Should blood cultures be ordered for a patient with uncomplicated cellulitis, considering factors such as age, immune status, history of vascular disease, intravenous (IV) drug use, presence of prosthetic devices, or recent travel?

Blood Cultures in Cellulitis: A Risk-Stratified Approach

Do not routinely order blood cultures for uncomplicated cellulitis—they are positive in only 5% of cases and rarely change management. 1

When Blood Cultures Are NOT Indicated

For typical uncomplicated cellulitis, blood cultures are unnecessary and represent low-value care. 1 The most recent European guidelines (2024) explicitly recommend against routine blood culture collection in emergency department patients with cellulitis/erysipelas, with a weak recommendation based on very low-quality evidence. 1

Key characteristics of uncomplicated cellulitis where cultures should be avoided:

• Immunocompetent patient without systemic toxicity 1
• No SIRS criteria (fever <38°C, heart rate <90, respiratory rate <24, WBC <12,000) 1
• No significant comorbidities 2, 3
• Localized infection without purulent drainage 1

The bacteremia rate in uncomplicated cellulitis is only 4.8%, and when positive, organisms are almost exclusively gram-positive cocci (Streptococcus species and Staphylococcus aureus) already covered by standard empiric therapy. 2 Even in complicated cellulitis with significant comorbidities, blood cultures changed antibiotic management in only 6 of 314 patients (1.9%). 3

When Blood Cultures ARE Indicated

Obtain blood cultures in the following high-risk scenarios:

Immunocompromised States (Strong Recommendation)

• Active chemotherapy or malignancy 1, 3
• Neutropenia (absolute neutrophil count <500 cells/mm³) 1
• Severe cell-mediated immunodeficiency (HIV with CD4 <200, solid organ transplant recipients) 1
• Drug-induced immunosuppression 1

Prosthetic Devices

• Intravascular prosthesis, pacemaker, or valvular prosthesis 1
• Ipsilateral orthopedic implant (odds ratio 10.8 for bacteremia) 4

High-Risk Exposures

• Immersion injuries (freshwater or saltwater exposure) 1
• Animal bites 1
• Injection drug use 1

Systemic Toxicity/SIRS Criteria

• Fever >38°C with shaking chills 5, 4
• Hypotension or hemodynamic instability 1
• Altered mental status 1
• WBC ≥13,000 cells/µL (independent predictor of bacteremia) 5, 4

Specific Comorbidities Associated with Bacteremia

• Lymphedema (25% bacteremia rate vs. 10.8% overall) 4
• Liver cirrhosis 4, 6
• Chronic kidney disease 4
• Alcoholism (odds ratio 4.9 for bacteremia) 2, 6
• Chronic obstructive pulmonary disease (odds ratio 25.4) 6
• Diabetes mellitus (odds ratio 4.4) 6

Elderly Patients with Additional Risk Factors

The bacteremia rate increases dramatically with age: 8.5% in patients <65 years vs. 25.3% in those ≥65 years. 5 For elderly patients (≥65 years) with cellulitis PLUS shaking chills OR WBC ≥13,000 cells/µL, obtain blood cultures. 5 The most common pathogen in elderly bacteremic cellulitis is Streptococcus dysgalactiae (62.5%), with gram-negative bacteremia occurring in 8.3% of cases. 5

Clinical Impact of Positive Blood Cultures

When bacteremia is detected in cellulitis, it predicts:

• Longer hospitalization (mean 10.5 days vs. 4.9 days without bacteremia) 4, 6
• Higher recurrence rates 4
• No difference in mortality 4

Common Pitfall to Avoid

Do not reflexively order blood cultures simply because a patient is hospitalized. 7 Hospitalization alone does not indicate complicated cellulitis requiring blood cultures—the decision should be based on specific risk factors outlined above, not admission status. 1, 7