What are the steps to take when a blood culture is suspected to be contaminated?

Steps to Take When Blood Cultures Are Suspected to Be Contaminated

When a blood culture is suspected to be contaminated, follow a systematic approach to differentiate true bacteremia from contamination, including obtaining repeat cultures from separate peripheral sites, evaluating the organism isolated, and assessing clinical signs of infection. 1

Identifying Contaminated Blood Cultures

Common indicators of contamination:

• Growth of typical skin flora (especially coagulase-negative staphylococci, Bacillus spp., Micrococcus spp., Propionibacterium spp., or other Gram-positive bacilli) in only one of multiple blood culture sets 1, 2
• Discordant results between cultures (positive culture from catheter but negative from peripheral vein) 1
• Absence of clinical signs of infection that would correlate with the isolated organism 2
• Late growth in culture (typically after 48-72 hours) 3

Risk factors for contamination:

• Blood cultures drawn from existing intravascular devices rather than fresh venipuncture 1
• Poor skin antisepsis technique before venipuncture 1
• Samples obtained from sites with compromised skin integrity (burns, dermatological conditions) 1
• Blood cultures drawn from femoral sites 1

Immediate Steps When Contamination is Suspected

1. Obtain repeat blood cultures before initiating antimicrobial therapy if not already started 1

   • Draw at least two sets from separate peripheral venipuncture sites 1
   • Each set should include 20-30 mL of blood per culture 1
   • Mark bottles appropriately to reflect the site from which samples were obtained 1

2. Use proper technique for blood collection 1:

   • Employ a dedicated phlebotomy team when available 1
   • Prepare skin with alcohol, tincture of iodine, or alcoholic chlorhexidine (>0.5%) 1
   • Allow adequate contact and drying time for the antiseptic 1
   • If drawing through a catheter, clean the hub with alcohol, tincture of iodine, or alcoholic chlorhexidine 1

3. Communicate with the microbiology laboratory 1:

   • Inform them of the suspected contamination
   • Discuss whether special culture systems or extended incubation periods are needed
   • Request comparison of isolates from initial and repeat cultures

Diagnostic Approach to Differentiate True Bacteremia from Contamination

Clinical assessment:

• Evaluate for presence of fever, chills, hypotension, or signs of sepsis 4
• Consider Shapiro criteria for true bacteremia risk assessment 4:
  • Major criteria: temperature >39.5°C, indwelling vascular catheter, clinical suspicion of endocarditis
  • Minor criteria: temperature 38.3-39.4°C, age >65 years, chills, vomiting, hypotension, neutrophilia >80%, WBC >18k, bands >5%, platelets <150k, creatinine >2.0 4

Laboratory methods:

• For suspected catheter-related bloodstream infection (CRBSI) 1:

  • Compare cultures from catheter and peripheral vein
  • Quantitative blood cultures: ≥3-fold higher colony count from catheter hub vs. peripheral vein indicates CRBSI
  • Differential time to positivity (DTP): growth from catheter hub sample ≥2 hours before peripheral vein sample indicates CRBSI

• For non-catheter suspected contamination 2, 3:

  • Multiple positive cultures with the same organism strongly suggests true bacteremia
  • Single positive culture with a common contaminant organism suggests contamination

Management Decisions

1. If contamination is confirmed 2, 5:

   • Avoid unnecessary antimicrobial therapy
   • Document contamination in medical record
   • Review blood culture collection techniques with staff

2. If true bacteremia cannot be ruled out 1:

   • For high-risk patients or those with clinical signs of infection, consider empiric antimicrobial therapy
   • Perform additional diagnostic testing as indicated (e.g., echocardiography for suspected endocarditis)
   • Reassess need for continued antibiotics when repeat culture results are available

3. For catheter-related concerns 1:

   • Consider catheter removal if it is the suspected source of infection
   • Culture catheter tip if removed (using roll plate technique for short-term catheters)
   • For long-term catheters, culture both insertion site and catheter hub

Prevention of Future Contamination

• Implement standardized blood culture collection protocols 1, 5
• Use dedicated phlebotomy teams when available 1
• Ensure proper skin antisepsis with adequate contact time 1
• Avoid drawing cultures through existing catheters when possible 1
• Consider obtaining multiple cultures within the first 24 hours of fever onset to help differentiate true bacteremia from contamination 1