Skip to textbook content
Doctor’s PassportTextbook
TopicsAll topics

Loading topic navigation…

Textbook navigation
TopicsAll topics

Loading topic navigation…
← Dermatology indexChapter sections
OverviewAetiologyPathophysiologyClinical features and red flagsInvestigationsDifferential diagnosisManagementComplicationsMonitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
Full textbook13 sectionsOpen only what you need
Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Cellulitis and erysipelas

Dermatology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

RapidFull
!
Disproportionate pain may signal necrotising infection

Rapid progression, severe pain beyond visible change, crepitus, bullae, skin anaesthesia, necrosis or systemic shock is not uncomplicated cellulitis.

Action: Start ABCDE and sepsis care, obtain immediate surgical review and give broad intravenous antimicrobials without delaying operative source control for imaging.

Overview

Recognise acute bacterial skin and soft-tissue infection, exclude common mimics and limb-threatening disease, choose oral or intravenous antibiotics by severity and review response within the expected trajectory.

Assess the whole patient before the limb. Record temperature, pulse, blood pressure, respiratory rate, oxygen saturation, mental state, hydration and pain. Expose and inspect the full area with consent, looking between toes and beneath dressings for a portal and palpating for fluctuance, crepitus and joint-centred tenderness. Erythema may be muted or violaceous in darker skin, so warmth, oedema, tenderness and edge progression carry extra weight.

Most uncomplicated cases can take oral antibiotics. Site and exposure matter: infection near eyes or nose needs co-amoxiclav-based cover and specialist threshold; bites, water exposure, diabetic foot disease and surgical wounds follow different guidance. A penicillin-allergy label must be characterised because flucloxacillin is the preferred narrow treatment and vague intolerance should not automatically drive broader antibiotics.

Review expectations explicitly. Fever and pain should begin settling within two to three days, although colour and swelling can persist beyond the antibiotic course. Worsening after 48 hours, severe pain, new blistering, systemic illness or an expanding collection triggers reassessment of diagnosis, depth, adherence and microbiology. Repeated cellulitis needs oedema and portal management rather than prophylaxis by default.

Aetiology

01

Streptococcal entry

Beta-haemolytic streptococci enter through fissures, wounds, ulcers or often an unapparent break and spread through dermis and superficial lymphatics.

02

Staphylococcal contribution

Staphylococcus aureus is more likely with penetrating trauma, purulence, ulceration or an abscess, which must be sought because drainage changes management.

03

Barrier and lymphatic risk

Tinea pedis, eczema, oedema, lymphoedema, venous disease, obesity and previous cellulitis impair barrier or local immune clearance and promote recurrence.

04

Exposure-specific organisms

Animal or human bites, fresh or salt water, injection, surgery and immune compromise broaden microbiology and require exposure-specific specialist or local guidance.

Pathophysiology

  1. 1
    Dermal inflammatory spread

    Bacterial invasion triggers cytokines, vasodilatation and oedema, creating expanding warmth, tenderness, swelling and colour change with an indistinct edge.

  2. 2
    Superficial lymphatic involvement

    Erysipelas affects upper dermis and lymphatics, producing a raised, more sharply demarcated plaque and sometimes lymphangitic streaking.

  3. 3
    Systemic response

    Toxin and inflammatory mediator release can cause fever, tachycardia, hypotension, delirium and organ dysfunction even before extensive skin change appears.

  4. 4
    Recurrence cycle

    Inflammation further damages lymphatics, increasing chronic oedema and creating susceptibility to subsequent episodes unless portals and swelling are treated.

Clinical features and red flags

Unilateral expanding inflammation

Tender warmth, oedema and ill-defined colour change spreading from a portal supports cellulitis, particularly when systemic symptoms accompany it.

Raised erysipelas edge

A bright, dusky or violaceous raised plaque with a sharp border and fever reflects superficial dermal and lymphatic infection.

Purulent centre

Fluctuance, pointing or spontaneous pus suggests an abscess requiring drainage assessment and microbiological sampling.

Necrotising departureRed flag

Pain out of proportion, rapid spread, skin anaesthesia, crepitus, haemorrhagic bullae, necrosis or shock demands immediate surgery.

Orbital warningRed flag

Painful eye movements, reduced vision, proptosis, diplopia or ophthalmoplegia with periocular swelling indicates possible orbital cellulitis.

Red flags requiring action

  • Sepsis physiology, necrotising features, orbital or facial danger-site infection, compartment syndrome, septic joint, osteomyelitis, severe immunocompromise, rapidly spreading disease or inability to take oral treatment requires hospital assessment.

Investigations

  1. 01
    ABCDE and sepsis assessmentFirst step
    Why
    Identify systemic severity and determine hospital or intravenous-treatment need.
    Interpretation and limitations
    Normal observations support outpatient care but do not neutralise disproportionate pain or rapidly evolving local danger signs.
  2. 02
    Portal, joint and abscess examination
    Why
    Find a treatable entry site and alternative source-control diagnosis.
    Interpretation and limitations
    Fluctuance prompts drainage; severe passive joint pain prompts septic-arthritis assessment; toe-web fissures need antifungal and barrier care.
  3. 03
    Blood tests and blood cultures
    Why
    Assess organ dysfunction and bacteraemia in severe, atypical or systemically unwell infection.
    Interpretation and limitations
    Routine bloods are unnecessary in mild disease; cultures should precede antibiotics when feasible but never delay treatment of sepsis.
  4. 04
    Culture of pus or open portal
    Why
    Guide treatment in purulence, unusual exposure, recurrent infection or non-response.
    Interpretation and limitations
    Do not swab intact erythema; provide exposure, prior antibiotics and MRSA history so laboratory results can be interpreted clinically.

Differential diagnosis

01

Venous stasis dermatitis

Bilateral chronic lower-leg erythema or brown change, itch, scale and oedema without fever favours venous eczema; cellulitis is usually unilateral and acutely tender.

02

Deep-vein thrombosis

Risk factors, calf swelling and tenderness can overlap, but surface warmth or redness cannot confirm or exclude thrombosis; use the validated VTE pathway.

03

Acute gout or septic arthritis

Pain centred on a joint with restricted passive movement requires aspiration-led assessment; overlying erythema should not lead to a skin-only diagnosis.

04

Contact dermatitis

Geometric exposure, itch, vesicles and a sharply product-related distribution without systemic illness supports dermatitis rather than bacterial spread.

05

Necrotising soft-tissue infection

Disproportionate pain, rapid progression, crepitus, bullae, anaesthesia or shock requires immediate surgical management and must never be observed as routine cellulitis.

Additional chapter-specific clues

Bilateral mimic pattern

Symmetrical chronic lower-leg change with itch, scale and haemosiderin staining is much more likely venous dermatitis than bilateral cellulitis.

Management

01Uncomplicated outpatientStart narrow oral treatmentFirst stepCellulitis is localised, the patient is stable, oral treatment is feasible and no danger-site or unusual exposure applies.+
  1. 1Mark and document extent, portal, pain and observations, then start oral flucloxacillin for five to seven days after allergy assessment.
  2. 2Advise elevation, fluids, analgesia and treatment of toe-web fungal disease, eczema or wounds while avoiding compression during intolerable acute pain.
  3. 3Arrange review if not improving within two to three days and immediate help for systemic decline, rapid spread, blistering or severe pain.
02Severe or dangerous siteEscalate to hospital careEscalationSepsis, rapid progression, necrotising features, orbital disease, severe immunocompromise or inability to take oral treatment is present.+
  1. 1Begin ABCDE, sepsis investigations and analgesia, and obtain urgent surgical, ophthalmic or specialty review according to the threatened structure.

Complications

01

Sepsis and metastatic infection

Bacteraemia can lead to shock or seed joints, bone and endocardium, especially with systemic toxicity or significant comorbidity.

02

Abscess formation

A focal fluctuant or increasingly tender centre suggests a collection that needs ultrasound or drainage rather than antibiotic escalation alone.

03

Lymphatic damage

Repeated episodes worsen lymphoedema and chronic swelling, which in turn increases future infection risk and functional limitation.

04

Orbital and intracranial spread

Infection around an eye or central face can threaten vision or extend intracranially and requires urgent specialist assessment.

Monitoring and follow-up

  • Review improvement in fever, pain, function and edge progression within two to three days, allowing residual colour to resolve more slowly.
  • Escalate immediately for hypotension, confusion, disproportionate pain, crepitus, bullae, necrosis, eye symptoms or rapid expansion.
  • At non-response, examine again for abscess, septic joint, osteomyelitis and a non-infective mimic before broadening antibiotics.
  • After recovery, document and treat portals, chronic oedema, venous disease, eczema and tinea pedis to reduce recurrence.
  • For intravenous therapy, review daily for oral step-down using physiology, absorption, source control and microbiology rather than colour alone.

Special situations

Bilateral is a warning

True bilateral lower-leg cellulitis is uncommon; chronic venous or eczematous inflammation deserves deliberate reconsideration.

Colour lags behind cure

Persistent erythema or pigmentation after pain and systemic features improve does not automatically justify more antibiotics.

Find the toe webs

Maceration and fissures between toes are common recurrent lower-limb portals and require direct examination.

Drainage beats spectrum

An occult abscess fails even broad antibiotics if the collection is not identified and drained.

Pain can outrun skin

Severe deep pain before dramatic surface change is an early necrotising-infection clue, especially in darker skin.

Common pitfalls

  1. 01

    Diagnosing bilateral chronic red legs as cellulitis and exposing venous dermatitis to repeated antibiotics.

  2. 02

    Waiting for obvious necrosis before calling surgery in a patient with rapidly progressive disproportionate pain.

  3. 03

    Swabbing intact skin and treating a colonising result as the cause of deep infection.

  4. 04

    Extending therapy solely because pigmentation remains after pain, fever and spread have resolved.

  5. 05

    Ignoring oedema, eczema and interdigital tinea after the acute episode and inviting recurrence.

Practice

Two practice questions

Your practice draft is saved in this browser.

Question 1 of 2
0 correct
DermatologyOriginal SBA

Stable unilateral lower-leg infection

A stable adult has acute unilateral lower-leg warmth, tenderness and expanding erythema without abscess, sepsis or penicillin allergy. Which oral regimen is NICE first choice?

Choose one answer, then submit
← Rapid synopsisBack to overviewAll Dermatology topics →
Sources and review status3 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • NICE NG141 Cellulitis and erysipelas antimicrobial prescribingExact adult regimens, review timing, referral and reassessment.
  • NHS cellulitisRecognition, recovery trajectory, urgent symptoms and recurrence prevention.
  • Flucloxacillin 500 mg capsules SmPCAdministration, contraindications and adverse effects.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom

Key points

  • Cellulitis is an acute, usually unilateral, tender warm expanding infection of dermis and subcutis; erysipelas is more superficial, raised and sharply demarcated.
  • Mark the edge with date and time when appropriate, but judge deterioration by physiology, pain, function and new features as well as millimetres of redness.
  • Exclude an abscess, necrotising infection, septic joint, orbital disease and common non-infective mimics before issuing routine oral antibiotics.
  • NICE first-choice adult oral treatment is flucloxacillin 500 mg to 1 g four times daily for five to seven days.
  • Symptoms can worsen during the first 48 hours and skin may take time to normalise; improvement should begin within two to three days.
  • Elevation, analgesia, hydration and treatment of tinea pedis, eczema, wounds and oedema support recovery and recurrence prevention.
  • Do not routinely swab intact skin; culture pus, an open wound or a portal when unusual exposure, non-response or MRSA risk makes the result actionable.
  • Refer to hospital for severe illness, rapid spread, oral-treatment failure with danger features, significant immune compromise or infection near the eye or nose.
05
Ultrasound or cross-sectional imaging
Why
Detect occult collection, gas, foreign body, osteomyelitis or deep extension when examination is uncertain.
Interpretation and limitations
Imaging assists source definition but must not delay surgical exploration when necrotising infection is clinically suspected.
  • 2Give intravenous antibiotics based on NICE, exposure and local policy without delaying source control; use co-amoxiclav-based cover for near-eye or nose infection.
  • 3Reassess frequently for organ dysfunction, collection and tissue viability, stepping down to oral therapy only after clear clinical improvement.
    • 03Failure or recurrenceReopen diagnosis and preventionDisease worsens after 48 hours, fails a completed course or repeatedly affects the same limb.+
    1. 1Check adherence, allergy-driven spectrum, portal, abscess, joint or bone involvement, DVT and dermatitis rather than simply lengthening the prescription.
    2. 2Culture an actionable lesion and use targeted bloods or imaging; refer if severe, spreading or unable to resolve uncertainty safely.
    3. 3After recovery, treat tinea and eczema, optimise oedema and venous care, review wounds and consider specialist-guided prophylaxis only for frequent proven episodes.
    Key medicines and prescribing safety3 treatments · regimens, roles and cautions+
    NICE first-choice narrow oral antistaphylococcal and antistreptococcal treatment.

    Flucloxacillin capsules

    Take 500 mg to 1 g orally four times daily for five to seven days for uncomplicated adult cellulitis.

    Check penicillin hypersensitivity, liver history, renal status and interactions; take on an empty stomach and review jaundice, severe diarrhoea or allergy urgently.

    A NICE alternative for uncomplicated adult cellulitis after allergy and interaction review.

    Clarithromycin tablets

    Take 500 mg orally twice daily for five to seven days when a true penicillin allergy makes it appropriate.

    Review QT prolongation, macrolide interactions, hepatic disease, pregnancy and local resistance; nausea and diarrhoea are common.

    NICE first-choice broader cover at facial sites where additional organisms and complications matter.

    Co-amoxiclav tablets

    Take 500/125 mg orally three times daily for seven days for infection near the eyes or nose when outpatient treatment is appropriate.

    Avoid in serious penicillin allergy or previous co-amoxiclav jaundice, adjust for renal impairment and maintain a low threshold for specialist review.