Is coagulase-negative staphylococci (CoNS) usually a contaminant in blood cultures?

Coagulase-Negative Staphylococci in Blood Cultures: Usually Contaminants

Coagulase-negative staphylococci (CoNS) are most commonly contaminants rather than true pathogens when isolated from blood cultures, with approximately 65-94% of CoNS-positive blood cultures representing contamination rather than true bloodstream infection. 1, 2, 3

Distinguishing Contamination from True Infection

Key Indicators of True Bacteremia

• Multiple positive blood cultures: A high proportion of positive blood cultures from multiple sites is the best indication of true CoNS bacteremia 1
• Clinical signs of infection: The presence of systemic inflammatory response syndrome (SIRS) criteria significantly increases the likelihood of true infection 4
• Presence of intravascular devices: Patients with central venous catheters are 5.38 times more likely to have true CoNS bacteremia 4

Laboratory Indicators

• Time to positivity: Blood cultures that become positive in ≤16 hours suggest high bacterial load (>100 CFU/ml) and true infection, while those becoming positive after >20 hours typically represent contamination (<10 CFU/ml) 5
• Quantitative blood cultures: Higher colony counts (>100 CFU/ml) correlate with true infection, while low counts (<10 CFU/ml) typically represent contamination 5

Clinical Algorithm for Interpreting CoNS Blood Cultures

Likely True Infection if:

• Two or more blood cultures positive for the same CoNS species within 5 days 2
• One positive blood culture plus:
  • ≥3 SIRS criteria (fever/hypothermia, tachycardia, tachypnea, leukocytosis/leukopenia) 4
  • Or 2 SIRS criteria plus presence of a central venous catheter 4
• Time to positivity ≤16 hours 5

Likely Contamination if:

• Single positive blood culture with negative concurrent cultures 1
• Time to positivity >20 hours 5
• Low colony count (<10 CFU/ml) 5
• Absence of clinical signs of infection 4

Management Implications

When CoNS is Likely a Contaminant:

• Avoid unnecessary antibiotic therapy, particularly vancomycin 1
• Obtain additional blood cultures (when not on antibiotics) to confirm absence of bacteremia 1
• Continue to monitor patient clinically 2

When CoNS is Likely a True Infection:

• For uncomplicated catheter-related bloodstream infection (CRBSI):
  • Treat with antibiotics for 5-7 days if catheter is removed 1
  • Treat for 10-14 days with antibiotic lock therapy if catheter is retained 1
• Consider catheter removal, especially with short-term catheters 1

Special Considerations

Exception: Staphylococcus lugdunensis

• Despite being a CoNS, S. lugdunensis should be managed similarly to S. aureus due to its higher virulence and ability to cause endocarditis and metastatic infections 1

Hospital Department Variations

• Emergency departments typically have higher rates of CoNS contamination (up to 3.3% of blood cultures) 3
• Inappropriate vancomycin use for contaminants is more common in ward patients 3

Pitfalls to Avoid

• Don't automatically treat a single positive CoNS blood culture with vancomycin 1
• Don't assume all CoNS species have equal clinical significance (S. lugdunensis is more virulent) 1
• Don't overlook the importance of proper blood culture collection technique in reducing contamination rates 1
• Don't dismiss CoNS in immunocompromised patients or those with prosthetic devices, as these populations are at higher risk for true CoNS infections 6