Do I need to order a CT (computed tomography) scan of the lower legs and an ultrasound (US) of the lower legs for cellulitis?

Imaging for Cellulitis of the Lower Legs

For uncomplicated cellulitis of the lower legs, neither CT nor ultrasound is routinely indicated as initial imaging studies. 1, 2

Initial Evaluation Approach

• Plain radiographs should be the initial imaging study if any imaging is needed, to:
  • Exclude alternative diagnoses (fractures, tumors)
  • Identify radiodense foreign bodies
  • Detect soft tissue gas
  • Evaluate for underlying osteomyelitis 1

When to Consider Ultrasound

Ultrasound is appropriate in specific scenarios:

1. Suspected abscess formation requiring drainage
2. Uncertain diagnosis based on clinical examination
3. Concern for necrotizing fasciitis with findings such as:
   • Diffuse subcutaneous thickening
   • Fluid accumulation >4mm along deep fascial layer
   • Fascial thickening ≥3mm 1, 2

Ultrasound has demonstrated excellent utility for detecting:

• Soft tissue fluid collections and abscesses (sensitivity 88-93%, specificity 93-97%)
• Foreign bodies (especially radiolucent ones like wood or plastic)
• Joint effusions 1

When to Consider CT

CT is generally not indicated for uncomplicated cellulitis and should be reserved for:

1. Suspected deep tissue infection not adequately visualized by ultrasound
2. Concern for necrotizing fasciitis with systemic toxicity
3. Detection of soft tissue gas when necrotizing infection is suspected (89% sensitivity, 93% specificity) 1

A recent study found that CT for extremity cellulitis had a low yield (5.5%) for detecting deep infection, suggesting potential overuse 3.

Clinical Pearls and Pitfalls

• Don't delay treatment while waiting for imaging if there is high clinical suspicion of severe infection 1, 2
• Misdiagnosis is common: Venous insufficiency, eczema, deep vein thrombosis, and gout are frequently mistaken for cellulitis 4
• Risk factors for recurrent cellulitis include:
  • Previous episodes of cellulitis (OR 31.04)
  • Presence of S. aureus/beta-hemolytic streptococci in toe webs (OR 28.97)
  • Leg erosions or ulcers (OR 11.80)
  • Prior saphenectomy (OR 8.49) 5

Algorithm for Imaging Decision

1. Clinical diagnosis of uncomplicated cellulitis: No imaging needed
2. If uncertain diagnosis or complications suspected:
   • Start with plain radiographs
   • If abscess suspected → Ultrasound
   • If necrotizing fasciitis or deep tissue involvement suspected → Consider CT or MRI
3. If foreign body suspected:
   • Radiographs for radiodense objects
   • Ultrasound for radiolucent objects 1, 2

Remember that cellulitis is primarily a clinical diagnosis, and imaging should be used judiciously to evaluate for complications or alternative diagnoses rather than to confirm the presence of cellulitis itself.