Is Non-Purulent Cellulitis Associated with Leukocytosis?
Non-purulent cellulitis can be associated with leukocytosis, but this finding is neither universal nor required for diagnosis—leukocytosis when present (WBC ≥14,000 cells/mm³ or left shift ≥6%) strongly suggests underlying bacterial infection and warrants careful assessment, but its absence does not exclude cellulitis. 1
Evidence Supporting the Association
Leukocytosis as a Marker of Bacterial Infection
An elevated total WBC count (≥14,000 cells/mm³) has a likelihood ratio of 3.7 for detecting documented bacterial infection in older adults with suspected infection, with or without fever. 1
An elevated total band count (≥1,500 cells/mm³) demonstrates the highest likelihood ratio of 14.5 for bacterial infection, while a left shift (≥6% band neutrophils) has a likelihood ratio of 4.7. 1
In elderly patients with cellulitis, leukocytosis (WBC ≥13,000 cells/µL) is an independent risk factor for bacteremia, along with shaking chills. 2
Leukocytosis has been associated with increased mortality in long-term care facility residents with nursing home–acquired pneumonia (WBC ≥15,000 cells/mm³) and bloodstream infection (WBC ≥20,000 cells/mm³). 1
Clinical Context and Interpretation
In the absence of fever, leukocytosis and/or left shift warrant careful assessment for bacterial infection in any long-term care facility resident with suspected infection. 1
Without fever, leukocytosis, or left shift, additional diagnostic tests may not be indicated due to low potential yield, though nonbacterial infections cannot be excluded. 1
The presence of leukocytosis in cellulitis does not automatically indicate MRSA involvement—in non-purulent cellulitis, β-hemolytic streptococci or methicillin-sensitive Staphylococcus aureus are the predominant pathogens even when WBC is elevated. 3, 4
Key Clinical Distinctions
Non-Purulent vs. Purulent Cellulitis
Non-purulent cellulitis patients are typically older and more likely to have lower limb involvement, tinea pedis, stasis dermatitis, and higher Charlson comorbidity scores compared to purulent cellulitis patients. 4
In hospitalized Taiwanese adults, the etiological agent was identified in only 35.5% of non-purulent cases, with β-hemolytic streptococci the most frequent cause (70.2%). 4
Purulent cellulitis was associated with longer hospital stays and duration of antimicrobial therapy, but the presence of purulence—not leukocytosis—was the positive predictor of MRSA. 4
When to Obtain Blood Cultures
Blood cultures are positive in only ~5% of typical uncomplicated cellulitis cases and are generally unnecessary. 5
However, in elderly patients (≥65 years), the bacteremia rate reaches 25.3%, with shaking chills and WBC ≥13,000 cells/µL as independent risk factors—two routine sets of blood cultures are recommended in this population. 2
Blood cultures should be obtained in patients with severe systemic features, malignancy, neutropenia, severe immunodeficiency, or unusual predisposing factors. 5
Practical Algorithm for Assessment
Step 1: Assess for Systemic Toxicity
- Check for fever >38°C, heart rate >90 bpm, respiratory rate >24 breaths/min, hypotension, or altered mental status—these define systemic toxicity and trigger more intensive evaluation. 5
Step 2: Laboratory Evaluation Based on Clinical Presentation
In uncomplicated cellulitis without systemic signs, no laboratory studies are required. 5
When systemic toxicity is present, obtain complete blood count with differential, blood cultures, serum creatinine, bicarbonate, CPK, and CRP. 5
Step 3: Interpret Leukocytosis in Context
If WBC ≥14,000 cells/mm³ or left shift ≥6%, there is high probability of underlying bacterial infection—proceed with appropriate antibiotic therapy. 1
If WBC is normal but clinical cellulitis is present, this does not exclude infection—cellulitis remains a clinical diagnosis based on expanding erythema, warmth, tenderness, and edema. 5, 3
Step 4: Consider Hospitalization Criteria
- Admit patients with marked left-shift on differential, CRP >13 mg/L, hypotension, elevated creatinine, low bicarbonate, or CPK ≥2-3× upper limit of normal. 5
Common Pitfalls to Avoid
Do not delay antibiotic treatment waiting for laboratory confirmation—cellulitis is a clinical diagnosis and leukocytosis is supportive but not required. 3, 6
Do not reflexively add MRSA coverage based solely on leukocytosis—purulence, not elevated WBC, is the predictor of MRSA in cellulitis. 4
Do not obtain blood cultures or extensive laboratory testing for typical uncomplicated cellulitis in younger patients—this represents unnecessary resource utilization. 5
Do not assume absence of leukocytosis excludes serious infection—in the absence of fever, leukocytosis, or left shift, nonbacterial infections cannot be excluded. 1