What is the treatment for a suspected Staphylococcal (Staph) species contamination in a blood culture?

Management of Suspected Staphylococcal Blood Culture Contamination

When a single blood culture grows Staphylococcus species (particularly coagulase-negative staphylococci), obtain additional blood cultures from different sites before initiating antibiotics to distinguish true bacteremia from contamination. 1, 2

Distinguishing Contamination from True Infection

Key Clinical Indicators

Multiple positive blood cultures from different anatomical sites strongly indicate true bacteremia rather than contamination. 1, 2 The probability of contamination is 65-94% when coagulase-negative staphylococci (CoNS) appear in blood cultures. 2

• Single positive culture with concurrent negative cultures: Likely represents contamination and does not require antibiotic therapy 1, 2
• Multiple positive cultures (≥2 sets) from different sites: Indicates true bacteremia requiring treatment 1, 2
• Growth in only one bottle of a set: Suggests contamination 1

Critical Pitfall to Avoid

Do not automatically initiate vancomycin therapy for a single positive CoNS blood culture. 2 This leads to unnecessary antibiotic exposure, increased costs, and potential adverse effects. Instead, obtain repeat blood cultures to confirm bacteremia before starting treatment. 1, 2

Species-Specific Management

Coagulase-Negative Staphylococci (CoNS)

For suspected contamination (single positive culture):

• Obtain additional blood cultures from peripheral sites 1, 2
• Withhold antibiotics pending repeat culture results 2
• Monitor for clinical signs of infection (fever, hemodynamic instability, presence of intravascular devices) 1

For confirmed CoNS bacteremia (multiple positive cultures):

• Remove short-term catheters if present 1
• Treat for 5-7 days if catheter removed 1, 3, 2
• Treat for 10-14 days with systemic therapy plus antimicrobial lock therapy if catheter retained 1, 3
• Use vancomycin 15-20 mg/kg IV every 8-12 hours (adjusted for renal function) as first-line therapy 3, 4

Important Exception: Staphylococcus lugdunensis

Manage S. lugdunensis similarly to S. aureus despite being coagulase-negative. 2 This species has higher virulence and can cause endocarditis and metastatic infections requiring aggressive treatment. 2

Staphylococcus aureus

Even a single positive blood culture for S. aureus should be treated as true bacteremia, not contamination. 1, 3, 5

Immediate actions required:

• Initiate empiric vancomycin or daptomycin while awaiting susceptibility results 3, 5
• Remove all intravascular catheters 1, 3
• Obtain repeat blood cultures at 48-72 hours to document clearance 1, 3
• Perform transthoracic echocardiography (minimum evaluation) 1

Laboratory Approach to Reduce Contamination

Proper Collection Technique

Collect at least two blood culture sets (ideally 60 mL total blood) from different anatomical sites sequentially without time intervals. 1 Each set should include one aerobic and one anaerobic bottle with 10 mL blood per bottle. 1

Key collection practices:

• Disinfect venipuncture site with chlorhexidine or 2% iodine tincture 1
• Avoid catheter-drawn cultures when possible due to higher contamination risk 1
• Ensure proper bottle filling (10 mL per bottle) to optimize yield 1

Laboratory Reporting Standards

Laboratories should have abbreviated workup protocols for common contaminants. 1 When CoNS or other skin flora grow in a single culture, laboratories may provide abbreviated identification without full susceptibility testing, with a comment instructing clinicians to contact the laboratory if clinically significant. 1

Contamination rates should not exceed 3% in well-functioning systems. 1

Rapid Diagnostic Testing

Rapid molecular PCR testing can identify Staphylococcus species and detect mecA gene within 75 minutes, reducing time to appropriate therapy by approximately 39 hours. 1, 3, 6, 7 However, rapid tests should only be used with concomitant standard blood cultures, not as standalone diagnostics. 1

Benefits of rapid PCR:

• Earlier switch from vancomycin to beta-lactams for methicillin-susceptible S. aureus (MSSA), improving outcomes 1, 6
• Reduced unnecessary anti-MRSA therapy for CoNS contamination 6
• Faster appropriate therapy initiation (5.2 hours vs 49.8 hours for MSSA) 6

Clinical Algorithm for Management

Step 1: Initial blood culture result shows gram-positive cocci in clusters

• Obtain at least one additional blood culture set from a different site 1, 2
• Assess for clinical signs of infection (fever, hemodynamic instability, indwelling devices) 1

Step 2: Determine if single or multiple positive cultures

• Single positive: Likely contamination; withhold antibiotics and await repeat cultures 1, 2
• Multiple positive: Treat as true bacteremia 1, 2

Step 3: Species identification

• S. aureus: Initiate treatment immediately, remove catheters, obtain echocardiography 1, 3
• CoNS (except S. lugdunensis): Confirm with multiple cultures before treating 1, 2
• S. lugdunensis: Treat as S. aureus 2

Step 4: Antibiotic selection based on susceptibility

• MSSA: Switch to cefazolin or antistaphylococcal penicillin (nafcillin) 3, 8, 5
• MRSA: Continue vancomycin or use daptomycin 3, 4, 5
• CoNS: Vancomycin for confirmed bacteremia 3, 4

Step 5: Source control

• Remove infected catheters, particularly for S. aureus 1, 3
• Drain abscesses if present 1
• Evaluate for metastatic foci in S. aureus bacteremia 1, 5