How to diagnose necrotizing fasciitis?

How to Diagnose Necrotizing Fasciitis

The diagnosis of necrotizing fasciitis relies primarily on high clinical suspicion combined with immediate surgical exploration when suspected, as the most definitive diagnostic feature is the appearance of necrotic fascia and subcutaneous tissues at operation. 1, 2

Clinical Diagnostic Features

The following clinical signs should raise immediate suspicion for necrotizing fasciitis:

• Pain severity disproportionate to physical findings is the hallmark early sign 2, 3
• Hard, wooden feel of subcutaneous tissue on palpation 2, 3
• Systemic toxicity with fever, hypotension, or altered mental status 2
• Edema or tenderness extending beyond the visible erythema 2, 3
• Crepitation (gas in tissues) 1, 2
• Bullous lesions or skin necrosis/ecchymosis 2, 3
• Failure to respond to initial antibiotic therapy within 24-48 hours 1, 2

Early diagnosis is missed or delayed in 85-100% of cases because initial presentation often resembles benign cellulitis. 4 The key is maintaining a high index of suspicion, particularly in patients with diabetes, immunocompromise, or recent trauma. 4, 5

Imaging Studies

Ultrasound (First-Line Bedside Tool)

• Perform bedside ultrasound immediately when necrotizing fasciitis is suspected 1
• Diagnostic findings: diffuse subcutaneous thickening with fluid accumulation >4 mm in depth along the deep fascial layer 1
• Sensitivity 88.2%, specificity 93.3%, diagnostic accuracy 91.9% 1

CT Scanning

• CT has 100% sensitivity and 81% specificity for identifying necrotizing soft tissue infections 1
• Useful when ultrasound is equivocal, but do not delay surgical exploration to obtain CT imaging 1

MRI

• MRI is the most sensitive imaging modality but should not be used as first-choice under emergency conditions 1, 3
• Key findings: thick (≥3 mm) abnormal signal on fat-suppressed T2, involvement of ≥3 compartments 1

Surgical Exploration (Gold Standard)

When clinical suspicion exists, proceed directly to surgical exploration rather than waiting for imaging confirmation. 1, 2 This approach is both diagnostic and therapeutic.

The "Finger Test"

• Perform under local anesthesia with a 2-cm incision down to deep fascia 1
• Positive findings indicating necrotizing fasciitis:
  • Minimal resistance to finger dissection along fascial planes 1
  • Absence of bleeding 1
  • Thin, brownish "dishwater" exudate (not true pus) 1, 2
  • Necrotic tissue present 1

Intraoperative Findings (Definitive Diagnosis)

The most important diagnostic features at operation include: 1, 2

• Fascia appears swollen and dull gray with stringy areas of necrosis 1, 2
• Extensive undermining of surrounding tissues 1, 2
• Easy dissection of tissue planes with gloved finger or blunt instrument 1, 2
• Absence of true pus even during deep dissection 1, 2

Gram Stain and Culture

• Obtain Gram stain of deep tissue exudate immediately during exploration 1
• Gram-positive cocci in chains suggest Streptococcus species 1
• Culture specimens from deep tissues, not superficial wounds 1
• Blood cultures should be obtained but are not required for diagnosis 3

Frozen Section Biopsy (Limited Role)

Frozen section biopsy should only be used when clinical or radiographic findings are not diagnostic. 1, 3 This technique is impractical in most settings and delays definitive surgical debridement. 1 If suspicion is high enough to perform biopsy, the diagnosis is usually evident on gross inspection. 1

Critical Timing Considerations

Any delay in diagnosis beyond 24 hours after admission significantly increases mortality. 6 In one series, mortality was 75% (3 of 4 patients) when surgical debridement was delayed more than 24 hours. 6

The decision to operate should be based on: 1

• No response to antibiotics after reasonable trial (24-48 hours)
• Profound toxicity, fever, hypotension, or advancement during antibiotic therapy
• Any skin necrosis with easy fascial dissection
• Gas in affected tissues

Common Diagnostic Pitfalls

• Mistaking early necrotizing fasciitis for simple cellulitis or abscess 4, 5
• Waiting for imaging results when clinical suspicion is high 3
• Relying on superficial wound cultures rather than deep tissue specimens 1
• Delaying exploratory incision in equivocal cases - if necrotizing infection is absent, the small exploratory incision carries minimal morbidity 1