What labs can be ordered for a patient with cellulitis?

Laboratory Tests for Patients with Cellulitis

For most patients with typical cellulitis, laboratory tests are not routinely recommended, as diagnosis is primarily based on clinical examination. 1

Diagnostic Approach Based on Clinical Presentation

Routine Cases (Mild, Uncomplicated Cellulitis)

• No laboratory tests required for typical cases of cellulitis without systemic signs of infection 1
• Clinical diagnosis based on:
  • Expanding erythema, warmth, tenderness, and swelling
  • Characteristic "orange peel" appearance (peau d'orange) due to superficial cutaneous edema

When to Order Laboratory Tests

Blood Cultures

• Not routinely recommended for uncomplicated cellulitis (strong recommendation, moderate evidence) 1
• Should be obtained in patients with:
  • Malignancy on chemotherapy 1
  • Neutropenia 1
  • Severe cell-mediated immunodeficiency 1
  • Immersion injuries 1
  • Animal bites 1
  • Intravascular prosthesis, pacemaker, or valvular prosthesis 1
  • Severe systemic features (high fever, hypotension) 1

Tissue Aspirates, Biopsies, or Swabs

• Not routinely recommended for typical cellulitis 1
• Consider in the same high-risk patients as listed for blood cultures 1
• Low yield: Studies show aspirate cultures are positive in only about 33% of cases 2, 3

Laboratory Tests for Specific Clinical Scenarios

Patients with Systemic Signs of Infection

• Complete blood count with differential 1
• Blood cultures 1
• C-reactive protein 1
• Creatinine 1
• Bicarbonate 1
• Creatine phosphokinase 1

Patients with Suspected Diabetes

• Serum glucose 1
• Hemoglobin A1c 1
• Urine ketones 1

Elderly Patients

• Consider blood cultures - higher yield in patients ≥65 years (25.3% vs 8.5% in younger patients) 4
• Especially with:
  • Shaking chills
  • White blood count ≥13,000 cells/μL 4

Indications for Hospital Admission and More Extensive Testing

Hospitalization and more extensive laboratory evaluation should be considered for patients with:

• Systemic inflammatory response syndrome (SIRS) 1
• Altered mental status 1
• Hemodynamic instability 1
• Signs of deeper or necrotizing infection 1
• Poor adherence to therapy 1
• Severe immunocompromise 1
• Failure of outpatient treatment 1

Warning Signs Requiring Urgent Evaluation

Laboratory tests should be performed urgently when the following warning signs are present:

• Pain disproportionate to physical findings 1
• Violaceous bullae 1
• Cutaneous hemorrhage 1
• Skin sloughing 1
• Skin anesthesia 1
• Rapid progression 1
• Gas in the tissue 1

Common Pitfalls

1. Overreliance on laboratory tests: Diagnosis of cellulitis is primarily clinical; laboratory tests have low yield in uncomplicated cases 5

2. Failure to identify high-risk patients: Immunocompromised patients and those with severe systemic symptoms need more extensive laboratory evaluation 1

3. Missing underlying conditions: Undiagnosed diabetes is common in patients with cellulitis and should be screened for 1

4. Inappropriate use of blood cultures: Blood cultures have low yield (approximately 2-8%) in uncomplicated cellulitis but higher yield in elderly and immunocompromised patients 2, 4

5. Delaying treatment: Laboratory results should not delay initiation of appropriate antibiotic therapy in patients with clear clinical signs of cellulitis 1