Is a positive blood culture with Staphylococcus epidermidis always contamination?

Staphylococcus epidermidis in Blood Cultures: Not Always Contamination

A positive blood culture with Staphylococcus epidermidis is not always contamination and requires careful clinical assessment to determine its significance. While S. epidermidis is commonly a contaminant, it can represent true bacteremia, particularly in patients with indwelling devices, prosthetic valves, or immunocompromised states.

Distinguishing True Infection from Contamination

Clinical Criteria Suggesting True Infection:

• Multiple positive blood cultures with the same organism (identical strain)
• Presence of indwelling vascular catheters or prosthetic devices
• Immunocompromised status
• No alternative source of infection
• Clinical signs of infection (fever, chills, hypotension)

Laboratory Criteria Suggesting True Infection:

• Isolation of identical S. epidermidis strains from multiple blood cultures
• Growth in multiple culture bottles
• Early time to positivity (<24 hours)
• Quantitative blood cultures with >5:1 ratio (catheter vs. peripheral)
• Differential time to positivity >2 hours (catheter culture becoming positive earlier than peripheral)

High-Risk Clinical Scenarios

S. epidermidis is particularly likely to represent true infection in:

• Patients with prosthetic heart valves (endocarditis)
• Patients with intravascular devices (catheter-related bloodstream infections)
• Patients with cerebrospinal fluid shunts
• Immunocompromised patients
• Patients with recent surgery involving implanted materials

Diagnostic Approach

1. Obtain multiple blood cultures (at least 2 sets from separate venipuncture sites) before starting antibiotics 1
2. Compare catheter and peripheral blood cultures when catheter-related infection is suspected 1
3. Consider removing the catheter if it's the suspected source and culturing the tip
4. Apply laboratory criteria to distinguish contamination from true infection

Management Considerations

When S. epidermidis is isolated from blood cultures:

• Single positive culture with no risk factors: Likely contamination, especially if other cultures are negative 1
• Multiple positive cultures with identical strains: Treat as true infection 2
• Positive culture with indwelling device: Consider catheter removal and appropriate antimicrobial therapy 1
• Prosthetic valve or implant: Evaluate for endocarditis or device-related infection 1

Common Pitfalls

1. Dismissing all S. epidermidis isolates as contaminants - This can lead to missed diagnoses of serious infections, especially in high-risk patients
2. Overtreatment of likely contaminants - This contributes to antimicrobial resistance and unnecessary costs
3. Failure to remove infected devices - Most S. epidermidis infections involving prosthetic materials require device removal for cure 3
4. Inadequate antimicrobial therapy - S. epidermidis is frequently methicillin-resistant and may require vancomycin or other appropriate agents 4

Important Considerations

• The CDC/HICPAC guidelines specifically warn against "treatment in response to a single blood culture positive for coagulase-negative staphylococcus, if other blood cultures taken during the same time frame are negative (i.e., if contamination of the blood culture is likely)" 1
• According to the Infectious Diseases Society of America, laboratories should have policies for abbreviating the work-up of common blood culture contaminants, including coagulase-negative staphylococci 1
• In the Modified Duke Criteria for endocarditis diagnosis, a single positive culture for coagulase-negative staphylococci is specifically excluded from the major microbiological criteria 1

Remember that proper collection techniques can reduce contamination rates, which should not exceed 3% 1. Proper skin antisepsis and collection technique are essential to minimize false-positive results.