Can growth in one blood culture bottle be considered a contaminant?

Growth in One Blood Culture Bottle: Contaminant or Pathogen?

Growth in a single blood culture bottle is most likely a contaminant, especially when the organism is a common skin commensal, but clinical context must be considered before dismissing it entirely. 1

Understanding Blood Culture Contamination

Blood cultures are the gold standard for detecting bacteremia and fungemia, but contamination during collection is common. According to guidelines:

• Contamination rates should not exceed 3% 1
• Common contaminants include coagulase-negative staphylococci, viridans group streptococci, diphtheroids, and Bacillus species (other than B. anthracis) 1
• Laboratories should have policies for abbreviated workup and reporting of common contaminants 1

Differentiating Contaminants from True Pathogens

Factors suggesting contamination:

• Growth in only one bottle of multiple sets 1
• Organism is a common skin commensal 1
• Delayed time to positivity (>48 hours)
• Patient lacks clinical signs of infection
• Multiple different organisms growing in different bottles

Factors suggesting true bacteremia:

• Same organism growing in multiple bottles/sets
• Organism is a known pathogen (not typical skin flora)
• Early time to positivity
• Patient has clinical signs of infection
• Presence of intravascular devices 1
• Immunocompromised status 2

Best Practices for Blood Culture Collection

To minimize contamination and maximize diagnostic yield:

• Collect at least two sets of blood cultures from different anatomical sites 1
• For adults, collect 20-30 mL per set (10 mL per bottle) 1
• Use proper skin disinfection with chlorhexidine or 2% iodine tincture 1
• Ideally use dedicated phlebotomy teams 1
• No time interval is required between sets in critically ill patients 1
• Catheter-drawn blood cultures have higher contamination rates 1

Clinical Approach to Single Positive Blood Culture

1. Identify the organism - Is it a common contaminant or known pathogen?
2. Review clinical context - Does the patient have signs/symptoms of infection?
3. Assess risk factors - Does the patient have indwelling devices or immunosuppression?
4. Consider repeating cultures - If clinical suspicion for infection is high
5. Consult infectious disease specialist - For difficult cases or unusual organisms

Special Considerations

• In immunocompromised patients, even common skin commensals growing in a single bottle may represent true infection 2
• Some pathogens like HACEK organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) may grow in only a fraction of bottles but still represent true infection 1
• Certain organisms like Brevibacterium species, typically considered contaminants, can cause true bacteremia in immunocompromised hosts 2

Conclusion

While a single positive blood culture bottle growing a common skin commensal is most likely a contaminant, clinical judgment is essential. The decision to treat should be based on the organism identified, patient's clinical status, and risk factors for infection.