria

• Hospital admission: Eron III–IV, failed oral therapy, diagnostic uncertainty
• Surgery: suspected necrotising fasciitis (urgent)
• Vascular: recurrent cellulitis with chronic venous insufficiency
• Dermatology: if diagnostic uncertainty persists

• Uncomplicated cellulitis: excellent prognosis with appropriate antibiotics
• Expected improvement within 48–72 hours (erythema may initially spread before improving — common in first 24 hours)
• Recurrence: 20–30% within 3 years
• Prophylactic penicillin (PATCH trial): reduces recurrence by ~45%
• Necrotising fasciitis: mortality 20–40% even with treatment
• Post-cellulitis lymphoedema: may develop or worsen (perpetuating cycle)
• Hospital admission ~2–5 days average; OPAT reduces length of stay

Eron Classification of Cellulitis Severity

PATCH Trial (Prophylactic Antibiotics for Cellulitis)