Is Pseudoclavibacter faecalis usually considered a contaminant when it grows out of 1 of 2 blood cultures?

Pseudoclavibacter faecalis in Blood Cultures: Contaminant vs. Pathogen

Pseudoclavibacter faecalis growing in 1 of 2 blood cultures should NOT be automatically dismissed as a contaminant, particularly in immunocompromised patients or those with prosthetic devices, as this organism has been documented as a true pathogen causing serious infections including infective endocarditis and bacteremia with fatal outcomes.

Clinical Significance Assessment

When to Consider True Pathogen

Pseudoclavibacter species can cause genuine bloodstream infections, particularly in specific clinical contexts:

• Prosthetic heart valves or cardiac devices: A case of infective endocarditis due to Pseudoclavibacter species required 42 days of vancomycin therapy in a patient with prior mitral valve surgery 1

• Immunocompromised hosts: Fatal bacteremia has been documented in a patient with COPD and immunosuppression 2, and otitis media occurred in a child with tuberculosis 3

• Multiple positive cultures: When the same organism grows in multiple blood culture bottles drawn at different times, this strongly suggests true bacteremia rather than contamination 1

Standard Contamination Criteria

The CDC guidelines classify certain organisms as common skin contaminants, requiring specific interpretation criteria 4:

• Single positive culture with typical contaminants (diphtheroids, Bacillus spp., Propionibacterium spp., coagulase-negative staphylococci) generally requires two or more positive blood cultures from separate venipunctures to establish true bacteremia 4

• However, if clinical signs of infection are present AND the physician institutes appropriate antimicrobial therapy based on a single positive culture from a patient with an intravascular line, this can meet criteria for laboratory-confirmed BSI 4

Algorithmic Approach to Interpretation

Step 1: Assess Clinical Context

• High-risk features favoring true infection 1, 2, 3:
  • Prosthetic cardiac valves or devices
  • Immunocompromised state
  • Fever, sepsis, or hemodynamic instability
  • Recent cardiac surgery
  • Chronic lung disease

Step 2: Evaluate Microbiologic Data

• Repeat blood cultures immediately from a peripheral vein (separate venipuncture) 4
• If the second set is also positive for Pseudoclavibacter, this strongly indicates true bacteremia 1
• Differential time to positivity: If available, blood drawn from a catheter that becomes positive >2 hours before peripheral blood suggests catheter-related infection 4

Step 3: Clinical Decision Making

If only 1 of 2 cultures is positive:

• Do NOT automatically dismiss as contamination in high-risk patients 1, 2
• Obtain additional blood cultures (at least 2 more sets) before making final determination 4
• Consider empiric therapy if patient is clinically unstable while awaiting repeat cultures 4

If repeat cultures are negative:

• Contamination is more likely, but clinical judgment remains paramount 5
• The contamination rate for potential contaminants in 1 set of blood cultures can be as high as 95%, but drops to 87% when 2 sets are obtained 5

Critical Pitfalls to Avoid

Common Errors

1. Automatic dismissal as contaminant: Unlike typical skin flora, Pseudoclavibacter has documented pathogenicity in vulnerable hosts 1, 2, 3

2. Inadequate repeat culturing: The IDSA strongly recommends obtaining at least 2 blood culture sets from separate venipunctures when evaluating potential contaminants 4

3. Ignoring clinical context: Patients with true bloodstream infections have significantly higher 14-day mortality (23.8% vs 7.3%) compared to pseudobacteremia 5

Special Considerations

• Identification challenges: Pseudoclavibacter requires molecular methods (16S rRNA sequencing) for definitive identification, as conventional biochemical tests are inadequate 1, 2

• Antibiotic susceptibility: The organism is typically susceptible to vancomycin, penicillin, and linezolid but may be resistant to clindamycin and tetracycline 1

• Device-related infections: If prosthetic material is present and infection is confirmed, prolonged antibiotic therapy (6 weeks or more) may be necessary 1

Practical Management

Immediate actions when Pseudoclavibacter grows in 1 of 2 cultures:

• Obtain 2 additional sets of blood cultures from separate peripheral venipunctures before initiating antibiotics 4
• Assess for prosthetic devices, immunosuppression, or other high-risk features 1, 2
• If patient is hemodynamically unstable or has high-risk features, consider empiric therapy with vancomycin while awaiting repeat culture results 1
• Contact the microbiology laboratory to request molecular identification if not already performed 1, 2

The key distinction: While many gram-positive bacilli are contaminants, Pseudoclavibacter's documented ability to cause serious infections in specific populations means it requires more careful evaluation than routine dismissal 1, 2, 3.