Laboratory Testing for Fever with Multiple Furuncles
For a patient with fever and multiple furuncles, you should obtain a complete blood count with differential, looking specifically for leukocytosis >14,000 cells/mm³ or a left shift (band neutrophils ≥6% or absolute band count >1,500/mm³), as these findings warrant careful assessment for bacterial infection even in the absence of high fever. 1
Essential Laboratory Tests
Complete Blood Count with Differential
- Order a CBC with differential for all patients with suspected infection and fever 1
- An elevated WBC count (>14,000 cells/mm³) or left shift (percent band neutrophils ≥6% or total band neutrophil count >1,500/mm³) indicates likely bacterial infection requiring aggressive treatment 1
- The absolute band count >1,500/mm³ has the highest likelihood ratio for bacterial infection, even more reliable than total WBC count alone 1
Blood Cultures
- Blood cultures are generally NOT recommended for uncomplicated furuncles, as they have low yield in this setting 1
- Blood cultures should only be obtained if you suspect bacteremia based on: 1
- High fever with systemic toxicity
- Multiple lesions with extensive surrounding cellulitis
- Severely impaired host defenses (diabetes, immunosuppression)
- Cutaneous gangrene
When Cultures ARE Indicated
Wound Cultures
- Gram stain and culture are rarely necessary for simple furuncles 1
- Consider wound culture only in these specific circumstances: 1
- Multiple lesions present
- Initial antibiotic treatment has failed
- Extensive surrounding cellulitis
- Suspicion of MRSA (community outbreaks, healthcare exposure)
- Severely immunocompromised patient
Nasal Carriage Screening
- For recurrent furunculosis, obtain nasal swab cultures to identify S. aureus carriage 1, 2, 3
- The prevalence of nasal staphylococcal colonization is 20-40% in the general population, and carriers are at higher risk for recurrent infections 1
- Screen household contacts if recurrent infections occur, as transmission within families is common 1, 3
Additional Testing Based on Clinical Context
Metabolic Panel
- Consider basic metabolic panel if the patient appears systemically ill or has risk factors for diabetes 1
- Carbuncles (coalescent furuncles) tend to develop in diabetic patients, making glucose assessment relevant 1
Inflammatory Markers
- C-reactive protein (CRP) can help distinguish bacterial infection when clinical probability is low to intermediate 1, 4
- However, if bacterial infection probability is already high based on clinical findings, CRP adds little value 1
What NOT to Order
- Do NOT routinely order blood cultures for uncomplicated furuncles - they have extremely low yield and are not cost-effective 1, 5
- Do NOT obtain surface swab cultures - they are not indicated for diagnosis of furuncles and will only grow colonizing flora 1
- Do NOT order imaging studies unless you suspect deeper infection (abscess, necrotizing fasciitis) or osteomyelitis 1
Clinical Pitfalls to Avoid
- Do not rely solely on fever presence - bacterial infection can occur with normal temperature, making the CBC with differential critical 1
- Do not treat based on clinical appearance alone without obtaining at least a CBC when systemic signs are present 1
- Do not forget to assess for MRSA risk factors - community-acquired MRSA is increasingly common in skin infections, particularly in outbreak settings (prisons, sports teams, close-contact groups) 1
- Do not overlook the need for carrier screening in recurrent cases - failure to eradicate nasal carriage leads to treatment failure 1, 2, 3
Special Circumstances Requiring Expanded Testing
If the patient has any of the following, expand your laboratory evaluation: 1, 6
- Systemic signs of severe infection (fever >38°C, tachycardia, hypotension) - add blood cultures, lactate
- Immunosuppression - add blood cultures, consider HIV testing if not previously done
- Diabetes or suspected diabetes - add hemoglobin A1c, comprehensive metabolic panel
- Suspected necrotizing infection - add lactate, creatine kinase, imaging studies