How do you differentiate between purulent and nonpurulent cellulitis?

Distinguishing Between Purulent and Nonpurulent Cellulitis

The key distinction between purulent and nonpurulent cellulitis is the presence of a collection of pus, with purulent cellulitis having a focal collection requiring drainage, while nonpurulent cellulitis presents as diffuse skin inflammation without a drainable focus. 1

Clinical Characteristics

Purulent Cellulitis

• Defining features:
  • Presence of a drainable collection of pus
  • Often presents as abscess, furuncle, or carbuncle
  • Commonly caused by Staphylococcus aureus (particularly MRSA)
  • Focal area of fluctuance, induration, or pointing
  • May have surrounding erythema and inflammation

Nonpurulent Cellulitis

• Defining features:
  • Diffuse spreading skin infection without purulent drainage or abscess
  • Typically caused by beta-hemolytic streptococci or methicillin-susceptible S. aureus
  • Presents with erythema, warmth, tenderness, and swelling
  • No focal collection of pus
  • May have lymphangitis and regional lymphadenopathy

Diagnostic Approach

Visual Examination

• Look for:
  • Purulent: Focal swelling, fluctuance, or visible collection of pus
  • Nonpurulent: Diffuse erythema with indistinct borders, warmth, and edema

Physical Examination

• Fluctuance test: Gentle palpation with two fingers to detect fluid movement suggestive of abscess (purulent)
• Skin texture: Purulent infections may have a central point or "head"
• Orange peel appearance (peau d'orange): More common in nonpurulent cellulitis due to superficial cutaneous edema 1

Point-of-Care Ultrasound

• Particularly useful in unclear cases 2
• Can detect:
  • Fluid collections indicating purulent infection
  • Cobblestoning pattern in nonpurulent cellulitis
  • Deep tissue involvement

Treatment Implications

The distinction between purulent and nonpurulent cellulitis is crucial for determining appropriate treatment:

Purulent Cellulitis

• Primary treatment: Incision and drainage of the purulent collection 1
• Antibiotic considerations:
  • For mild cases: Drainage alone may be sufficient
  • For moderate/severe cases: Add antibiotics with MRSA coverage (TMP-SMX, doxycycline, or clindamycin)

Nonpurulent Cellulitis

• Primary treatment: Antimicrobial therapy targeting streptococci and MSSA 1
• Antibiotic choices:
  • First-line: Penicillin, amoxicillin, dicloxacillin, or cephalexin
  • MRSA coverage generally not needed unless purulent areas develop or risk factors present 3

Common Pitfalls

1. Misdiagnosing deep abscesses: Some purulent collections may not be immediately apparent on visual inspection but require ultrasound for detection
2. Overlooking necrotizing infections: Watch for signs of deeper infection such as:
   • Severe pain out of proportion to skin findings
   • Rapid progression
   • Bullae, skin sloughing, or crepitus
   • Systemic toxicity 1
3. Confusing noninfectious conditions with cellulitis: Consider pseudocellulitis mimics such as:
   • Venous stasis dermatitis
   • Contact dermatitis
   • Lymphedema
   • Deep vein thrombosis 3

Algorithmic Approach to Differentiation

1. Initial assessment:

   • Is there visible pus, drainage, or abscess formation?
   • Is there a focal collection or fluctuance on palpation?
   • If yes to either → purulent cellulitis
   • If no to both → likely nonpurulent cellulitis

2. If diagnosis unclear:

   • Perform point-of-care ultrasound to detect occult fluid collections
   • Consider aspiration of suspected collections

3. Assess severity (for both types):

   • Mild: No systemic signs of infection
   • Moderate: Systemic signs present (fever, tachycardia, etc.)
   • Severe: Failed prior treatment, immunocompromised, or signs of deeper infection 1

By carefully distinguishing between purulent and nonpurulent cellulitis, clinicians can select the most appropriate treatment strategy, improving outcomes and reducing unnecessary antibiotic use.