How can I differentiate cellulitis from an infected wound and what is the appropriate management?

Differentiating Cellulitis from Infected Wounds

Cellulitis is a diffuse spreading infection of the dermis and subcutaneous tissue without purulent collections, while an infected wound presents with purulent drainage or localized abscess formation—this distinction is critical because cellulitis requires antibiotics alone, whereas infected wounds often need surgical drainage in addition to antimicrobials. 1

Key Clinical Distinctions

Cellulitis Characteristics

• Diffuse spreading erythema with poorly defined borders extending beyond any visible wound 2, 3
• No purulent drainage or pus collections—the term cellulitis should be reserved for infections without abscess formation 4
• Yielding subcutaneous tissues on palpation—you can still feel the underlying fascial planes and muscle groups 1
• Rapidly spreading warmth, swelling, and tenderness over hours to days 4, 2
• May have systemic fever, often preceding skin changes 4, 3
• Possible lymphangitis (red streaking) and regional lymphadenopathy 4

Infected Wound Characteristics

• Purulent secretions (pus) are diagnostic of wound infection 1
• Localized to the wound site with defined margins 1
• At least 2 cardinal signs of inflammation: redness, warmth, swelling/induration, pain or tenderness 1
• May have wound dehiscence or breakdown 1
• Probing the wound edges permits ready dissection beyond margins if deeper infection present 1

Critical Diagnostic Maneuvers

Physical Examination Techniques

• Palpate the subcutaneous tissues: In cellulitis they remain soft and yielding; if they feel wooden-hard, suspect necrotizing fasciitis 1
• Use a sterile blunt probe to assess wound depth, detect foreign bodies, abscess cavities, or palpable bone (which has a characteristic stony feel) 1
• Assess for purulence: Any purulent drainage confirms wound infection rather than simple cellulitis 1
• Look for a "peau d'orange" (orange-peel) appearance suggesting superficial edema typical of erysipelas/cellulitis 4, 3

Timing Considerations

• Early infection (within 48 hours) suggests virulent organisms like β-hemolytic streptococci or Clostridium species 1
• Late infection (days 4-6 postoperatively) is typically polymicrobial 1

Management Algorithm

For Cellulitis (No Purulent Collections)

• Antimicrobial therapy targeting streptococci is the primary treatment 2, 5
• Oral antibiotics (penicillin, amoxicillin, cephalexin, or dicloxacillin) for β-hemolytic streptococci and methicillin-sensitive S. aureus 2, 6, 5
• 5-day course is as effective as 10-day course if clinical improvement seen at day 5 2
• Elevation of affected area accelerates improvement by promoting drainage 2
• MRSA coverage generally not needed for non-purulent cellulitis despite rising community rates 5

For Infected Wounds

• Obtain cultures before antibiotics when possible—tissue specimens are superior to swabs 1
• Debridement is essential: Remove all necrotic tissue, callus, and foreign material to expose viable tissue 1
• Measure wound dimensions and drainage characteristics to track healing 1
• Antibiotics are adjunctive to drainage in immunocompetent patients with simple abscesses 7
• For contaminated/dirty abdominal wounds, assume anaerobic coinfection regardless of culture results 1

When Cultures Are Indicated

• Cellulitis: Generally not needed as pathogens are predictable (aerobic gram-positive cocci); cultures positive in <40% of cases 7
• Infected wounds: Obtain tissue specimens (not swabs) before antibiotics, especially for deep infections, immunocompromised patients, or treatment failures 1, 7
• Swab specimens yield more organisms than deep tissue but may miss true pathogens 1

Critical Pitfalls to Avoid

Misdiagnosis of Cellulitis

• Only 33% of erythematous skin diagnosed as cellulitis is actually cellulitis 8
• Common mimics include venous stasis dermatitis, contact dermatitis, eczema, lymphedema, and deep vein thrombosis 6, 5
• Always consider if erythema is secondary to a deeper process like abscess, necrotizing fasciitis, or osteomyelitis 8

Failure to Recognize Deeper Infections

• Necrotizing fasciitis presents initially as cellulitis but progresses with wooden-hard subcutaneous tissues, skin anesthesia, and systemic toxicity 1
• Failure to respond to initial antibiotics suggests deeper tissue plane involvement 1
• Use ultrasound as first-line imaging; CT if ultrasound non-diagnostic 1

Wound Assessment Errors

• Not all ulcers are infected—infection requires purulence OR ≥2 inflammatory signs 1
• Failing to probe wounds misses deep abscesses, foreign bodies, and bone involvement 1
• Inadequate debridement leaves necrotic tissue that perpetuates infection 1

Special Circumstances

High-Risk Wounds

• Contaminated wounds (penetrating trauma <4 hours, major sterile technique breach): 15.2% infection rate 1
• Dirty wounds (purulent inflammation, perforation, trauma >4 hours): 40% infection rate 1
• These require assumption of polymicrobial infection including anaerobes 1

Recurrent Cellulitis Prevention

• Manage venous insufficiency and lymphatic obstruction 2
• Consider 5-day intranasal mupirocin twice daily for decolonization 2
• Each cellulitis episode causes permanent lymphatic damage, potentially leading to lymphedema 3