Synopsis
Recognise non-purulent bacterial skin infection, grade severity, distinguish common mimics and prescribe safe narrow therapy while addressing portals of entry and recurrence.
- Cellulitis is a spreading infection of dermis and subcutaneous tissue; erysipelas is more superficial, raised and sharply demarcated.
- Typical findings are unilateral warmth, erythema, tenderness and swelling, sometimes with lymphangitis or systemic upset.
- Bilateral red legs are rarely bilateral cellulitis; venous stasis, lipodermatosclerosis and oedema are common mimics.
- Outline the margin and document observations, pain, portal of entry and baseline limb function before treatment.
- For uncomplicated adult disease, NICE first choice is flucloxacillin 500 mg to 1 g four times daily for five to seven days.
- Clarithromycin 500 mg twice daily is an option for penicillin allergy; erythromycin 500 mg four times daily is preferred in pregnancy when a macrolide is needed.
- Initial redness may extend during the first 24 to 48 hours despite effective therapy, but worsening physiology or disproportionate pain is not expected.
- Address tinea pedis, eczema, wounds, oedema and venous disease because antibiotics alone do not prevent recurrence.
Key red flags
Pain out of proportion or rapidly advancing oedema suggests necrotising soft-tissue infection.
Hypotension, confusion, tachypnoea, mottling or oliguria indicates sepsis-related organ dysfunction.
Crepitus, haemorrhagic bullae, skin anaesthesia or dusky necrosis requires immediate surgical assessment.
Periorbital swelling with painful eye movements, visual change or proptosis suggests orbital cellulitis.
Hand infection, human bite, diabetic foot disease or severe immune compromise requires a different specialist pathway.
Fever, rigors, tachycardia, hypotension, delirium or reduced urine output indicates systemic involvement and possible sepsis.
Disproportionate pain, rapid spread, anaesthesia, bullae, crepitus or skin necrosis should override a reassuring superficial label.
Infection near an eye, hand, joint, surgical implant, diabetic ulcer or bite requires anatomy-specific assessment and often specialist input.
Investigation priorities
Define extent, severity, portal of entry and a reproducible baseline for reassessment.
Management branches
Uncomplicated cellulitis is likely, physiology is stable and oral absorption and follow-up are reliable.
- Document the site, margin, pain, observations, portal of entry, allergy reaction, renal function and factors that alter organism risk.
- Give flucloxacillin 500 mg to 1 g orally four times daily for five to seven days when suitable, using a guideline-listed allergy alternative when needed.
- Elevate an affected limb, provide analgesia and treat fissures, tinea, eczema, wound care and oedema without applying topical antibiotics to intact skin.
- Arrange review if symptoms worsen rapidly or fail to begin improving within two to three days and provide sepsis and necrotising-infection safety-net advice.