Is growth in 1 of 4 blood cultures at 69 hours considered contamination or a true infection?

Interpreting Growth in 1 of 4 Blood Cultures at 69 Hours

Growth in 1 of 4 blood cultures at 69 hours is most likely to represent contamination rather than a true bloodstream infection, especially when the organism is a common skin contaminant. 1

Diagnostic Criteria for True Bloodstream Infections vs. Contamination

• Growth in only one of multiple blood culture sets, especially when detected after prolonged incubation (>48 hours), strongly suggests contamination rather than true bacteremia 1
• According to the Infectious Diseases Society of America (IDSA) guidelines, a definitive diagnosis of catheter-related bloodstream infection (CRBSI) requires that the same organism grow from at least one percutaneous blood culture and from a culture of the catheter tip 2
• Alternatively, diagnosis requires two blood samples (one from a catheter hub and another from a peripheral vein) that meet criteria for quantitative blood cultures or differential time to positivity (DTP) 2
• For quantitative blood cultures, a colony count from the catheter hub that is at least 3-fold greater than the colony count from a peripheral vein best defines a true CRBSI 2
• For DTP, growth of microbes from a catheter hub sample at least 2 hours before growth is detected in a peripheral vein sample best defines CRBSI 2

Factors Supporting Contamination in This Case

• The growth occurred in only 1 of 4 cultures, which is a classic pattern for contamination 1
• The delayed time to positivity (69 hours) further supports contamination, as true pathogens typically grow more rapidly 2
• Common contaminants include coagulase-negative staphylococci, Bacillus spp., Micrococcus spp., Propionibacterium spp., and other gram-positive bacilli 1

Clinical Approach to Suspected Contamination

• Assess the patient's clinical status - absence of fever, hypotension, or other signs of sepsis supports contamination 2
• Consider the identity of the organism - certain organisms like coagulase-negative staphylococci are common contaminants but can also cause true infections in specific settings (e.g., with indwelling devices) 1
• Evaluate the number of positive cultures - multiple positive cultures with the same organism suggest true infection rather than contamination 1
• Consider the time to positivity - delayed growth (>48 hours) often indicates contamination 2

Management Recommendations

• If contamination is suspected based on clinical assessment and culture results, avoid unnecessary antimicrobial therapy 1
• Document the likely contamination in the medical record 1
• If clinical suspicion for true infection remains, consider obtaining repeat blood cultures before initiating antimicrobial therapy 1
• For patients with indwelling catheters, paired blood cultures from the catheter and a peripheral site should be obtained to help differentiate contamination from true infection 2

Common Pitfalls to Avoid

• Treating all positive blood cultures as true infections can lead to unnecessary antibiotic use, increased risk of adverse effects, and antimicrobial resistance 1
• Failure to consider the clinical context when interpreting blood culture results may lead to inappropriate management decisions 2
• Not obtaining adequate blood volume or proper collection technique can increase contamination rates and complicate interpretation 1
• Ignoring the timing of growth - late growth (>48-72 hours) often indicates contamination or low bacterial load 2

Prevention of Blood Culture Contamination

• Proper skin antisepsis with alcohol, tincture of iodine, or alcoholic chlorhexidine with adequate contact and drying times 1
• Use of dedicated phlebotomy teams when available 1
• Proper venipuncture technique rather than drawing cultures through existing catheters when possible 1
• Collection of adequate blood volume (20-30 mL per culture set in adults) 1