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

• 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

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

1. 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.
2. Baddour LM. Cellulitis and skin abscess in adults: Treatment. UpToDate. 2025.
3. 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.
4. 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.

For the intern

At 3am, your priority is distinguishing necrotizing fasciitis from simple cellulitis. If your patient has pain out of proportion to exam, rapidly spreading erythema, crepitus, bullae, hemodynamic instability, or a Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score of 6 or higher, call surgery immediately—don't wait for imaging. For suspected necrotizing infection, start vancomycin plus piperacillin-tazobactam (add clindamycin for streptococcal coverage) while awaiting the operating room. For uncomplicated cellulitis, mark the borders with a pen, start IV cefazolin, and reassess in the morning. If you see a fluctuant abscess, arrange incision and drainage—antibiotics alone won't cure it. Drawing blood cultures in simple cellulitis is low-yield and can wait until morning rounds unless the patient is septic.

Common pitfalls

• Mistaking bilateral leg erythema for cellulitis: Bilateral presentation is almost always stasis dermatitis, not infection. True cellulitis is typically unilateral.
• Delaying surgery for imaging in suspected necrotizing fasciitis: The decision to operate is clinical. Imaging causes dangerous delays when you already have high suspicion based on exam and labs.
• Treating abscesses with antibiotics alone: Incision and drainage is the primary treatment. Antibiotics without drainage will fail.
• Missing early necrotizing fasciitis: Pain out of proportion is often the only early finding. Crepitus and skin necrosis appear later when tissue destruction is already extensive.

Make it stick (one 80-word vignette)

A 58-year-old male with diabetes presents with severe left leg pain. Exam shows erythema and swelling, but his pain is "10/10, way worse than it looks." You notice a small bulla forming. His white blood cell count is 22,000, CRP 200, sodium 130, and creatinine 1.8. His LRINEC score is 8. You immediately call surgery. They agree to take him to the operating room. You start broad-spectrum antibiotics, including clindamycin. The surgeon finds extensive fascial necrosis. Your prompt recognition saved his leg, and possibly his life.

If you remember one thing

Pain out of proportion to exam should immediately raise suspicion for necrotizing fasciitis; prioritize rapid surgical consultation over extensive workup. Always outline cellulitis borders to objectively track treatment response.

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