What tests are needed for the diagnosis of cellulitis?

Diagnostic Tests for Cellulitis

For uncomplicated cellulitis with mild signs and symptoms, laboratory tests are generally unnecessary as an etiologic diagnosis is often difficult and not required for management. 1

Clinical Assessment

• Evaluate severity of infection based on:
  • Systemic signs of toxicity (fever/hypothermia, tachycardia >100 beats/min, hypotension)
  • Pain disproportionate to physical findings
  • Skin manifestations (violaceous bullae, cutaneous hemorrhage, skin sloughing, skin anesthesia)
  • Rapid progression
  • Gas in tissue

Laboratory Tests

For patients with systemic toxicity or moderate-severe infection:

• Blood cultures
• Complete blood count with differential
• Serum creatinine
• Serum bicarbonate
• Creatine phosphokinase (CPK)
• C-reactive protein (CRP)

Indications for hospitalization:

• Hypotension and/or elevated creatinine
• Low serum bicarbonate
• Elevated CPK (2-3 times upper limit of normal)
• Marked left shift in differential
• CRP >13 mg/L 1

Microbiological Diagnosis

• Obtain specimens for culture before initiating antibiotic therapy when possible 1
• Blood cultures have low yield in typical cellulitis (positive in only ~5% of cases) but may be valuable in elderly patients (21.7% positive rate, particularly in those ≥65 years) 1, 2
• Tissue samples provide more accurate results than superficial swabs 1
• Culture methods:
  • Needle aspiration of inflamed skin (low sensitivity)
  • Punch biopsy (yields organisms in 20-30% of cases)
  • Tissue culture from debrided material 1

Imaging Studies

• Imaging studies should not delay surgical consultation and intervention when necrotizing infection is suspected 1
• Plain radiographs: Limited utility, may show soft tissue gas in advanced cases
• Ultrasound: Useful to differentiate cellulitis from abscess or necrotizing fasciitis 1
• CT scan: Higher sensitivity than plain films for identifying early necrotizing infections; can detect fat stranding, fluid collections, and gas along fascial planes 1
• MRI: Most sensitive for detecting deep tissue involvement, particularly for necrotizing fasciitis 1

Differential Diagnosis Considerations

For atypical presentations or when concerned about necrotizing infection, consider:

• LRINEC score (Laboratory Risk Indicator for Necrotizing Infection):
  • CRP >150 mg/L
  • WBC >15,000/μL
  • Hemoglobin <13.5 g/dL
  • Serum sodium <135 mmol/L
  • Creatinine >1.6 mg/dL
  • Glucose >180 mg/dL
  • Score ≥8 suggests 75% risk of necrotizing soft tissue infection 1

Special Considerations

• For diabetic patients with foot wounds, consider:
  • Wound assessment with sterile blunt probe to detect depth, foreign bodies, or bone involvement
  • Vascular assessment (Doppler studies, ABI, TcpO2)
  • Peripheral neuropathy assessment using 10-g monofilament 1

Practical Approach

1. For mild, uncomplicated cellulitis: Clinical diagnosis without laboratory tests
2. For moderate infections or patients with comorbidities: CBC, basic metabolic panel
3. For severe infections or systemic toxicity: Full laboratory panel including blood cultures, CPK, CRP
4. For suspected deep tissue involvement: Appropriate imaging and surgical consultation

Remember that most cellulitis is caused by streptococci (often group A) or Staphylococcus aureus, with specific risk factors potentially suggesting other pathogens 1, 3.