Cellulitis: workup and management
Acute spreading skin infection. Separate purulent (abscess, usually S. aureus) from non-purulent (usually streptococcal) infection, exclude mimics and necrotizing infection, and choose oral or IV therapy by severity.
Working diagnosis
Adult · US guidelines
Draft · awaiting physician review
01Orient
What's new in the evidence
- IDSA 2014 SSTI guideline remains current; 5 days is enough for most uncomplicated cellulitis
- Point-of-care ultrasound is increasingly used to find occult abscess
- Ceftobiprole was FDA-approved in 2024 for skin infections and S. aureus bacteremia
Differential diagnoses
- Non-purulent cellulitis or erysipelas
- Abscess or purulent cellulitis
- Necrotizing fasciitis
- Deep vein thrombosis
- Stasis dermatitis (often bilateral)
- Gout or septic arthritis
- Contact dermatitis, lipodermatosclerosis
02Assess
Risk scores and decision tools
- ALT-70 score to separate cellulitis from pseudocellulitis
- LRINEC score is not sensitive enough to rule out necrotizing fasciitis
- SIRS/sepsis criteria for site of care
Key history questions
- Onset and spread; pain out of proportion
- Skin breaks, bites, injection drug use
- Diabetes, immunosuppression, peripheral vascular disease, lymphedema
- Prior MRSA
- Systemic symptoms: fever, rigors
Examination findings
- Mark the border of erythema
- Fluctuance or purulent drainage
- Bullae, crepitus, skin necrosis, anesthesia (necrotizing infection)
- Bilateral involvement (suggests a mimic)
- Tinea pedis between toes (portal of entry)
03Investigate
Initial investigations
- Clinical diagnosis in most cases
- CBC, BMP, CRP if systemically unwell
- Blood cultures if systemic signs or immunocompromised
- Culture of purulent drainage
Advanced investigations
- Bedside ultrasound for occult abscess
- Venous duplex if DVT possible
- CT or urgent surgical exploration if necrotizing infection suspected (do not delay surgery for imaging)
04Treat
Immediate management
- Suspected necrotizing fasciitis: emergency surgical consult plus broad-spectrum antibiotics (vancomycin, piperacillin-tazobactam, clindamycin)
- Incision and drainage for abscess
First-line treatment
- Non-purulent, oral: cephalexin 500 mg four times daily
- Non-purulent, IV: cefazolin 2 g IV every 8 h
- Purulent or MRSA risk, oral: TMP-SMX or doxycycline
- Purulent, IV: vancomycin
- Usual duration 5 days, extended if not improving
- Elevate the limb
Second-line treatment
- Clindamycin for severe beta-lactam allergy
- Treat tinea pedis and edema to reduce recurrence
Specialist referrals
- General surgery for necrotizing infection or large abscess
- Infectious diseases for recurrent or complicated infection
05Disposition
Admission criteria
- Sepsis or systemic toxicity
- Rapid progression or suspected necrotizing infection
- Immunosuppression
- Failure of oral therapy or unable to take oral drugs
Safe discharge criteria
- No systemic toxicity
- Tolerating oral antibiotics
- Border marked and review arranged at 48-72 h
Follow-up timing
- Review at 48-72 h
Patient instructions
- Expect some redness to look worse in the first 24-48 h
- Return for spreading redness beyond the line, severe pain, blistering or fever
Monitoring
- Daily border check
- Signs of necrotizing infection
Clinical pearls and pitfalls
- Bilateral red legs are rarely cellulitis; think stasis dermatitis
- Routine MRSA cover is not needed for non-purulent cellulitis
RefGuidelines and landmark evidence
For education only. Typical adult starting points; confirm doses against renal function, allergies, interactions and your institution's protocols. Not a substitute for clinical judgment or senior review.