What labs and imaging studies are recommended for diagnosing and managing cellulitis?

Laboratory and Imaging Studies for Cellulitis

Blood cultures and tissue sampling are not routinely recommended for typical cases of cellulitis, but should be considered in patients with severe systemic features, immunocompromise, unusual predisposing factors, or when operative intervention is needed. 1, 2

Diagnostic Approach

Clinical Diagnosis

• Cellulitis is primarily diagnosed clinically based on the presence of erythema, warmth, swelling, and tenderness 2
• The diagnosis does not routinely require laboratory testing or imaging in typical cases 1, 2

Laboratory Studies

Blood Cultures

• Not recommended for routine, uncomplicated cellulitis (positive in only ~5% of cases) 1, 2
• Should be obtained in patients with:
  • Malignancy on chemotherapy 1, 2
  • Neutropenia 1
  • Severe cell-mediated immunodeficiency 1
  • Immersion injuries or animal bites 1
  • Severe systemic features (high fever, hypotension) 2
  • SIRS (Systemic Inflammatory Response Syndrome) 1

Risk Factors for Bacteremia

• Specific conditions associated with higher risk of bacteremia include:
  • Age ≥65 years 3
  • Liver cirrhosis 4, 3
  • Chronic kidney disease 4
  • Lymphedema 4
  • Presence of orthopedic implants in affected limb 4
  • Leukocytosis >13.5 × 10^6/μL 4
  • SIRS 3

Tissue Sampling

• Cultures of cutaneous aspirates, biopsies, or swabs are not routinely recommended 1
• Consider tissue sampling in the same high-risk patients who warrant blood cultures 1
• When obtaining cultures, tissue samples are preferred over superficial swabs as they provide more accurate results 1, 2
• Skin aspiration may yield pathogens in cellulitis but is generally insensitive 1

Imaging Studies

When to Consider Imaging

• Imaging is not routinely required for typical cellulitis 2, 5
• Consider imaging when there is concern for:
  • Deeper infection or necrotizing process 1
  • Abscess formation 1
  • Foreign bodies 1
  • Bone or joint involvement 1

Imaging Modalities

• Ultrasonography (especially high resolution):
  • Useful for detecting deep soft-tissue abscesses or sinus tracts 1
  • Can help distinguish cellulitis from abscess 1
• CT scanning:
  • May help detect deep soft-tissue abscesses or sinus tracts 1
• Plain radiographs:
  • Best for initial assessment of potential bone involvement 1
• MRI:
  • Most sensitive for detecting bone involvement 1
  • Provides anatomic information about sinus tracts, abscesses, or muscle involvement 1
• Nuclear medicine scans:
  • Highly sensitive but less specific than MRI 1
  • May be useful in select cases 1

Management Implications

Hospitalization Criteria

• Outpatient therapy is recommended for patients without:
  • SIRS 1
  • Altered mental status 1
  • Hemodynamic instability 1
• Hospitalization is recommended if:
  • Concern for deeper or necrotizing infection 1
  • Poor adherence to therapy 1
  • Severe immunocompromise 1
  • Outpatient treatment is failing 1

Wound Assessment

• For cellulitis associated with wounds:
  • Measure and record wound size, extent of surrounding cellulitis, and quality/quantity of drainage 1
  • Use a sterile, blunt metal probe to measure depth and extent of wounds 1
  • Note any foreign bodies, soft-tissue abscesses, or communications with joint cavities 1

Treatment Duration

• A 5-day course of antimicrobial therapy is recommended for typical cellulitis 1, 2
• Treatment should be extended if infection has not improved within this time period 1

Special Considerations

Diabetic Foot Infections

• More comprehensive evaluation is needed for diabetic patients with foot infections 1
• Additional assessments should include:
  • Vascular status (using Doppler studies, ABI, ankle/toe pressures) 1
  • Neuropathy assessment (using 10-g monofilament test) 1
  • More aggressive wound assessment and debridement 1

Recurrent Cellulitis

• Examine interdigital toe spaces for fissuring, scaling, or maceration that may harbor pathogens 1
• Address predisposing factors such as edema or underlying cutaneous disorders 1, 5
• Consider decolonization regimens for recurrent Staphylococcus aureus infections 1