What is the recommended approach to assess lower‑extremity cellulitis?

Assessment of Lower Extremity Cellulitis

The assessment of lower extremity cellulitis is primarily clinical, based on the presence of expanding erythema, warmth, tenderness, and edema, with laboratory testing and imaging reserved only for patients with systemic toxicity or concern for deeper infection. 1, 2

Clinical Diagnostic Criteria

The diagnosis of cellulitis is fundamentally clinical and does not require laboratory confirmation in uncomplicated cases. 3, 4 Look for these specific features:

• Poorly demarcated unilateral erythema with associated warmth and tenderness—this distinguishes cellulitis from erysipelas, which has sharply demarcated borders 4, 5
• Expanding area of involvement over hours to days, which the patient can often describe 3
• Edema and induration of the affected area, reflecting deep dermal and subcutaneous tissue involvement 3, 6
• Absence of purulent drainage or fluctuance—if present, this suggests abscess requiring drainage rather than simple cellulitis 2, 7

Essential History Elements

Obtain these specific historical details to guide assessment:

• Preceding skin trauma: minor abrasion, insect bite, injection site, or any portal of entry 8, 9
• MRSA risk factors: penetrating trauma, injection drug use, known MRSA colonization, athletes, prisoners, military recruits, long-term care residents, or prior MRSA infection 2, 3
• Predisposing conditions: tinea pedis, chronic edema, venous insufficiency, lymphedema, obesity, diabetes, or immunosuppression 7, 9
• Timing and progression: rapid progression over hours suggests necrotizing infection requiring emergent surgical evaluation 1, 6

Physical Examination Priorities

Measure and document the extent of erythema to track progression or improvement—this is the single most important monitoring parameter. 8 Specifically assess:

• Interdigital toe spaces for tinea pedis, fissuring, scaling, or maceration—these are common portals of entry and must be treated to prevent recurrence 2, 7
• Presence of purulent drainage or exudate—this mandates MRSA coverage regardless of other risk factors 2, 7
• Warning signs of necrotizing infection: severe pain disproportionate to examination, skin anesthesia, rapid progression, "wooden-hard" subcutaneous tissues, violaceous bullae, cutaneous hemorrhage, skin sloughing, or gas in tissue 1, 2
• Systemic toxicity signs: fever >38°C, heart rate >90 bpm, respiratory rate >24 breaths/min, hypotension, or altered mental status 1, 7

Laboratory Testing—When and What to Order

For uncomplicated cellulitis without systemic signs, no laboratory testing is required. 1, 3 Blood cultures are positive in only 5% of cases and are unnecessary for typical presentations. 8

Obtain laboratory studies only when systemic toxicity is present (fever, tachycardia, hypotension, altered mental status): 1

• Blood cultures and susceptibility testing 1
• Complete blood count with differential 1
• Creatinine, bicarbonate, creatine phosphokinase, and C-reactive protein 1

Hospitalization should be considered if any of these laboratory abnormalities are present: 1

• Hypotension or elevated creatinine
• Low serum bicarbonate
• Elevated creatine phosphokinase (2-3 times upper limit of normal)
• Marked left shift on differential
• C-reactive protein >13 mg/L

Imaging and Invasive Diagnostic Procedures

Imaging is not routinely required for typical cellulitis. 9 Consider specific modalities only in these scenarios:

• Ultrasound: if clinical uncertainty exists regarding abscess formation—any purulent collection requires incision and drainage as primary treatment 2, 8
• Plain radiographs or MRI: when osteomyelitis is suspected based on probing to bone or chronic non-healing wounds, particularly in diabetic patients 8
• Gram stain and culture of needle aspiration or punch biopsy specimens: only in patients with hypotension, elevated creatinine, low bicarbonate, elevated CPK, marked left shift, or C-reactive protein >13 mg/L 1

Distinguishing Cellulitis from High-Risk Mimics

Emergent surgical consultation is mandatory if any of these features suggest necrotizing fasciitis or deeper infection: 1, 6

• Pain disproportionate to physical findings 1
• Violaceous bullae or cutaneous hemorrhage 1
• Skin sloughing or anesthesia 1
• Rapid progression over hours 1
• Gas in tissue on palpation or imaging 1
• "Wooden-hard" subcutaneous tissues 2

Other critical mimics to exclude include: 6

• Deep vein thrombosis: unilateral leg swelling with calf tenderness—obtain D-dimer and venous duplex ultrasound if suspected
• Septic bursitis or joint: focal fluctuance over bursa or joint effusion—requires arthrocentesis
• Necrotizing fasciitis: systemic toxicity with rapid progression—requires emergent surgical exploration

Hospitalization Criteria

Admit patients with any of the following: 7, 8

• Systemic inflammatory response syndrome (SIRS): fever, tachycardia, tachypnea, or abnormal WBC 7
• Hemodynamic instability or altered mental status 7
• Severe immunocompromise or neutropenia 1
• Concern for deeper or necrotizing infection 7
• Failure of outpatient treatment after 24-48 hours 8
• Poor adherence anticipated with outpatient therapy 7

Special Population Considerations

Diabetic patients require comprehensive vascular assessment: 8

• Ankle-brachial index to detect peripheral vascular disease 8
• 10-g monofilament testing for loss of protective sensation 8
• Tissue specimens (not swabs) for culture if infection is present 8
• Broader antimicrobial coverage for polymicrobial diabetic foot infections 8

Elderly patients should be assessed for: 8

• Underlying venous insufficiency and lymphedema 8
• Chronic edema as a major predisposing factor 8

Common Pitfalls to Avoid

• Do not obtain blood cultures or extensive laboratory testing for typical uncomplicated cellulitis—this represents unnecessary resource utilization 1, 8
• Do not delay surgical consultation if any warning signs of necrotizing infection are present—these infections progress rapidly and require debridement 1
• Do not assume bilateral lower extremity erythema is cellulitis—true cellulitis is almost always unilateral; bilateral presentation suggests venous stasis dermatitis or other non-infectious etiology 2, 4
• Do not miss abscess formation—any fluctuance requires drainage as primary treatment, not antibiotics alone 2