Cellulitis vs Necrotizing Fasciitis
Cellulitis and Necrotizing Fasciitis are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.
Cellulitis vs Necrotizing Fasciitis at a glance
- Cellulitis: Acute bacterial infection of the dermis and subcutaneous tissue, most often caused by beta-hemolytic streptococci or Staphylococcus aureus.
- Necrotizing Fasciitis: Rapidly progressive, life-threatening deep soft tissue infection requiring emergent surgical debridement and broad-spectrum antibiotics.
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Side-by-side comparison
| Feature | Cellulitis | Necrotizing Fasciitis |
|---|---|---|
| At a glance | Acute bacterial infection of the dermis and subcutaneous tissue, most often caused by beta-hemolytic streptococci or Staphylococcus aureus. | Rapidly progressive, life-threatening deep soft tissue infection requiring emergent surgical debridement and broad-spectrum antibiotics. |
| Classic presentation | Unilateral lower-leg cellulitis is extremely common; bilateral lower-leg 'cellulitis' is almost never cellulitis — consider stasis dermatitis, lipodermatosclerosis, or contact dermatitis.; Localized warmth, erythema, edema, tenderness — usually a single extremity; Spreading borders without sharp demarcation; Fever, chills, malaise in… | Patient with severe pain disproportionate to exam, rapidly spreading edema, hemorrhagic bullae, and systemic toxicity — emergency surgical exploration regardless of imaging.; Pain out of proportion to physical findings — hallmark early sign; Rapid progression of erythema, swelling, induration over hours; Systemic toxicity: fever,… |
| Workup / key labs | Clinical diagnosis. IDSA stratifies into mild, moderate, severe and purulent vs non-purulent for treatment selection.; CBC, BMP, lactate if systemic toxicity; Blood cultures only if severe sepsis, immunocompromise, water exposure, animal/human bites, or unusual organism suspected; Wound culture if purulent drainage or abscess (incise… | Clinical diagnosis confirmed at surgical exploration: gray, necrotic, easily dissected fascia ('finger test'); lack of bleeding; foul-smelling discharge. Definitive diagnosis is surgical.; CBC (leukocytosis with bandemia), CMP (AKI, hyponatremia <135), CRP, lactate, coagulation studies; Blood cultures, wound cultures (deep tissue at… |
| Imaging | Bedside ultrasound to differentiate cellulitis (cobblestoning) from abscess (anechoic collection); Plain films for foreign body, soft tissue gas (necrotizing infection), or osteomyelitis; CT/MRI if necrotizing fasciitis suspected (but do NOT delay surgical consultation for imaging) | Plain films may show soft tissue gas (not always present); CT with contrast (most useful): fascial thickening, gas, fluid tracking along fascia, lack of fascial enhancement; MRI sensitive but rarely available emergently; Bedside ultrasound: subcutaneous gas, fluid; DO NOT delay surgical consultation for imaging in a deteriorating patient |
| First-line treatment | Non-purulent cellulitis (likely beta-hemolytic strep):; • Mild outpatient: oral beta-lactam — cephalexin 500 mg PO QID, dicloxacillin 500 mg PO QID, or penicillin VK; • Moderate: IV cefazolin or ceftriaxone; • Severe (systemic toxicity, rapid progression): IV vancomycin + piperacillin-tazobactam pending evaluation for necrotizing… | Emergent surgical debridement is the cornerstone — every hour of delay increases mortality (Wong 2003); Broad-spectrum empiric antibiotics immediately:; • Vancomycin OR linezolid OR daptomycin (MRSA coverage; linezolid also blocks toxin production); • PLUS piperacillin-tazobactam OR carbapenem (meropenem, imipenem) for gram-negative and… |
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