🦵 Cellulitis is an acute bacterial infection of the dermis and subcutaneous tissue causing hot, tender, spreading erythema. It is usually unilateral and most often due to Streptococcus pyogenes (non-purulent cellulitis) or Staphylococcus aureus (more likely if purulent/abscess). “Bilateral red legs” are common in practice, but if both legs are red and cool/normal temperature (or mainly itchy/scaly), cellulitis is less likely-think venous eczema/stasis dermatitis or lipodermatosclerosis.

🧠 Pathophysiology (why it looks the way it does)

Entry point (often tiny): toe-web fissures (tinea pedis), eczema cracks, ulcers, insect bites, trauma, injection sites.
Innate immune response drives erythema, warmth and swelling: vasodilatation + capillary leak → oedema; neutrophil recruitment → tenderness.
Lymphatic involvement is common: lymphangitis (red streaks) and reactive lymphadenopathy; recurrent episodes can damage lymphatics → chronic oedema → higher recurrence risk.

🦠 Common pathogens

Non-purulent cellulitis: usually β-haemolytic streptococci (esp. Group A).
Purulent infection/abscess: more likely Staphylococcus aureus.
Diabetic/ischaemic limb or foul-smelling wounds: consider polymicrobial infection (including anaerobes) and seek senior/micro advice.

⚠️ Risk factors

Skin barrier disruption: eczema, psoriasis, ulcers, athlete’s foot, trauma, surgery.
Chronic oedema/lymphoedema, chronic venous insufficiency, obesity.
Diabetes, peripheral arterial disease, immunosuppression, liver/renal disease.
Previous cellulitis episode (risk of recurrence increases).

Cellulitis of the leg showing erythema and swelling

👩‍⚕️ Clinical features

Acute onset of unilateral erythema, warmth, swelling, and tenderness.
Ill-defined margins are typical (erysipelas is usually more sharply demarcated).
Possible systemic features: fever, rigors, malaise, tachycardia.
Look for: toe-web maceration (tinea), eczema, ulcers, bite marks, wounds, foreign body.
🖊️ Practical tip: mark the edge with a pen + date/time to monitor spread.

🧩 Differential diagnosis (common mimics)

Venous eczema / stasis dermatitis (often bilateral, itchy, scaly, chronic oedema, less tender/less hot).
Lipodermatosclerosis (chronic venous disease; “inverted champagne bottle” legs; painful, indurated, brown haemosiderin staining).
DVT (unilateral swelling/pain; skin may be less erythematous/hot than cellulitis; consider Wells + ultrasound if uncertain).
Superficial thrombophlebitis (linear tender cord along vein).
Gout/pseudogout (very focal joint pain/swelling; systemic upset can mimic infection).
Contact dermatitis/drug eruption (itch predominant; often sharply demarcated; vesicles).
Erythema nodosum (tender nodules, usually shins; systemic triggers).
🚨 Serious mimics: necrotising fasciitis, septic arthritis, osteomyelitis.

🚨 Red flags (think “not simple cellulitis”)

Pain out of proportion, rapidly progressive swelling/erythema, skin necrosis, bullae, crepitus.
Marked systemic illness: hypotension, confusion, persistent tachycardia, high NEWS2.
Failure to improve after 24–48 hours of appropriate antibiotics (some early progression can occur, but trend should stabilise).
High-risk sites: face/orbit, hand, perineum.
Immunocompromise, severe PAD/critical limb ischaemia, diabetic foot with deep infection signs.

🧪 Diagnostic approach (UK practical)

Cellulitis is largely a clinical diagnosis.
Record observations (temperature, HR, BP, RR, SpO₂) and assess for sepsis.
Check limb neurovascular status if swelling is marked.
Assess for abscess (fluctuance/pus) - if present, incision & drainage may be key.

🔬 Investigations

Bloods: FBC, CRP (helpful for severity/trend), U&E (antibiotic dosing/AKI risk), glucose/HbA1c if relevant.
Blood cultures: consider if febrile/systemically unwell or immunocompromised.
Swab: only if there is a wound, ulcer, or discharge/pus.
Ultrasound: if abscess suspected or DVT is a realistic alternative diagnosis.
X-ray: if concern for foreign body, gas, or osteomyelitis (clinical context dependent).

📊 Cellulitis severity (Eron classification) + what it means

Class	Typical features	Practical management
I	No systemic toxicity; no significant comorbidity affecting treatment.	Oral antibiotics + self-care; safety-net and review if not improving.
II	Systemically unwell or significant comorbidity (e.g., diabetes, venous disease, chronic oedema).	Consider IV/oral depending on severity and ability to take PO; consider SDEC/ambulatory IV.
III	Marked systemic upset (e.g., tachycardia, tachypnoea, hypotension) or unstable comorbidities.	Hospital assessment; IV antibiotics; monitor closely; exclude deep infection.
IV	Sepsis or life-threatening infection (e.g., necrotising fasciitis).	Resus + urgent senior/surgical input; broad-spectrum IV antibiotics per local policy.

🛏️ Supportive care (often forgotten, but high impact)

🦵 Elevate the affected limb to reduce oedema and pain.
🧦 Manage swelling: consider compression once acute infection is improving (and only if arterial supply is adequate).
💊 Analgesia (paracetamol ± NSAID if appropriate) and encourage fluids.
🧼 Treat the portal of entry: antifungal for tinea pedis, emollients/steroid for eczema when infection controlled, wound care for ulcers.
🖊️ Mark borders and document size/appearance; photos can help follow progression.

💊 Antibiotic treatment (adult) - typical UK first line

Scenario	First choice	Penicillin allergy (examples)	Notes
Mild–moderate cellulitis (oral)	Flucloxacillin (PO)	Clarithromycin (PO) (or doxycycline depending on local guidance/patient factors)	Review if not improving within 2–3 days; course length commonly 5–7 days, extend if slow response.
Severe/systemically unwell (IV)	Flucloxacillin (IV)	IV alternative (e.g., clarithromycin/other per local guideline)	Switch to oral when clinically improving; reassess diagnosis if deterioration.
Purulence/abscess suspected	Drainage + antibiotics guided by severity	As above	Source control matters: fluctuance/pus → think abscess rather than “simple” cellulitis.

🏥 When to admit / seek urgent specialist input

Sepsis features or Eron class III–IV.
Concern for necrotising fasciitis, deep abscess, septic arthritis, osteomyelitis.
Unable to tolerate oral antibiotics, significant vomiting, poor social support, or rapid progression.
High-risk anatomical sites (face/orbit/hand/perineum) or immunocompromised patient.

🔁 Recurrent cellulitis (prevention)

Address modifiable risks: treat tinea pedis, optimise eczema care, reduce chronic oedema (compression/lymphoedema input), weight management.
Consider referral for venous/lymphoedema assessment if recurrent episodes or chronic swelling.
Some patients with frequent recurrences may need prophylactic antibiotics via local/lymphoedema pathways.

📚 References (UK)

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Cellulitis and Erysipelas ✅