Cellulitis
An acute bacterial infection of the dermis and subcutaneous tissue causing spreading inflammation; the essential clinical task is to distinguish uncomplicated cellulitis from sepsis, orbital disease, abscess and necrotising fasciitis.
Definition
Cellulitis is an acute bacterial infection of the dermis and subcutaneous tissue, usually presenting with spreading warmth, swelling, tenderness and erythema.
Epidemiology
Common, usually affecting a lower limb. Risk increases with breaks in the skin, tinea pedis, ulcers, eczema, oedema, lymphoedema, venous disease, diabetes, immobility, previous cellulitis, injecting drug use and immunosuppression.
Pathophysiology
Bacteria enter through a skin barrier defect and trigger acute inflammation in the dermis and subcutaneous tissue. Vasodilation and inflammatory cell recruitment produce warmth, erythema, swelling and pain; oedema and impaired lymphatic clearance make recurrence more likely.
First principles
Cellulitis is a diffuse tissue infection, not a collection
A breach in the skin barrier allows bacteria to spread through the dermis and subcutaneous tissue, producing ill-defined warmth, erythema, swelling and tenderness. A discrete fluctuant or purulent collection suggests an abscess and needs a source-control assessment rather than antibiotics alone.1,2
The portal of entry and host factors change the risk
The entry site may be a wound, fissure, ulcer, eczema, injection site or tinea pedis, and it may be too small to see. Oedema, lymphoedema, venous disease, diabetes, immobility and immunosuppression impair local defence and increase the risk of severe or recurrent infection.1,2
The usual organisms are covered by narrow first-line treatment
Streptococcus pyogenes and Staphylococcus aureus are the usual organisms in non-surgically acquired cellulitis, but the site, exposure history, previous cultures and MRSA status may change the likely pathogens. This is why uncomplicated disease is treated with a narrow first-choice agent, while unusual exposures or severe infection need specialist or local microbiology advice.1,3
Cellulitis and erysipelas are related but not identical patterns
Cellulitis involves deeper dermis and subcutaneous tissue and usually has an ill-defined edge. Erysipelas is more superficial and classically has a raised, sharply demarcated margin. The initial antibiotic approach is similar, but describing the pattern helps distinguish it from dermatitis, venous disease and other non-infectious redness.1,2
Necrotising fasciitis is a different, time-critical disease
Pain that is much greater than the visible skin findings, rapid progression, altered sensation, bullae, dusky or black skin, systemic toxicity or shock should raise concern for infection of the deep fascia. This requires immediate hospital treatment, broad-spectrum intravenous antibiotics and urgent surgical assessment for debridement; do not manage it as routine cellulitis.1,4
Presentation
Usually unilateral spreading warmth, swelling, tenderness and erythema, often of a lower limb, with variable systemic upset; the rate of spread, pain severity and physiology matter more than the colour alone.1,5,2,4
Cardinal features
- Unilateral spreading warmth, swelling and tenderness with an ill-defined margin
- Erythema that may be less obvious on brown or black skin
- A wound, fissure, ulcer, eczema, tinea pedis or other possible portal of entry
- Fever, malaise, rigors or regional lymphangitis in more significant infection
- Lower-limb involvement, often with oedema or venous or lymphatic disease
- A sharply demarcated raised edge suggesting erysipelas rather than deeper cellulitis
Red flags
- Sepsis physiology: hypotension, tachypnoea, hypoxia, confusion, oliguria, high NEWS2 or rapidly worsening observations
- Pain out of proportion to the visible findings or new numbness or anaesthesia
- Rapid spread, crepitus, bullae, dusky, purple or black skin, or systemic toxicity
- Periorbital or nasal involvement, visual symptoms, painful eye movements or proptosis
- A fluctuant collection, severe focal tenderness or concern for a deep or joint infection
- Failure to start improving after 2 to 3 days, or significant worsening at any time
- An unusual exposure, immunosuppression, diabetes or an unsafe ability to obtain urgent review
Investigations
Clinical diagnosis and structured severity assessment
Cellulitis is primarily a clinical diagnosis. Examine the whole affected area, identify a portal of entry, assess pain and rate of spread, record observations and consider a local severity tool such as Eron or Dundee only as an adjunct rather than an automatic admission rule.
Expected finding: Spreading warmth, swelling and tenderness with an ill-defined inflammatory edge; severe systemic features, disproportionate pain or rapid progression suggest a more dangerous alternative.
1,3Observations, NEWS2 and perfusion assessment when systemically unwell
Record temperature, heart rate, respiratory rate, blood pressure, oxygen saturation, mental state, urine output and capillary refill. Use NEWS2 and the current adult sepsis pathway when infection is accompanied by physiological deterioration.
Expected finding: Tachycardia, hypotension, tachypnoea, hypoxia, confusion, oliguria or raised lactate indicate increased risk and should trigger escalation rather than outpatient cellulitis treatment.
1,5Targeted blood tests for systemic illness or admission
Send FBC, CRP, U&E and liver tests when severity, comorbidity or admission makes them clinically useful; add blood gas and lactate, glucose, cultures and other tests according to sepsis risk and the suspected alternative diagnosis. Do not use inflammatory markers alone to rule in cellulitis or rule out necrotising fasciitis.
Expected finding: Inflammatory marker elevation or leukocytosis may support severity assessment, but normal results do not make a dangerous clinical picture safe.
1,5,4Swab or other microbiology only when it can change treatment
Consider a swab from broken skin, particularly if there is a penetrating injury, water exposure, infection acquired outside the UK or a persistent wound. Review previous cultures and MRSA status; do not routinely swab intact skin or allow sampling to delay treatment.
Expected finding: The swab may be negative or may identify an organism that supports narrowing or changing antibiotics when the patient is not improving.
1Targeted assessment for mimics and focal complications
Look for venous stasis or lipodermatosclerosis, eczema, contact dermatitis, gout, superficial thrombophlebitis and DVT. Use venous ultrasound, joint aspiration, imaging for osteomyelitis or other tests only when the history and examination make those diagnoses plausible.
Expected finding: A bilateral chronic pattern, itch, a clear venous distribution, a calf DVT risk profile or focal joint findings should redirect the pathway rather than prompt repeated antibiotics.
1,2Urgent eye and specialist assessment for periorbital disease
Infection near the eye or nose needs hospital or specialist advice because of the risk of orbital or intracranial complications. Assess vision, ocular movements, pain, proptosis and systemic features, and use imaging or ophthalmology and ENT pathways as directed by the receiving team.
Expected finding: Visual loss, painful or restricted eye movements, proptosis, ophthalmoplegia or severe headache indicates a complication beyond uncomplicated cellulitis.
1,2Immediate surgical assessment when necrotising fasciitis is possible
Do not wait for a normal early test or a definitive scan before seeking urgent senior surgical review when the pain, spread, skin changes or physiology suggest necrotising fasciitis. Tissue and blood cultures are obtained as part of hospital management without delaying treatment.
Expected finding: Early disease may have little visible change despite severe pain; bullae, anaesthesia, dusky or black skin and shock are late or advanced warning signs.
1,4Management
| Step | Detail | Source |
|---|---|---|
| Assess severity, exclude dangerous mimics and start the sepsis pathway when indicated | Use ABCDE, observations and NEWS2 when clinically appropriate. Examine the whole limb or affected area, assess pain and spread, and actively consider abscess, DVT, dermatitis, venous disease, orbital cellulitis, osteomyelitis, septic arthritis, necrotising fasciitis and sepsis. A systemically unwell patient needs urgent hospital assessment and sepsis management rather than routine community treatment.1,5 | NICE NG141 and NICE NG253 |
| Mark the edge, elevate the affected limb and document the baseline | Consider drawing around the edge with a single-use surgical marker so progression can be assessed. Record the site, size, pain, skin findings, observations and likely portal of entry. Elevate an affected limb when possible to reduce swelling; remember that erythema may be less visible on darker skin tones.1,2 | NICE NG141 and NHS cellulitis guidance |
| Treat uncomplicated adult cellulitis with oral flucloxacillin first line | If the patient can take oral treatment and is not severely unwell, use flucloxacillin 500 mg to 1 g four times daily for 5 to 7 days, with the exact prescription checked against the current BNF, renal and hepatic function, allergy history and local policy. A longer course, up to 14 days in total, may be needed after clinical review; do not expect all skin changes to disappear by day 5 to 7.1,6 | NICE NG141 and BNF flucloxacillin |
| Use an appropriate alternative for true penicillin allergy or pregnancy | For penicillin allergy or when flucloxacillin is unsuitable, NICE lists clarithromycin 500 mg twice daily or doxycycline 200 mg on day 1 followed by 100 mg once daily for 5 to 7 days in total. If a macrolide is needed in pregnancy, erythromycin is preferred; check the BNF and specialist or local antimicrobial guidance for pregnancy, interactions and contraindications.1,7,8 | NICE NG141, BNF clarithromycin and BNF doxycycline |
| Treat infection near the eyes or nose as a specialist-risk site | Seek specialist advice or hospital assessment for infection immediately around the eye or nose, including periorbital cellulitis. NICE lists co-amoxiclav 500/125 mg three times daily orally or 1.2 g three times daily intravenously for 7 days; for penicillin allergy or if unsuitable, clarithromycin with metronidazole is the alternative pathway. Check current BNF dosing and local specialist guidance.1,9,10 | NICE NG141, BNF co-amoxiclav and BNF metronidazole |
| Use an IV or ambulatory pathway for severe or non-oral disease | Refer or admit people who are severely unwell, cannot take oral antibiotics, have spreading infection despite oral treatment, lymphangitis, an unusual pathogen risk or a serious complication. If IV treatment is used, NICE lists flucloxacillin 1 to 2 g four times daily as first choice for adults; alternatives and MRSA cover require local microbiology or specialist advice. Review IV therapy by 48 hours and switch to oral treatment when possible.1,6 | NICE NG141 and BNF flucloxacillin |
| Treat the portal of entry and the predisposing condition | Inspect between the toes and around ulcers, wounds, eczema, injections and devices. Treat tinea pedis, manage eczema and wounds, address diabetes and venous disease, and manage oedema or lymphoedema. Removing the ongoing portal of entry is part of treating the episode and reducing recurrence.1,2 | NICE NG141 and NHS cellulitis guidance |
| Escalate immediately for suspected necrotising fasciitis or another deep complication | Disproportionate pain, rapid progression, new sensory loss, bullae, crepitus, dusky or black skin, severe systemic upset or shock requires immediate hospital treatment and senior surgical review. Give broad-spectrum intravenous antibiotics according to the local emergency protocol and arrange urgent operative assessment for debridement; antibiotics are not a substitute for source control.1,4 | NICE NG141 and NHS necrotising fasciitis guidance |
| Reassess early and change the pathway if the patient is not improving | Explain that symptoms can worsen during the first 48 hours, but the patient should start to improve within 2 to 3 days. Reassess sooner for significant worsening, systemic illness, pain out of proportion or spread beyond the initial area. Review microbiology and previous antibiotics, reconsider the diagnosis, look for an abscess or deeper infection and narrow or change antibiotics when results and the clinical course support it.1,2 | NICE NG141 and NHS cellulitis guidance |
| Prevent recurrent cellulitis and give explicit safety-netting | Do not routinely prescribe prophylactic antibiotics. After at least 2 episodes in the previous 12 months treated in hospital or under specialist advice, a specialist may consider prophylaxis using shared decision-making; NICE lists phenoxymethylpenicillin 250 mg twice daily or erythromycin 250 mg twice daily for penicillin allergy, with review at least every 6 months. Give clear return advice for rapid worsening, fever or rigors, dizziness, confusion, purple or black skin, severe pain, eye symptoms or failure to improve within 2 to 3 days.1,2,4 | NICE NG141 and NHS cellulitis guidance |
Illustrations
Differentials
Venous stasis dermatitis or lipodermatosclerosis
Often bilateral or chronic, with venous skin changes and less acute systemic illness.
Contact or eczematous dermatitis
Itch, a sharp exposure-related distribution, scale or vesicles without a convincing infectious course.
Deep vein thrombosis
Unilateral swelling and pain with thromboembolic risk factors and no clear portal of entry; assess using the local VTE pathway.
Abscess
A discrete fluctuant or purulent collection needing drainage or source-control assessment.
Necrotising fasciitis
Pain out of proportion, rapid progression, sensory loss, bullae, dusky or black skin, systemic toxicity or shock; urgent surgical emergency.
Erysipelas
More superficial infection with a raised, sharply demarcated edge.
Complications
- Abscess or deeper soft-tissue infection
- Sepsis and bacteraemia
- Orbital cellulitis or intracranial spread when the eye or nose is involved
- Osteomyelitis or septic arthritis
- Necrotising fasciitis
- Recurrent cellulitis and progressive lymphoedema
Prognosis
Most uncomplicated cases improve with appropriate oral antibiotics, although pain and skin colour or swelling may take longer to settle. Severe infection can progress to sepsis, deep infection or necrotising fasciitis, and recurrence is more likely when oedema, skin disease or another portal of entry is not addressed.
Guidelines
- Cellulitis and erysipelas: antimicrobial prescribing (NG141) (NICE, 2019)
- Suspected sepsis in people aged 16 or over (NG253) (NICE, 2025)
References
- NICE NG141, Cellulitis and erysipelas: antimicrobial prescribing, recommendations (NG141)
- NHS, cellulitis (NHS cellulitis)
- NICE NG141, summary of the evidence (NG141 evidence)
- NHS, necrotising fasciitis (NHS necrotising fasciitis)
- NICE NG253, Suspected sepsis in people aged 16 or over: managing suspected sepsis (NG253)
- BNF, flucloxacillin (BNF flucloxacillin)
- BNF, clarithromycin (BNF clarithromycin)
- BNF, doxycycline (BNF doxycycline)
- BNF, co-amoxiclav (BNF co-amoxiclav)
- BNF, metronidazole (BNF metronidazole)
Evidence checked: 2026-08-03
This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.