How do you differentiate Necrotizing Soft Tissue Infection (NSTI) from cellulitis?

Differentiating Necrotizing Soft Tissue Infection (NSTI) from Cellulitis

Necrotizing soft tissue infections (NSTIs) should be distinguished from cellulitis primarily through clinical findings, with severe pain disproportionate to physical findings being the most critical differentiating factor. 1, 2

Key Clinical Features to Differentiate NSTI from Cellulitis

High Suspicion Clinical Findings

• Severe pain that seems disproportional to the clinical findings 1, 2
• Failure to respond to initial antibiotic therapy 1
• Hard, wooden feel of subcutaneous tissue extending beyond apparent skin involvement 1, 2
• Systemic toxicity, often with altered mental status 1
• Edema or tenderness extending beyond the cutaneous erythema 1, 2
• Crepitus (indicating gas in tissues) 1
• Bullous lesions 1, 2
• Skin necrosis or ecchymoses 1

Systemic Signs More Common in NSTI

• Fever (patient-reported history of fever is significantly associated with NSTI) 1, 3
• Tachycardia 1
• Hypotension 1
• Shock 1

Laboratory Assessment

• The Laboratory Risk Indicator for Necrotizing Infection (LRINEC) score can support diagnosis but lacks sensitivity 1, 4
• LRINEC assigns points for:
  • C-reactive protein > 150 mg/L
  • WBC count > 15,000/μL
  • Hemoglobin < 13.5 g/dL
  • Serum sodium < 135 mmol/L
  • Creatinine > 1.6 mg/dL
  • Glucose > 180 mg/dL 1
• A score ≥ 8 indicates 75% risk of NSTI 1
• Elevated lactate on hospital presentation is significantly associated with NSTI rather than cellulitis 5, 3

Imaging Studies

• Imaging should never delay surgical consultation or intervention when clinical suspicion is high 1, 6
• CT with intravenous contrast is the preferred imaging modality, showing:
  • Fascial thickening
  • Lack of fascial enhancement (highly specific for NSTI)
  • Fat stranding
  • Fluid and gas collections along fascial planes 1, 6
• CT has demonstrated 100% sensitivity and 81% specificity for identifying NSTI in some studies 1, 6
• MRI has the highest sensitivity (93%) for detecting fascial fluid and edema but may delay treatment 6, 2
• Ultrasound may be useful in unstable patients (sensitivity 88.2%, specificity 93.3%) 1, 6
• Plain radiographs have limited value and should not be used to rule out NSTI 1, 6

Risk Factors More Associated with NSTI

• Male gender 5, 3
• Intravenous substance use 3
• Homelessness 5

Definitive Diagnosis

• Surgical exploration remains the gold standard diagnostic method 2, 7
• Key surgical findings include:
  • Swollen, dull gray fascia with stringy areas of necrosis
  • Thin, brownish exudate (not frank pus)
  • Extensive undermining of surrounding tissues
  • Tissue planes easily dissected with a gloved finger or blunt instrument 1, 2
• The "finger test" - a 2-cm incision down to deep fascia under local anesthesia - can provide rapid diagnosis with minimal risk 2

Common Pitfalls

• Most cases of NSTI are initially diagnosed as cellulitis 1, 4
• Clinical scoring systems are more useful for excluding NSTI than confirming it 1, 2
• Subcutaneous gas may not be present in earlier stages or in pure aerobic infections 1
• Rapidly progressive soft tissue infections should always be treated as necrotizing infections from the beginning 1
• The clinical picture may worsen very quickly, sometimes during a few hours 1

Remember that early recognition and surgical intervention are critical for improving outcomes in NSTI, as time to operative debridement is an important determinant of survival 1, 4.