What are the diagnostic tests for cellulitis?

From the Guidelines

Diagnostic tests for cellulitis are not routinely recommended, and the diagnosis is primarily made based on clinical examination, including the presence of erythema, warmth, swelling, and tenderness in the affected area. The diagnosis of cellulitis relies heavily on clinical judgment, and there is no single definitive test for this skin infection 1.

Clinical Examination

The clinical examination is the most important element in diagnosing cellulitis, and it includes assessing the affected area for signs of infection such as erythema, warmth, swelling, and tenderness.

Laboratory Tests

Blood tests, including complete blood count (CBC), C-reactive protein (CRP), or erythrocyte sedimentation rate (ESR), may be ordered to assess inflammation levels and identify any potential bacterial infection 1.

• Blood cultures are recommended for patients with severe infection, systemic symptoms, or immunocompromise to identify any bacteria in the bloodstream.
• Wound cultures from any open lesions or drainage may help identify the causative organism, though these aren't always reliable.

Imaging Studies

In cases where the diagnosis is unclear or complications are suspected, imaging studies like ultrasound can help distinguish cellulitis from deeper infections such as abscess formation 1.

• MRI or CT scans might be used in complex cases to evaluate the extent of infection or to rule out osteomyelitis (bone infection).
• For patients with recurrent cellulitis, testing for predisposing conditions like diabetes or immunodeficiency may be warranted.

Diagnostic Approach

These diagnostic approaches help clinicians confirm cellulitis and distinguish it from other conditions that may present similarly, such as deep vein thrombosis, contact dermatitis, or gout 1.

• The recommended duration of antimicrobial therapy is 5 days, but treatment should be extended if the infection has not improved within this time period 1.
• Elevation of the affected area and treatment of predisposing factors, such as edema or underlying cutaneous disorders, are recommended 1.

From the Research

Diagnostic Tests for Cellulitis

• Cellulitis is a clinical diagnosis based on the history of present illness and physical examination, and it lacks a gold standard for diagnosis 2.
• The diagnosis of cellulitis can be confirmed using a combination of:
  • History and physical examination
  • Laboratory tests, such as blood cultures and complete blood counts
  • Imaging studies, such as ultrasound or magnetic resonance imaging (MRI)
  • Skin biopsy, in some cases 3, 4
• Clinical scoring assessments, in vivo imaging techniques, and laboratory assessments are also used to diagnose non-purulent cellulitis 5.
• Microbiological diagnosis is often unobtainable due to poor sensitivity of culture specimens, but cultures of aspirates from the advancing edge of cellulitis, skin biopsy specimens, and blood can be used to identify potential microbial pathogens 6.

Challenges in Diagnosing Cellulitis

• Cellulitis can be difficult to diagnose due to a number of infectious and non-infectious clinical mimickers, such as venous stasis dermatitis, contact dermatitis, eczema, lymphedema, and erythema migrans 2.
• Clinical presentation with acute onset of redness, warmth, swelling, and tenderness and pain is typical, but can also be seen in other conditions 2.
• Undertaking a holistic patient assessment, skin assessment, and thorough clinical history is important in the diagnosis of cellulitis 4.

Laboratory Tests

• Blood cultures and complete blood counts can be used to support the diagnosis of cellulitis 3, 4.
• Skin biopsy can be used to confirm the diagnosis of cellulitis, especially in cases where the diagnosis is uncertain 3, 6.
• Aspirate, biopsy, or blood cultures can be positive in patients with apparent primary lesions, and can provide microbiologic information that can guide treatment 6.