When should blood cultures be repeated in cases of suspected bacteremia or sepsis?

When to Repeat Blood Cultures in Suspected Bacteremia or Sepsis

Blood cultures should be repeated only when there is clinical suspicion of continuing or recurrent bacteremia/fungemia, or for test of cure 48-96 hours after initiation of appropriate antimicrobial therapy, and should always be obtained in pairs rather than as single specimens. 1

Initial Blood Culture Collection

• Obtain 3-4 blood cultures within the first 24 hours of fever onset 1, 2
• Draw cultures before starting antimicrobial therapy whenever possible 1, 3
  • Antibiotics significantly reduce culture positivity (50.6% positive without antibiotics vs. 27.7% with antibiotics) 3
• Each culture should contain 20-30 mL of blood (10-30 mL per bottle) 1, 2
• Use strict aseptic technique with 2% chlorhexidine in 70% isopropyl alcohol 1, 2
• For patients with intravascular catheters, obtain at least one culture peripherally and one through the catheter 1, 2

Indications for Repeat Blood Cultures

Recommended:

1. Clinical suspicion of persistent or recurrent bacteremia 1, 2

   • Persistent fever
   • Hemodynamic instability
   • New signs of infection

2. Test of cure for specific scenarios (48-96 hours after starting appropriate therapy) 1, 2:

   • Staphylococcus aureus bacteremia (every 24-48 hours until clearance documented) 2
   • Endovascular infections (7.66 times higher risk of persistent bacteremia) 4
   • Epidural infections (26.99 times higher risk of persistent bacteremia) 4

3. Inadequate source control 4

   • Patients with persistent bacteremia are less likely to have achieved source control within 48 hours

Not Recommended:

1. Routine follow-up in uncomplicated gram-negative bacteremia 5

   • Low yield (requires 17 follow-up cultures to detect 1 positive result)
   • Particularly low yield for E. coli, viridans group and β-hemolytic streptococci 4

2. Daily surveillance cultures without clinical indication 1

Special Considerations

For Patients on Antibiotics:

• If patient has received short-term antibiotics, wait at least 3 days after discontinuation before new cultures 2
• For long-term antibiotics, cultures may not become positive until 6-7 days after discontinuing treatment 2
• If cultures must be obtained while on antibiotics, use media with antibiotic-absorbing substances (BacT/Alert FAN and BACTEC Plus/F) 1, 2

Timing of Repeat Cultures:

• For test of cure: 48-96 hours after initiation of appropriate antimicrobial therapy 1, 2
• For S. aureus bacteremia: every 24-48 hours until documentation of clearance 2

Common Pitfalls to Avoid

1. Unnecessary repeat cultures - leads to increased healthcare costs, longer hospital stays, unnecessary consultations, and inappropriate antibiotic use 5

2. Single blood cultures - always obtain paired cultures for better diagnostic yield 1

3. Delaying antimicrobial therapy to obtain cultures - if obtaining cultures would substantially delay therapy (>45 minutes), prioritize starting antibiotics 1

4. Routine repeat cultures for all bacteremias - focus on high-risk scenarios (S. aureus, endovascular infections) 5, 4

5. "Pan-culturing" - culturing all potential sites without clinical indication leads to inappropriate antimicrobial use 1