Why Cellulitis Is Examined
Cellulitis is one of the most common skin infections requiring hospital admission, accounting for over 100,000 admissions in England annually. OSCEs examine it in prescribing stations, clinical reasoning (this patient has a swollen red leg — what is your management?), and acute assessment. Examiners test Eron classification, correct antibiotic choice, the DVT differential, and recognition of necrotising fasciitis — a surgical emergency.
⚠️ Red Flag
The most important differential in a red, swollen, painful limb is necrotising fasciitis. This is a deep fascial infection with mortality above 20% if surgical debridement is delayed. Features: pain disproportionate to appearance, wooden hard induration, rapid progression, septic shock, skin necrosis or gas on imaging. When suspected — call surgery immediately.
Pathophysiology and Common Organisms
Cellulitis is a bacterial infection of the dermis and subcutaneous tissues. Entry points: skin breaches, tinea pedis (athlete's foot — most common predisposing factor), leg ulcers, bites, wounds, IV drug use.
Organisms:
- Beta-haemolytic Streptococcus (Group A, C, G) — most common
- Staphylococcus aureus — including MRSA (particularly post-trauma, abscess-associated)
- Gram-negative organisms — immunocompromised, diabetic, water-exposure injuries
Eron Classification — Severity Staging
| Class | Features | Setting |
|---|---|---|
| I | No systemic features; no comorbidity | Community oral antibiotics |
| II | Systemically unwell (mild) OR significant comorbidity (lymphoedema, morbid obesity, venous insufficiency, diabetes) | Hospital or OPAT (outpatient parenteral) |
| III | Significantly systemically unwell (tachycardia, hypotension, raised inflammatory markers) OR severe comorbidity | Hospital IV antibiotics |
| IV | Sepsis or life-threatening infection (necrotising fasciitis) | ITU + urgent surgical debridement |
Antibiotic Treatment (PHE/NICE Guidelines)
Class I — Oral, Community
| Drug | Dose | Duration |
|---|---|---|
| Flucloxacillin | 500 mg QDS | 5-7 days |
| Cefalexin (if penicillin allergy — not anaphylaxis) | 500 mg QDS | 5-7 days |
| Clarithromycin (if penicillin anaphylaxis) | 500 mg BD | 5-7 days |
Class II-III — IV, Hospital
| Drug | Dose | Duration |
|---|---|---|
| Flucloxacillin | 1-2 g QDS IV | Until clinical improvement, then step down to oral |
| Co-amoxiclav | 1.2 g TDS IV | If atypical organisms, bites, water exposure, or perianal |
| Vancomycin | 15-20 mg/kg BD IV | If MRSA suspected or penicillin anaphylaxis + Gram-positive organisms |
💡 Tip
OPAT (Outpatient Parenteral Antibiotic Therapy) allows IV antibiotics at home or in a clinic for stable Class II-III cellulitis. This avoids unnecessary admission and is increasingly used in the NHS. Ertapenem once daily is a common OPAT agent for cellulitis.
Differential Diagnosis — Key Mimics
| Diagnosis | Features distinguishing from cellulitis |
|---|---|
| DVT | No erythema (or minimal); tenderness along deep vein; systemic features absent; Wells DVT score positive |
| Lipodermatosclerosis | Chronic; inverted champagne bottle deformity; woody induration; bilateral; no fever |
| Venous eczema | Bilateral; itchy; scaling; varicose veins; no fever or systemic features |
| Gout/pseudogout | Crystal arthropathy — joint effusion; previous attacks; hyperuricaemia |
| Necrotising fasciitis | Disproportionate pain; bullae, skin necrosis, crepitus; rapid progression; shock |
| Erysipelas | Superficial — sharply demarcated raised erythematous plaque; usually face or lower leg; caused by Group A Streptococcus; treat with phenoxymethylpenicillin |
Monitoring and Response Assessment
Mark the leading edge of erythema with a skin marker at assessment — essential for monitoring whether the infection is spreading or resolving. Document time and reassess at 24-48 hours.
| Response | Action |
|---|---|
| Erythema retreating, systemic features improving | Continue current antibiotic, step down to oral when stable |
| Erythema static at 48-72 hours | Consider broadening cover; review for abscess (USS); consider MRSA |
| Erythema advancing at 24-48 hours | Escalate urgently — consider necrotising fasciitis |
| Rapid deterioration, sepsis features | Call surgery; urgent MRI/CT; theatre as emergency |
Predisposing Factors and Prevention
Identify and treat the entry point:
- Tinea pedis (commonest) — treat with topical antifungal (clotrimazole) or oral terbinafine
- Leg ulcers — wound care, dressings
- Lymphoedema — compression therapy, skin care
Prophylactic antibiotics for recurrent cellulitis:
- Four or more episodes per year in the same limb — consider phenoxymethylpenicillin 250 mg BD long-term (PATCH trials)
- Also treat underlying predisposing factors
Frequently Asked Questions
"What is the difference between cellulitis and erysipelas?"
Erysipelas is a superficial skin infection involving the upper dermis and superficial lymphatics — caused almost exclusively by Group A Streptococcus (Streptococcus pyogenes). It has a characteristically sharply demarcated, raised, bright red plaque, often on the face or lower leg. Cellulitis involves the deeper dermis and subcutaneous tissue — the borders are less well-defined and it is often more painful. Both are treated with penicillin-based antibiotics, but erysipelas may respond faster.
"Why is tinea pedis the most common predisposing factor for leg cellulitis?"
Tinea pedis (athlete's foot) causes maceration and fissuring of the interdigital skin, creating a portal of entry for bacteria colonising the skin surface. Even small cracks or fissures between the toes are sufficient to allow Streptococcus or Staphylococcus to invade the dermis. Treating tinea pedis with topical antifungal agents significantly reduces the recurrence rate of cellulitis in patients with repeated episodes.
"How do you distinguish bilateral leg swelling from cellulitis?"
Cellulitis is almost always unilateral — bilateral leg swelling strongly suggests a systemic cause (heart failure, venous insufficiency, lymphoedema, hypoalbuminaemia, drugs) rather than infection. When a patient presents with bilateral erythema and swelling, consider bilateral lipodermatosclerosis, venous eczema, or stasis dermatitis — which are inflammatory but not infectious conditions and are overtreated with antibiotics. Cellulitis in a lymphoedematous limb may appear more symmetrical — use fever, systemic features, and local warmth to guide diagnosis.
"What clinical features suggest necrotising fasciitis rather than cellulitis?"
Necrotising fasciitis (NF) should be suspected when: pain is out of proportion to the visible skin changes; there is a wooden hard induration extending beyond the erythema; skin changes progress rapidly (hours); bullae or skin necrosis develop; there is crepitus (gas in soft tissues — pathognomonic); the patient develops septic shock. The Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score combines CRP, WCC, haemoglobin, sodium, creatinine, and glucose to stratify risk. Any suspicion = urgent surgical review. MRI is the most sensitive imaging; CT can show gas in soft tissues.
"When should blood cultures be taken in cellulitis?"
NICE guidance recommends blood cultures only if the patient is systemically unwell (Class III or IV), immunocompromised, or if the infection has not responded to initial antibiotics. Routine blood cultures in mild uncomplicated cellulitis have a very low yield (under 4%) and are not recommended. Wound swabs are useful only if there is an open wound, abscess, or ulcer — intact cellulitis skin swabs are not helpful.
Related Posts
- Deep Vein Thrombosis OSCE — differentiating DVT from cellulitis in the swollen leg
- Sepsis and NEWS Score OSCE — recognising septic cellulitis and escalation criteria
- Wound Care and Suturing OSCE — managing skin breaches that predispose to cellulitis