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

Is Coagulase-Negative Staphylococcus a Contaminant in Blood Cultures?

Coagulase-negative staphylococci (CoNS) are contaminants in approximately 65-94% of positive blood cultures, but can represent true bacteremia in specific clinical contexts—particularly when multiple blood cultures are positive or when risk factors such as indwelling catheters or prosthetic devices are present. 1

Clinical Algorithm for Interpretation

The determination of whether CoNS represents true bacteremia versus contamination requires systematic evaluation using multiple clinical and microbiological parameters:

Microbiological Criteria

Multiple positive blood cultures are the single most important indicator of true bacteremia:

• At least 2 blood cultures positive for CoNS within 48-72 hours (ideally from different sites including one peripheral draw) strongly suggests true bacteremia 1, 2
• A single positive blood culture with concurrent negative cultures is highly likely contamination (85% probability) and should not trigger vancomycin therapy 1, 2, 3
• Growth in both aerobic and anaerobic bottles occurs in 85% of true bacteremia cases versus only 30% of contaminated cultures 4
• Time to positivity <48 hours is significantly associated with true bacteremia, with catheter-related bloodstream infections typically showing growth <24 hours 4, 5

Clinical Risk Factors for True Bacteremia

The presence of specific risk factors substantially increases the likelihood that CoNS represents true infection:

• Indwelling central venous catheters or other intravascular devices (present in 87.9% of true CoNS bacteremia cases) 6, 2, 4
• Prosthetic heart valves or other prosthetic devices 2
• Hemodialysis patients 2
• Neutropenia 3
• Signs of sepsis syndrome (abnormal white blood cell count, fever, hypotension) 3
• Recent healthcare exposure or long-term care facility residence 2

Clinical Signs Supporting True Infection

• Fever with abnormal vital signs (temperature or blood pressure abnormalities) 3
• Clinical evidence of catheter-related infection (exit site infection, tunnel infection) 6
• Persistent bacteremia despite appropriate initial management 6

Management Based on Interpretation

When CoNS is Likely a Contaminant

Do not initiate vancomycin therapy based solely on a single positive CoNS blood culture 1, 2. This approach:

• Prevents unnecessary antibiotic exposure and vancomycin resistance 1, 2
• Avoids misclassification of nosocomial bloodstream infections 3
• Obtain additional blood cultures to confirm absence of bacteremia 1

When CoNS Represents True Bacteremia

Initiate vancomycin 40 mg/kg/day IV divided every 8-12 hours (maximum 2g daily) as empirical therapy, given that 58-87% of CoNS isolates are methicillin-resistant 2:

• Target vancomycin trough levels of 15-20 mcg/mL for serious infections 2
• Remove or replace central venous catheters if multiple blood cultures are positive, as catheter retention is associated with persistent bacteremia 2
• Duration: 10-14 days for uncomplicated bacteremia with prompt clinical response; 4-8 weeks for complicated infections (endocarditis, osteomyelitis, septic thrombosis) 2
• If catheter is removed, uncomplicated catheter-related bloodstream infection can be treated for 5-7 days 1
• De-escalate to nafcillin, oxacillin, or cefazolin if methicillin-susceptible 2

Critical Species Exception

Staphylococcus lugdunensis, despite being coagulase-negative, must be managed similarly to S. aureus due to its high virulence and ability to cause endocarditis and metastatic infections 6, 1. This species requires:

• At least two independent blood culture sets to differentiate contamination from true infection 6
• Aggressive treatment approach similar to S. aureus bacteremia 1
• Consideration of echocardiography to evaluate for endocarditis 6

Common Pitfalls to Avoid

• Do not automatically treat every positive CoNS blood culture with vancomycin—this is the most common error and drives unnecessary antibiotic use 1, 2
• Do not assume all CoNS species have equal clinical significance—S. lugdunensis requires more aggressive management 1
• Do not rely on antibiotic sensitivity patterns alone to differentiate bacteremia from contamination—they are not useful for this purpose 4
• Do not overlook proper blood culture collection technique—contamination rates are directly related to collection practices 1
• Do not dismiss CoNS in patients with prosthetic devices—these patients have substantially higher rates of true bacteremia 2

Special Populations Requiring Lower Threshold for Treatment

Patients with prosthetic heart valves or other prosthetic devices require:

• Higher suspicion for true pathogen status even with fewer positive cultures 2
• Longer duration of therapy (4-6 weeks) 2
• Infectious diseases consultation 2
• Consideration of valve involvement and need for surgical intervention 6