Residency · Residency · Internal Medicine
Skin and Soft Tissue Infections: Cellulitis, Abscess, and Necrotizing Fasciitis
Introduction
Skin and soft tissue infections (SSTIs) are among the most common reasons for hospital admission and emergency department visits. Accurate classification of SSTIs is essential because management ranges from simple oral antibiotics to emergent surgical debridement. The internist must rapidly distinguish uncomplicated infections from life-threatening necrotizing processes.
Classification of SSTIs
Purulent vs. Non-Purulent Infections
- Purulent SSTIs: abscess, furuncle, carbuncle; usually caused by Staphylococcus aureus including MRSA
- Non-purulent SSTIs: cellulitis, erysipelas; typically caused by beta-hemolytic streptococci
- The IDSA classifies SSTIs as mild, moderate, or severe based on systemic signs and host factors
Anatomic Depth
| Type | Depth | Key Features | Typical Organism |
|---|---|---|---|
| Erysipelas | Superficial dermis | Sharply demarcated, raised borders | Group A Streptococcus |
| Cellulitis | Deeper dermis/subcutaneous | Poorly defined margins | Beta-hemolytic streptococci |
| Abscess | Subcutaneous | Fluctuant, purulent | S. aureus (including MRSA) |
| Necrotizing fasciitis | Fascial planes | Rapid tissue destruction, crepitus | Polymicrobial or GAS |
- Erysipelas: superficial dermis with sharply demarcated, raised borders
- Cellulitis: deeper dermis and subcutaneous tissue with poorly defined margins
- Necrotizing fasciitis: infection along fascial planes with rapid tissue destruction
Cellulitis
Clinical Features
- Expanding area of erythema, warmth, swelling, and tenderness
- Unilateral lower extremity is the most common site
- Predisposing factors: lymphedema, tinea pedis, saphenous venectomy, obesity
- Mark the leading edge with a skin marker to track progression or response
Management
- Mild: oral antibiotics targeting streptococci (cephalexin, dicloxacillin)
- Moderate: IV antibiotics (cefazolin); add MRSA coverage if risk factors present
- Failure to improve in 48-72 hours should prompt re-evaluation for abscess, deeper infection, or alternative diagnosis
- Common mimics: stasis dermatitis, DVT, contact dermatitis, gout
Cutaneous Abscess
Diagnosis and Treatment
- Fluctuant, tender, erythematous nodule; often with surrounding cellulitis
- Incision and drainage (I&D) is the primary treatment; antibiotics alone are insufficient
- Point-of-care ultrasound improves diagnostic accuracy over clinical exam alone
- Packing is optional; evidence does not clearly support routine packing
When to Add Antibiotics
- Surrounding cellulitis greater than 2 cm, multiple lesions, immunocompromise
- Systemic signs of infection (fever, tachycardia)
- TMP-SMX or doxycycline for outpatient MRSA coverage
Necrotizing Fasciitis
Pathophysiology and Classification
- Type I: polymicrobial (anaerobes, gram-negatives, enterococci); often post-surgical or in diabetics
- Type II: monomicrobial, typically Group A Streptococcus; can occur in healthy individuals
- Mortality ranges from 20-40% even with optimal treatment
Clinical Red Flags
- Pain out of proportion to exam findings
- Rapidly spreading erythema, crepitus, bullae, skin necrosis
- Systemic toxicity: hemodynamic instability, altered mental status
- LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis): CRP, WBC, hemoglobin, sodium, creatinine, glucose; score >=6 raises suspicion
| LRINEC Component | Criteria | Points |
|---|---|---|
| CRP | >= 150 mg/L | 4 |
| WBC | 15,000-25,000/mcL | 1 |
| WBC | > 25,000/mcL | 2 |
| Hemoglobin | 11-13.5 g/dL | 1 |
| Hemoglobin | < 11 g/dL | 2 |
| Sodium | < 135 mEq/L | 2 |
| Creatinine | > 1.6 mg/dL | 2 |
| Glucose | > 180 mg/dL | 1 |
| Score >= 6 | Suspect necrotizing fasciitis | |
| Score >= 8 | Strongly predictive |
Management
- Emergent surgical exploration and debridement is the definitive treatment; do not delay for imaging
- Broad-spectrum antibiotics: vancomycin + piperacillin-tazobactam or carbapenem
- Add clindamycin for toxin suppression in streptococcal necrotizing fasciitis
- Serial return to OR for re-debridement every 24-48 hours until infection is controlled
Special Populations
- Diabetic foot infections: polymicrobial; probe-to-bone test for osteomyelitis; MRI is imaging of choice
- Injection drug users: consider MRSA, Clostridium species, and unusual pathogens
- Immunocompromised: broader differential including fungal and atypical mycobacterial infections
Key Clinical Pearls
- Always outline the border of cellulitis to objectively assess treatment response
- Purulent SSTIs require drainage; antibiotics alone are inadequate
- Pain out of proportion is the most important early clue to necrotizing fasciitis
- The decision to operate for suspected necrotizing fasciitis is clinical, not radiographic
- Bilateral lower extremity erythema is almost never cellulitis; consider stasis dermatitis
References
- Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. Clin Infect Dis. 2014;59(2):e10-e52.
- Baddour LM. Cellulitis and skin abscess in adults: Treatment. UpToDate. 2025.
- Wong CH, Khin LW, Heng KS, et al. The LRINEC (Laboratory Risk Indicator for Necrotizing Fasciitis) score: a tool for distinguishing necrotizing fasciitis from other soft tissue infections. Crit Care Med. 2004;32(7):1535-1541.
- Talan DA, Mower WR, Krishnadasan A, et al. Trimethoprim-sulfamethoxazole versus placebo for uncomplicated skin abscess. N Engl J Med. 2016;374(9):823-832.