Do I need to repeat a blood culture if one set is positive for anaerobes (anaerobic bacteria)?

Should You Repeat Blood Cultures When One Set is Positive for Anaerobes?

Yes, you should obtain repeat blood cultures when only one set is positive for anaerobes to distinguish true bacteremia from contamination and to assess treatment response.

Initial Interpretation of Single Positive Set

When you have only one positive blood culture set growing anaerobes, you face a critical diagnostic dilemma:

• At least two sets of blood cultures should always be collected initially from different anatomical sites to help distinguish true bacteremia from contamination 1, 2
• A single positive culture set—even with anaerobes—may represent contamination rather than true infection, though this is less common with obligate anaerobes than with skin commensals 1
• The standard of care requires multiple blood culture sets (minimum of two, ideally 60 mL total blood volume) to achieve adequate sensitivity and interpretability 1

When to Obtain Additional Cultures

If Only One Set Was Initially Drawn

Immediately obtain at least one additional blood culture set from a different site before making definitive treatment decisions 1:

• This helps confirm true bacteremia versus contamination
• Provides additional isolates for susceptibility testing
• Establishes baseline for assessing treatment response

If Two Sets Were Initially Drawn (One Positive, One Negative)

The clinical context determines your next steps:

• For catheter-related infections: The discordant result (catheter positive, peripheral negative) may indicate catheter-related bloodstream infection rather than contamination 1
• For suspected true bacteremia: Consider the organism type and clinical presentation
  • Obligate anaerobes (Bacteroides, Clostridium, Peptostreptococcus) are rarely contaminants 3, 4
  • If the patient has predictable risk factors for anaerobic infection (recent GI surgery, malignancy, immunosuppression, obvious anaerobic source), the positive culture is more likely clinically significant 4

Mandatory Repeat Cultures During Treatment

You must obtain repeat blood cultures 72 hours after initiating appropriate antimicrobial therapy if 1:

• The patient remains febrile or clinically unstable
• You are attempting catheter salvage (rather than removal)
• Blood cultures remain positive at 72 hours, indicating treatment failure and necessitating catheter removal

This is particularly critical because:

• Persistent bacteremia despite appropriate therapy indicates complicated infection (endocarditis, suppurative thrombophlebitis, metastatic infection) 1
• Anaerobic bacteremia that persists >72 hours requires 4-6 weeks of therapy 1

Clinical Context for Anaerobic Bacteremia

Understanding when anaerobes are likely helps interpret your results 4:

• Predictable clinical scenarios (84% of cases): GI tract pathology, recent abdominal surgery, malignancy, immunosuppression, abscess formation
• High-risk settings: ICU patients, bone marrow transplant recipients, patients with GI microbiota translocation 5
• Suspicious clinical features: Foul-smelling discharge, gas in tissues, necrotic tissue, proximity to mucosal surfaces 6

Common Pitfalls to Avoid

• Don't assume a single positive anaerobic culture is automatically significant without clinical correlation, though obligate anaerobes are less likely to be contaminants than CoNS 1, 3
• Don't delay repeat cultures if attempting catheter salvage—you need them at 72 hours to guide catheter removal decisions 1
• Don't forget that anaerobes represent <5% of blood culture isolates, so their recovery should prompt careful evaluation of the source 2, 3
• Don't rely solely on anaerobic bottles—most anaerobes will also grow in aerobic bottles, and aerobic bottles detect more pathogens overall 3, 7

Practical Algorithm

1. If only one set was drawn initially: Obtain at least one more set immediately from a different site 1
2. If two sets were drawn (one positive): Assess clinical context—if high suspicion for true infection based on risk factors, proceed with treatment but still consider additional cultures 4
3. At 72 hours of therapy: Obtain repeat cultures to document clearance or identify persistent bacteremia requiring catheter removal and extended therapy 1
4. If cultures remain positive at 72 hours: Remove catheter, evaluate for complicated infection (echocardiography for endocarditis), and plan 4-6 weeks of therapy 1