Diagnosing Cellulitis
Cellulitis is diagnosed clinically based on characteristic skin findings—expanding erythema, warmth, tenderness, and swelling—without the need for cultures, biopsies, or laboratory testing in typical uncomplicated cases. 1
Clinical Diagnostic Criteria
The diagnosis relies entirely on history and physical examination findings 1, 2:
- Rapidly spreading erythema with poorly demarcated borders 3
- Warmth over the affected area 2, 4
- Tenderness and pain on palpation 4, 5
- Edema and swelling of the involved tissue 3
- "Peau d'orange" appearance where superficial edema surrounds hair follicles, causing dimpling as follicles remain tethered to underlying dermis 3
Additional Clinical Features to Assess
Skin Changes
- Vesicles or bullae filled with clear fluid may develop 3
- Petechiae or ecchymoses can appear in inflamed skin 3
- Lymphangitis (red streaking) may be present 3
- Regional lymph node inflammation often accompanies the infection 3
Systemic Manifestations
While usually mild, systemic signs may include 3:
- Fever
- Tachycardia
- Confusion
- Hypotension
- Leukocytosis
Important caveat: These systemic symptoms may occur hours before skin abnormalities appear 3
When Laboratory Testing is NOT Needed
For typical cellulitis without systemic symptoms, blood cultures, tissue aspirates, and skin biopsies are unnecessary. 1 This is critical because:
- Blood cultures are positive in only 5% of cases 3
- Needle aspiration yields organisms in only 5-40% of cases with highly variable results 3
- Punch biopsy cultures are positive in only 20-30% of cases with very low bacterial concentrations 3
- The majority of cellulitis cases (85%) are non-culturable 2
Red Flags Requiring Deeper Investigation
If widespread petechiae/ecchymoses occur with systemic toxicity, consider necrotizing fasciitis rather than simple cellulitis. 3 Other warning signs include 1:
- Pain disproportionate to physical findings
- Violaceous bullae or cutaneous hemorrhage
- Skin sloughing or anesthesia
- Rapid progression
- Gas in tissue or crepitus
- Hard, wooden feel of subcutaneous tissue extending beyond visible skin involvement
- Edema or tenderness extending beyond the erythema
Essential History Elements
Identify predisposing factors and entry sites 3:
- Skin barrier disruption: trauma, preexisting infections (impetigo, ecthyma), ulceration, fissured toe webs from tinea pedis or maceration, inflammatory dermatoses (eczema)
- Lymphatic compromise: prior saphenous venectomy, axillary node dissection, pelvic surgery with lymph node dissection
- Host factors: obesity, venous insufficiency, lymphedema
- Special exposures: animal bites (Pasteurella), fresh water immersion (Aeromonas), salt water exposure (Vibrio), occupational exposures
Critical pitfall: Examine interdigital toe spaces for maceration or tinea pedis, as streptococci colonizing these areas are a frequent source of lower extremity cellulitis 3
Differential Diagnosis Considerations
Cellulitis is frequently misdiagnosed 4, 5. Common mimics include 6, 7:
- Venous stasis dermatitis
- Contact dermatitis
- Deep vein thrombosis
- Lymphedema
- Eczema
- Drug reactions
- Insect stings
If cellulitis does not respond to appropriate first-line antibiotics within 48 hours, reconsider the diagnosis or evaluate for resistant organisms, immunosuppression, chronic liver disease, or chronic kidney disease. 2
Diagnostic Algorithm
- Assess for cardinal features: expanding erythema, warmth, tenderness, swelling 1, 2
- Examine for entry site: check toe webs, look for trauma, bites, or preexisting skin conditions 3
- Evaluate for red flags suggesting necrotizing infection 1
- Consider alternative diagnoses if presentation is atypical 7
- No laboratory testing needed for typical uncomplicated cases 1
- Initiate empiric antibiotic therapy targeting streptococci 1, 4