Determining Necrotizing Fasciitis: Diagnostic Criteria and Approach
Necrotizing fasciitis is primarily determined by clinical presentation, with a wooden-hard feel of subcutaneous tissues being pathognomonic, supported by laboratory findings such as the LRINEC score, and confirmed through surgical exploration when suspected. 1
Clinical Presentation and Initial Assessment
Key Clinical Features
- Edema extending beyond visible erythema
- Wooden-hard feel of subcutaneous tissues (pathognomonic)
- Skin discoloration and anesthesia of involved skin
- Bullous lesions and skin necrosis
- Broad erythematous tract along infection route
- Systemic toxicity: high fever, disorientation, altered mental status 1
Diagnostic Tests
- Probe test: Blunt instrument dissection of superficial fascial planes beyond wound margins 1
- Finger test: Surgical exploration revealing lack of resistance to finger dissection, presence of dishwater-gray exudate 2, 1
- Frozen-section biopsy: Early performance (within hours of symptom onset) can provide definitive diagnosis with significant mortality reduction 3
Laboratory Assessment
LRINEC Score
Points assigned for abnormalities in six variables:
- C-reactive protein > 150 mg/L
- White blood cell count > 15,000/μL
- Hemoglobin < 13.5 g/dL
- Serum sodium < 135 mmol/L
- Serum creatinine > 1.6 mg/dL
- Serum glucose > 180 mg/dL
A score ≥8 indicates 75% risk of necrotizing fasciitis 2
Important caveat: Recent evidence shows LRINEC lacks sensitivity for diagnosing necrotizing infections, with sensitivity of only 59.2% for scores <6 2, 4
Imaging Studies
Imaging should never delay surgical consultation and intervention 2, 1
Computed Tomography (CT)
- Most sensitive modality for detecting soft tissue gas (89% sensitivity, 93% specificity)
- Key findings: fascial thickening, fat stranding, fluid/gas collections along fascial planes 2
- 100% sensitivity, 81% specificity, 76% positive predictive value, 100% negative predictive value 2
Magnetic Resonance Imaging (MRI)
- Gold standard imaging method with highest soft tissue contrast 5
- Key findings: thick (≥3mm) abnormal signal intensity on fat-suppressed T2-weighted images, low signal intensity in deep fascia, non-enhancing portions in deep fascia 2
- 100% sensitivity, 86% specificity 4
Ultrasound
- Rapid bedside assessment
- Findings: diffuse subcutaneous thickening with fluid accumulation >4mm along deep fascial layer
- 88.2% sensitivity, 93.3% specificity, 91.9% diagnostic accuracy 2
Predisposing Factors
Risk Conditions
- Diabetes mellitus (significant risk factor for limb amputation) 1, 6
- Arteriosclerotic vascular disease
- Venous insufficiency with edema
- Obesity
- Immunocompromised states
- Alcoholism 1
Triggering Events
- Minor trauma (80% of cases)
- Insect bites
- Injection sites
- Boils or abscesses
- Chronic vascular ulcers
- Perianal or Bartholin gland abscesses 1
Diagnostic Algorithm
- High clinical suspicion based on presentation (wooden-hard feel, disproportionate pain)
- Calculate LRINEC score - useful if high (≥8) but cannot rule out if low
- Perform bedside tests (probe test, finger test) if wound present
- Obtain imaging (CT or MRI) if diagnosis uncertain but do not delay surgical consultation
- Perform frozen-section biopsy for definitive early diagnosis
- Immediate surgical exploration if clinical suspicion remains high regardless of other findings
Remember: Early diagnosis is missed or delayed in 85-100% of cases due to lack of specific clinical features in initial stages 7. Maintain a high index of suspicion, as this remains the most important tool for early diagnosis.