About This Page
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
- Nonpurulent cellulitis is usually streptococcal; treat mild disease with beta-lactam coverage such as cephalexin
- Purulent abscess is usually S aureus including MRSA; primary treatment is incision and drainage
- Add MRSA antibiotics for abscess with systemic signs, immunocompromise, extensive disease, recurrent infection, or failed I&D alone
- Necrotizing fasciitis = pain out of proportion, rapid progression, bullae, skin anesthesia, crepitus, systemic toxicity; urgent surgery is the next best step
- Empiric nec fasc antibiotics: vancomycin plus piperacillin-tazobactam or carbapenem plus clindamycin for toxin suppression
- Animal/human bites need polymicrobial coverage; amoxicillin-clavulanate is first-line outpatient therapy
Overview
Skin and soft tissue infections include impetigo, erysipelas, cellulitis, abscess, wound infection, bite infection, pyomyositis, and necrotizing fasciitis. The most important clinical distinction is purulent versus nonpurulent infection, because purulence implies S aureus and possible MRSA, while nonpurulent cellulitis is usually beta-hemolytic streptococci. Necrotizing fasciitis is a surgical emergency and should not be delayed for imaging or laboratory scoring when clinical suspicion is high.
Epidemiology
SSTIs are among the most common reasons for outpatient visits and emergency department antibiotic prescribing. Risk factors include skin trauma, injection drug use, diabetes, obesity, edema/lymphedema, venous insufficiency, tinea pedis, immunosuppression, prior MRSA, animal bites, water exposure, and recent surgery. Community-associated MRSA is a key cause of purulent abscesses in the United States.
Clinical Features
Symptoms
Cellulitis: unilateral erythema, warmth, swelling, tenderness, often on lower extremity
Erysipelas: sharply demarcated raised erythema with fever, usually streptococcal
Abscess: painful fluctuant nodule or collection with purulent drainage
Necrotizing fasciitis: severe pain out of proportion, rapidly spreading erythema, fever, toxicity
Clostridial myonecrosis: severe pain, crepitus, brown discharge, systemic toxicity after trauma
Bite infection: pain, swelling, purulence after cat, dog, or human bite; hand bites are high risk
Signs
Localized warmth, edema, erythema, tenderness without fluctuance in cellulitis
Fluctuance, pointing, or spontaneous pus in abscess
Bullae, ecchymosis, necrosis, skin anesthesia, crepitus, or wooden-hard induration in necrotizing infection
Lymphangitic streaking or tender regional lymphadenopathy
Hypotension, confusion, tachypnea, or organ dysfunction suggests sepsis
Investigations
First-line
Clinical diagnosisUncomplicated cellulitis and abscess are diagnosed clinically; routine cultures are not needed for mild cellulitis
UltrasoundUseful when abscess is uncertain; shows drainable fluid collection
Blood culturesFor severe systemic toxicity, immunocompromise, unusual exposures, neutropenia, immersion injury, or animal bite with sepsis
Second-line
CBC, BMP, CRP, CK, lactateAssess severity; CK may rise in myonecrosis; labs cannot exclude necrotizing fasciitis
Wound or abscess cultureRecommended for recurrent abscess, severe infection, immunocompromise, or treatment failure
Plain radiographMay show gas or foreign body but normal imaging does not exclude nec fasc
Specialist
CT or MRIMay define extent if diagnosis uncertain and patient stable; do not delay surgery when nec fasc is likely
Surgical explorationGold standard for necrotizing fasciitis; finger test may show easy fascial dissection and dishwater fluid
1
Nonpurulent cellulitis
- Mild: oral beta-lactam such as cephalexin, dicloxacillin, or amoxicillin-clavulanate
- Moderate with systemic signs: IV cefazolin or ceftriaxone
- Add MRSA coverage if penetrating trauma, injection drug use, purulence, MRSA elsewhere, nasal MRSA, or severe infection
- Elevate limb and treat predisposing conditions such as tinea pedis, edema, venous insufficiency
2
Purulent abscess
- Incision and drainage is primary treatment
- Add TMP-SMX, doxycycline, or clindamycin if systemic signs, extensive disease, immunocompromise, recurrent abscess, extremes of age, or failed I&D
- Severe purulent infection: IV vancomycin or alternative MRSA agent
3
Necrotizing fasciitis
- Immediate surgical consultation and operative debridement; repeat debridement often required
- Empiric antibiotics: vancomycin plus piperacillin-tazobactam, or vancomycin plus carbapenem
- Add clindamycin for toxin suppression in suspected group A strep or clostridial infection
- Do not rely on LRINEC score to rule out necrotizing infection
4
Special exposures
- Human or animal bite: amoxicillin-clavulanate; cover Pasteurella, anaerobes, streptococci, and oral flora
- Saltwater exposure: consider Vibrio vulnificus; doxycycline plus ceftazidime
- Freshwater exposure: consider Aeromonas; fluoroquinolone or third/fourth-generation cephalosporin depending severity
Complications
- Abscess formation: Cellulitis may evolve into a drainable collection
- Sepsis: Especially with necrotizing infection, diabetes, immunosuppression, or delayed therapy
- Necrotizing fasciitis: Rapid fascial destruction, shock, limb loss, death
- Osteomyelitis: Contiguous spread from ulcers, diabetic foot infection, or trauma
- Recurrent cellulitis: Driven by chronic edema, tinea pedis, venous disease, or obesity
USMLE Step 2 CK Exam Tips
- 1Abscess = incision and drainage first; antibiotics alone is a common wrong answer
- 2Nonpurulent cellulitis usually needs streptococcal coverage, not automatic MRSA coverage
- 3Pain out of proportion + bullae/crepitus/systemic toxicity = necrotizing fasciitis; next best step is surgery
- 4Do not wait for MRI before operating when nec fasc is clinically likely
- 5Clindamycin is added in nec fasc for toxin suppression, especially group A strep or clostridia
- 6Cat bite cellulitis = Pasteurella; treat with amoxicillin-clavulanate
- 7Hot tub folliculitis = Pseudomonas; saltwater wound sepsis = Vibrio vulnificus
practicetest your knowledge on skin & soft tissue infections (cellulitis, abscess, necrotizing fasciitis)Apply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — infectious diseases and beyond.
open q-bank