Diagnostic Approach to Blood Culture Contamination

Identifying Contaminated Blood Cultures

Growth of typical skin flora in only one of multiple blood culture sets is a common indicator of contamination, especially for coagulase-negative staphylococci, Bacillus spp., Micrococcus spp., Propionibacterium spp., or other Gram-positive bacilli 3
Discordant results between cultures, such as a positive culture from a catheter but negative from a peripheral vein, may indicate contamination 1

Blood cultures drawn from existing intravascular devices rather than fresh venipuncture increase the risk of contamination 4, 5
Poor skin antisepsis technique before venipuncture is a significant risk factor for contamination 5
Samples obtained from sites with compromised skin integrity, such as burns or dermatological conditions, are at higher risk for contamination 4

Obtain repeat blood cultures before initiating antimicrobial therapy, drawing at least two sets from separate peripheral venipuncture sites, with each set including 20-30 mL of blood per culture 1, 2, 3
Use proper technique for blood collection, including employing a dedicated phlebotomy team when available, preparing skin with alcohol, tincture of iodine, or alcoholic chlorhexidine, and allowing adequate contact and drying time for the antiseptic 2, 5, 7

For suspected catheter-related bloodstream infection (CRBSI), compare cultures from the catheter and peripheral vein, and consider quantitative blood cultures or differential time to positivity (DTP) to differentiate true bacteremia from contamination 2, 3
For non-catheter suspected contamination, multiple positive cultures with the same organism strongly suggest true bacteremia, while a single positive culture with a common contaminant organism suggests contamination 1

If contamination is confirmed, avoid unnecessary antimicrobial therapy and document contamination in the medical record 4, 7
If true bacteremia cannot be ruled out, consider empiric antimicrobial therapy for high-risk patients or those with clinical signs of infection, and perform additional diagnostic testing as indicated 2, 4, 8
For catheter-related concerns, consider catheter removal if it is the suspected source of infection, and culture the catheter tip if removed 2, 7

Implement standardized blood culture collection protocols, use dedicated phlebotomy teams when available, and ensure proper skin antisepsis with adequate contact time to prevent future contamination 1, 2, 5