🧭 When to suspect

Cellulitis is an acute bacterial infection of the deep dermis and subcutaneous tissue, usually entering through a break in the skin. Erysipelas is the more superficial form (upper dermis and superficial lymphatics) with a sharply demarcated, raised border, classically caused by Streptococcus pyogenes and often affecting the face. Both are managed identically and are most commonly caused by Streptococcus pyogenes and Staphylococcus aureus.

The classic picture is an acute, unilateral area of skin that is painful, hot, swollen and red, with a poorly demarcated border, usually on the lower limb, often with a visible portal of entry (athlete's foot, a wound, an ulcer or eczema). The patient may feel generally unwell with fever and rigors. Note that redness is less visible on brown or black skin, where warmth, swelling and tenderness become the more reliable signs.

Two skills matter in primary care: first, confirm it really is cellulitis – the red leg has many mimics and is the site where cellulitis is most often over-diagnosed; and second, stratify severity (the Eron classification) to decide between oral treatment in the community and intravenous antibiotics or admission.

Eron class	Typical features	Action / setting
Class I	No systemic toxicity; no uncontrolled comorbidity	Oral antibiotics – manage in the community
Class II	Systemically unwell, or well but with a comorbidity (diabetes, venous insufficiency, obesity, PAD) that may delay resolution	Oral antibiotics; consider IV / OPAT per local pathway
Class III	Significant systemic upset (acute confusion, tachycardia, tachypnoea, hypotension), unstable comorbidity, or limb-threatening infection	Admit for IV antibiotics
Class IV	Sepsis or life-threatening infection (e.g. necrotising fasciitis)	Emergency admission – urgent surgical / critical care

Raise suspicion in anyone with a break in the skin (cut, graze, bite, ulcer, tinea pedis), an underlying skin condition (eczema, psoriasis, leg ulcer), chronic oedema or lymphoedema, venous insufficiency, diabetes, obesity, immunosuppression, or previous cellulitis.

Source: NICE NG141 · DermNet

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