Definition

Cellulitis is an acute bacterial infection of the dermis and subcutaneous tissue, usually spreading rapidly with poorly demarcated edges.

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Common Causative Organisms

• Streptococcus pyogenes (Group A strep) – most common.

• Staphylococcus aureus – especially when abscess or purulence is present.

• MRSA – consider in recurrent, hospital-acquired, or IV drug users.

💡 PLAB Gem: If cellulitis has pus, boils, or abscesses → think Staph aureus.
If diffuse and spreading with lymphangitis → think Streptococcus pyogenes.

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Risk Factors

• Skin break: trauma, ulcer, surgery, insect bite.

• Lymphatic obstruction, venous insufficiency.

• Diabetes mellitus.

• Immunosuppression.

💡 Exam Tip: In PLAB, diabetics with foot ulcers are a classic scenario for cellulitis.

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Clinical Features

• Local signs: erythema, warmth, swelling, tenderness.

• Poorly demarcated edges (contrast with erysipelas, which has sharp borders).

• Systemic symptoms: fever, malaise, chills.

• May have regional lymphadenopathy.

⚠️ Red flag: rapidly spreading cellulitis with systemic toxicity → consider necrotising fasciitis.

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Differential Diagnosis

• Erysipelas – raised, well-demarcated edges (usually Group A strep).

• DVT – painful, swollen leg but no fever.

• Venous eczema – itching, not tender, chronic.

• Necrotising fasciitis – severe pain out of proportion, systemic toxicity, rapid progression.

💡 PLAB Trap: If the exam vignette says “very painful, rapidly spreading, crepitus or necrosis” → DO NOT choose cellulitis. Correct answer = necrotising fasciitis.

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Investigations (not always needed in mild cases)

• Usually clinical diagnosis.

• Blood cultures if systemic toxicity or immunocompromised.

• Swab of wound/pus if present.

• Imaging rarely required, unless to exclude deeper infection.

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Management

General

• Elevate affected limb.

• Analgesia.

Antibiotics (UK/PLAB guidance)

• Mild (no systemic symptoms):
  → Oral flucloxacillin (first-line).
  → If penicillin-allergic: Clarithromycin or Doxycycline.

• Moderate–severe (systemic features, rapid spread, facial cellulitis, immunocompromised):
  → IV flucloxacillin (hospital).
  → If penicillin-allergic: IV vancomycin or clindamycin.

• MRSA suspected: use vancomycin, linezolid, or daptomycin (depending on scenario).

💡 PLAB Gem:

• Flucloxacillin is the first-line oral antibiotic for cellulitis.

• Co-amoxiclav if facial cellulitis (covers anaerobes from mouth).

• Penicillin allergy → clarithromycin.

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Complications

• Abscess formation.

• Sepsis.

• Necrotising fasciitis.

• Recurrent cellulitis → chronic lymphoedema.

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PLAB-Style Question Examples

Q1. A 50-year-old diabetic man presents with a warm, red, swollen lower leg. He has a fever of 38.2°C but is otherwise stable. What is the most appropriate antibiotic?

• A. Oral flucloxacillin

• B. Oral co-amoxiclav

• C. Oral clarithromycin

• D. IV vancomycin

• E. IV clindamycin

✅ Answer: A. Oral flucloxacillin
(Stable patient, no systemic toxicity → oral flucloxacillin first-line.)

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Q2. A 45-year-old man presents with a red, swollen leg that is exquisitely tender. The pain is disproportionate to the skin changes. His BP is 85/50 mmHg and HR is 120. What is the most likely diagnosis?

• A. Cellulitis

• B. Erysipelas

• C. DVT

• D. Necrotising fasciitis

• E. Septic arthritis

✅ Answer: D. Necrotising fasciitis
(Severe pain out of proportion + hypotension + systemic toxicity = nec fasc, not cellulitis.)

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Key PLAB Takeaways

• Flucloxacillin = 1st line for cellulitis.

• Clarithromycin/doxycycline if penicillin allergic.

• Co-amoxiclav for facial cellulitis.

• Think nec fasc if pain >> skin findings or systemic shock.

• Differentiate cellulitis (poor margins) vs erysipelas (clear margins).