Cellulitis
Summary
Cellulitis is a common bacterial skin infection affecting the dermis and subcutaneous tissue. It typically presents with erythema, warmth, swelling, and pain, most commonly on the lower limbs. Common pathogens are Streptococcus pyogenes and Staphylococcus aureus. Management involves antibiotics (flucloxacillin first-line), limb elevation, and marking the extent of erythema. Recognition of necrotising fasciitis is critical.
Key Facts
- Definition: Bacterial infection of dermis and subcutaneous tissue
- Incidence: Common; 200+ per 100,000 per year
- Pathognomonic: Erythema + warmth + swelling + tenderness
- Gold Standard Investigation: Clinical diagnosis
- First-line Treatment: Flucloxacillin 500mg-1g QDS
- Prognosis: Good with treatment; recurrence common
Clinical Pearls
Red Flag Pearl: Crepitus, pain out of proportion, rapid spread = necrotising fasciitis - surgical emergency.
Eron Pearl: Eron classification guides IV vs oral treatment.
Bilateral Pearl: True bilateral cellulitis is rare - consider venous eczema.
| Class | Features | Treatment |
|---|---|---|
| I | No systemic toxicity, no comorbidities | Oral flucloxacillin |
| II | Systemic toxicity OR comorbidities | IV flucloxacillin |
| III | Significant toxicity or limb-threatening | IV + specialist input |
| IV | Necrotising fasciitis | Emergency surgery |
Algorithm
Treatment
| Severity | Antibiotic |
|---|---|
| Mild | Flucloxacillin 500mg-1g QDS PO x5-7d |
| Moderate/Severe | Flucloxacillin 1-2g QDS IV |
| Penicillin allergy | Clarithromycin or doxycycline |
Supportive
- Mark erythema extent
- Elevate limb
- Treat underlying cause (tinea, lymphoedema)
-
NICE guideline NG141. Cellulitis and erysipelas: antimicrobial prescribing. 2019.
-
Eron LJ et al. Managing skin and soft tissue infections. J Antimicrob Chemother. 2003;52(Suppl 1):i3-i17. PMID: 14662805
Viva Points
"Cellulitis: Strep/Staph infection. Flucloxacillin first-line. Eron classification guides IV vs oral. Red flags for necrotising fasciitis: crepitus, pain out of proportion, rapid spread."
Last Reviewed: 2026-01-01 | MedVellum Editorial Team