Cellulitis

cellulitis · skin and soft tissue infection · SSTI

One of the most over-diagnosed conditions in the ED - and in darker skin, simultaneously one of the most under-diagnosed.

GPBand C

Red flags

Pain out of proportion to the appearance, rapid progression, crepitus, haemorrhagic bullae or skin anaesthesia - necrotising soft tissue infection, a surgical emergency
Sepsis, hypotension or a rising lactate
Periorbital or orbital involvement - a different pathway, needing urgent imaging and ENT or ophthalmology
Failure to improve on appropriate antibiotics - reconsider the diagnosis before escalating the antibiotic
Lymphangitis, or a possible uncommon pathogen after a penetrating injury, exposure to water-borne organisms, or an infection acquired outside the UK - both are explicit NG141 1.1.14 referral triggers, and lymphangitis is easy to note and then not act on
A baby under 1 month - NG141 table 2 gives no antibiotic choice for this age group at all: antibiotic choice is based on specialist advice
Immunosuppression, diabetes with a foot ulcer, or a prosthetic joint beneath the affected area

Recognise

On light skin

Diffuse warm, tender, spreading erythema with a poorly defined edge, sometimes with oedema, blistering or lymphangitis, in a patient who may be febrile.

On brown and black skin

Erythema is the entire basis of the textbook description and it is frequently invisible in brown and black skin. Inflamed skin may appear darker, violaceous, greyish or simply shiny rather than red, and this produces error in both directions: cellulitis is missed and treated late, and once suspected it is harder to judge whether it is improving. Use the tone-independent signs - warmth compared with the same site on the other limb, swelling, tenderness and induration - and use the back of your hand to compare temperature side by side. When you mark the margin, use a skin marker and note the anatomical landmark in the notes as well, because a margin drawn on skin whose colour you cannot see is hard for the next clinician to interpret. Photograph with consent where local policy allows, since serial comparison is more reliable than recalled colour.

In any skin tone

Unilateral - bilateral lower-limb cellulitis is rare, and bilateral disease is usually venous eczema or stasis change
Warmth, swelling and tenderness, with a poorly demarcated advancing edge
Look for a portal of entry: tinea pedis between the toes, a wound, an ulcer, an insect bite, eczema
Fever and systemic upset support the diagnosis but are often absent
Pain out of proportion, rapid progression, crepitus or dusky change means stop and consider necrotising infection

Distribution

Almost always unilateral, most often a lower limb, with a diffuse and poorly demarcated margin. Bilateral lower-limb cellulitis is rare and should make you doubt the diagnosis.

Photographs

Early cellulitis of the right foot, photographed beside the unaffected left. The comparison is the examination. The right foot is swollen ac — Early cellulitis of the right foot, photographed beside the unaffected left. **The comparison is the examination.** The right foot is swollen across the dorsum and ankle, the skin is tight and shiny, and there is a patch of erythema over the mid-foot - all of which are far easier to call with the normal limb in the same frame. On brown and black skin the redness may be absent entirely and swelling, warmth, shininess and tenderness carry the diagnosis, so always expose and compare both legs.
Fitzpatrick II (Fitzpatrick type estimated from the photograph, not stated by the source) · cbinrva · Wikimedia Commons - Cellulitis of lower leg and foot.jpg · CC BY-SA 4.0

Provenance for every photograph in this library: image licences. How much of the corpus still has no darker-skin photograph: skin tone coverage.

Mimics

Necrotising fasciitis and necrotising soft tissue infection — The one that kills. Early necrotising infection looks exactly like cellulitis - disproportionate pain and rapid progression are the discriminators
Erysipelas — Erysipelas is more superficial, with a sharply demarcated raised edge, and is often facial
IgA vasculitis (Henoch-Schonlein purpura) — Palpable purpura over the legs is vasculitis, not infection
Tinea corporis, cruris and pedis — Tinea pedis is frequently the portal of entry and must be treated as well, or the cellulitis recurs
Diabetic foot ulcer and deep-seated foot infection — A hot swollen foot in a person with diabetes is a diabetic foot problem, not simple cellulitis - and acute Charcot arthropathy mimics infection exactly, often with the skin intact

What to do in the ED

Ask first whether this is cellulitis at all. Bilateral, chronic, itchy or non-tender presentations are usually venous eczema, lipodermatosclerosis or a deep vein thrombosis, and antibiotics will not help any of them. But that reasoning applies to a well patient only. Fever, tachycardia, a rising lactate or systemic upset overrides the pattern - bilateral cellulitis does occur, venous eczema and stasis change can be superinfected, and NG141 makes systemic illness a referral trigger whatever the distribution. Treat and escalate first, and settle the diagnosis afterwards
Consider and exclude necrotising infection before settling on cellulitis
Mark the margin with a skin marker, record the time and describe it against an anatomical landmark rather than by colour alone
Treat per NICE NG141 and local policy, choosing oral or intravenous route on severity rather than on appearance
Look for and treat the portal of entry, particularly interdigital tinea pedis - failing to do so is the commonest reason cellulitis recurs
Elevate the limb and give analgesia; these are part of treatment, not an afterthought
Arrange review within 48 hours for anyone managed at home, and say explicitly what failure to improve should prompt

Disposition

Oral antibiotics and planned review for uncomplicated disease in a well patient. Admit or refer for intravenous therapy for systemic upset, rapid progression, failure of oral treatment, significant comorbidity, or any suspicion of a deeper infection.

Safety-netting

Return immediately if the redness or swelling spreads beyond the marked line, if the pain becomes severe or out of keeping with how the skin looks, if the skin blisters or turns dusky or purple, if you develop a fever, shivering or feel very unwell, or if there is no improvement within 48 hours of starting antibiotics. Keep the limb raised whenever you can.

Local variation

Ambulatory intravenous antibiotic services and community pathways vary widely; know what yours accepts before committing to admission.

Sources

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