Is gram-positive cocci in an anaerobic bottle considered a contaminant?

Gram-Positive Cocci in Anaerobic Blood Culture Bottles: Contamination Assessment

Gram-positive cocci in a single anaerobic blood culture bottle should be presumed to be a contaminant, particularly if the organism displays grape-like clustering (suggesting coagulase-negative staphylococci), unless multiple blood culture sets from separate venipuncture sites are positive or the patient has specific high-risk features. 1

Clinical Interpretation Algorithm

Indicators of True Bacteremia (Not Contamination)

• Multiple positive blood culture sets from separate venipuncture sites growing the same gram-positive cocci 1
• Growth in both aerobic and anaerobic bottles from the same or different sets 1
• High-risk patient features including:
  • Presence of indwelling central venous catheters 2
  • Immunocompromised status 2
  • Clinical signs of sepsis with hemodynamic instability 1
  • Prosthetic devices or valves 1

Indicators of Contamination

• Single positive bottle only, especially if it shows gram-positive cocci in grape-like clusters (coagulase-negative staphylococci) 1
• Delayed time to positivity compared to other bottles in the same set 1
• Absence of clinical signs of infection despite positive culture 1
• Skin flora organisms such as coagulase-negative staphylococci, Micrococcus species, or Corynebacterium species 1

Critical Next Steps

Immediate Actions

• Do not delay reporting - even suspected contaminants should be reported to clinicians within 1 hour of Gram stain completion, as the guideline specifically notes that reporting practices changed to include "single positive bottle with Gram-positive cocci grape-like clustering" 1
• Collect repeat blood cultures from separate peripheral venipuncture sites if clinical suspicion for bacteremia remains high 1, 2
• Obtain at least 2 additional blood culture sets (ideally 60 mL total blood volume) sequentially from different anatomical sites 1

Species-Specific Considerations

Gram-positive anaerobic cocci (GPAC) such as Peptostreptococcus, Finegoldia, Parvimonas, and Anaerococcus species are legitimate pathogens when isolated from blood cultures, particularly in the context of: 3, 4

• Deep organ abscesses 3, 4
• Obstetric and gynecological infections 4
• Intraoral infections with hematogenous spread 4
• Polymicrobial bacteremia with other anaerobes 3, 5

Coagulase-negative staphylococci (appearing as grape-like clusters) are the most common blood culture contaminants and require multiple positive sets for clinical significance 1

Common Pitfalls to Avoid

• Dismissing all gram-positive cocci as contaminants - true GPAC bacteremia accounts for 25-30% of all anaerobic isolates from clinical specimens and can cause serious infections 3
• Failing to obtain adequate blood volume - suboptimal filling of blood culture bottles (less than 10 mL per bottle) decreases yield and may contribute to false interpretation 1
• Not collecting multiple sets - at least two sets from separate sites are essential to distinguish contamination from true bacteremia 1
• Ignoring clinical context - patients with deep-seated abscesses, particularly below the diaphragm, may have legitimate GPAC bacteremia 5

Quality Metrics

The target contamination rate for blood cultures should be less than 3%, and laboratories should monitor this as a critical key performance indicator 1