Laboratory Tests for Cellulitis Beyond CBC
For patients with cellulitis, blood cultures are not routinely recommended, but specific laboratory tests should be ordered based on clinical presentation, with C-reactive protein (CRP) being the most important additional test beyond CBC. 1
Core Laboratory Tests
- C-reactive protein (CRP): Important inflammatory marker that can help assess severity and monitor response to treatment 2
- Basic metabolic panel: To evaluate kidney function, especially if systemic symptoms are present 1
- Erythrocyte sedimentation rate (ESR): Can be useful for monitoring chronic inflammatory conditions 2
When to Order Blood Cultures
Blood cultures are not routinely recommended for typical cases of cellulitis 1, but should be considered in patients with:
- Malignancy on chemotherapy 1
- Neutropenia 1
- Severe cell-mediated immunodeficiency 1
- Immersion injuries 1
- Animal bites 1
- Systemic inflammatory response syndrome (SIRS) 1
- Diabetes mellitus 3
- Alcoholism 3, 4
- Chronic obstructive pulmonary disease 3
- Lymphedema 5
- Presence of ipsilateral orthopedic implants 5
- Total white blood cell count >13.5 × 10⁶/μL 5
- Liver cirrhosis 5
- Chronic kidney disease 5
Additional Tests Based on Clinical Presentation
- Cutaneous aspirates, biopsies, or swabs: Consider in immunocompromised patients or those with unusual presentations 1
- Wound cultures: Useful when purulent drainage is present 1
- Pulse oximetry: If respiratory rate >25 breaths/min to document hypoxemia 1
- Laboratory Risk Indicator for Necrotizing Infection (LRINEC) score: Consider when necrotizing infection is suspected, including serum sodium, creatinine, and glucose levels 1
Tests for Complicated Cases or Special Situations
- Imaging studies: Consider if there is concern for deeper infection, abscess, or necrotizing fasciitis 1
- Liver function tests: Particularly if systemic symptoms are present or if considering medications that may affect liver function 1
- Urinalysis: If systemic symptoms suggest possible concurrent urinary tract infection 1
Common Pitfalls to Avoid
- Ordering blood cultures in uncomplicated cellulitis has a low yield (4.8%) and rarely changes management 4
- Delaying antibiotic therapy while waiting for culture results can worsen outcomes 1
- Not considering underlying conditions that may predispose to bacteremia, which occurs in approximately 8-11% of hospitalized cellulitis patients 3, 5
Follow-up Testing
- Serial CRP measurements can be useful to monitor response to therapy, as CRP has a shorter half-life than ESR and will decrease more rapidly with effective treatment 2
- Consider repeat laboratory tests if clinical improvement is not observed within 48-72 hours of antibiotic initiation 1
Remember that laboratory testing should complement, not replace, thorough clinical assessment, and the decision to order specific tests should be guided by the severity of infection and patient risk factors 1.