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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Cellulitis is deeper and poorly demarcated; erysipelas is superficial with sharply demarcated raised borders
- Nonpurulent cellulitis is usually beta-hemolytic streptococci; purulence/abscess suggests S aureus including MRSA
- Mild nonpurulent outpatient therapy: cephalexin, dicloxacillin, or amoxicillin-clavulanate depending on scenario
- MRSA coverage when purulent infection, abscess, penetrating trauma, injection drug use, MRSA history, or systemic severe infection
- Pain out of proportion, bullae, crepitus, anesthesia, rapid progression, or shock = necrotizing fasciitis until proven otherwise
Overview
Cellulitis is an acute bacterial infection involving dermis and subcutaneous tissue, typically entering through breaks in skin such as tinea pedis fissures, ulcers, trauma, eczema, edema, or surgical wounds. Erysipelas is a more superficial infection of upper dermis and lymphatics, classically caused by group A Streptococcus, with a raised sharply demarcated border. Distinguishing nonpurulent cellulitis from purulent abscess is essential because abscess requires drainage and MRSA-active therapy may be needed.
Epidemiology
Cellulitis is a common reason for outpatient visits and hospital admissions. Risk factors include skin barrier disruption, venous insufficiency, lymphedema, obesity, diabetes, tinea pedis, chronic wounds, prior cellulitis, injection drug use, and immunosuppression. Lower extremities are the most common site. Bilateral red legs are more often venous stasis dermatitis, lipodermatosclerosis, edema, or another mimic rather than bilateral cellulitis.
Clinical Features
Symptoms
Acute unilateral skin pain, tenderness, warmth, swelling, and erythema
Fever, chills, malaise, or lymphangitic streaking
Purulent drainage, fluctuance, or focal abscess symptoms
Severe pain out of proportion to exam or rapidly progressive symptoms
Recurrent episodes in same limb suggest lymphedema, venous disease, or tinea pedis portal
Signs
Cellulitis: poorly demarcated erythema, warmth, edema, tenderness
Erysipelas: raised, sharply demarcated, bright red plaque, often on face or leg
Regional lymphadenopathy or lymphangitis
Fluctuance or pustule indicates abscess/purulent SSTI
Bullae, skin necrosis, crepitus, anesthesia, ecchymosis, hypotension, or confusion
Investigations
First-line
Clinical diagnosisMost uncomplicated cellulitis is diagnosed clinically; mark borders to monitor progression
Assess severity and portal of entryVitals, systemic toxicity, diabetes/immunosuppression, tinea pedis, ulcers, trauma, edema, abscess
Second-line
UltrasoundIf abscess is suspected but not obvious; detects drainable fluid collection
CBC, BMP, CRP, blood culturesNot needed in mild outpatient cellulitis; obtain if severe systemic features, immunocompromised, unusual exposures, or admission
Wound cultureIf purulent drainage, abscess, bite wound, immersion injury, or treatment failure
Specialist
CT/MRI/surgical explorationIf necrotizing infection, deep abscess, osteomyelitis, gas, or rapidly progressive severe infection suspected. Do not delay surgery for imaging if unstable
1
Mild nonpurulent cellulitis
- Oral beta-lactam active against streptococci: cephalexin, dicloxacillin, amoxicillin, or amoxicillin-clavulanate
- Typical duration 5 days if improving; extend if slow response
- Elevate limb, treat edema, provide analgesia, and treat portal such as tinea pedis
2
Purulent cellulitis or abscess
- Incision and drainage is primary treatment for abscess
- Add MRSA-active oral antibiotic when systemic signs, extensive disease, immunosuppression, failed I&D alone, or high-risk features: TMP-SMX, doxycycline, or clindamycin depending on local resistance and patient factors
- If both streptococcal and MRSA coverage needed, combine TMP-SMX/doxycycline with beta-lactam or use clindamycin if appropriate
3
Moderate-severe cellulitis
- Hospitalize for systemic toxicity, rapid progression, immunocompromise, inability to tolerate oral therapy, failed outpatient treatment, or concern for deep infection
- IV cefazolin or ceftriaxone for typical nonpurulent cellulitis; vancomycin if MRSA risk or severe infection
- Animal/human bites, water exposure, diabetic foot infection, or neutropenia require broader tailored coverage
4
Necrotizing fasciitis concern
- Urgent surgical consultation and operative debridement
- Broad-spectrum IV antibiotics: vancomycin plus piperacillin-tazobactam or carbapenem; add clindamycin for toxin suppression when group A strep/clostridial infection possible
- Do not wait for imaging if shock or rapidly progressive necrotizing infection is likely
Complications
- Abscess: Requires drainage; antibiotics alone may fail
- Bacteremia/sepsis: Especially in severe infection, immunosuppression, or delayed therapy
- Necrotizing fasciitis: Rapidly progressive life-threatening infection requiring surgery
- Recurrent cellulitis: Common with lymphedema, venous insufficiency, obesity, and tinea pedis
- Post-inflammatory edema/skin change: Can perpetuate recurrence risk
USMLE Step 2 CK Exam Tips
- 1Unilateral warm tender red leg = cellulitis; bilateral red swollen legs usually not cellulitis
- 2Nonpurulent cellulitis = streptococci; choose beta-lactam such as cephalexin
- 3Abscess = incision and drainage first, not antibiotics alone
- 4Purulence, injection drug use, MRSA history, or severe infection = add MRSA coverage
- 5Pain out of proportion + crepitus/bullae/shock = necrotizing fasciitis; immediate surgery
- 6Tinea pedis between toes is a common portal for recurrent leg cellulitis
- 7Erysipelas has sharply demarcated raised borders and is classically group A strep
practicetest your knowledge on cellulitis & erysipelasApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — dermatology and beyond.
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