When should blood cultures be repeated and how does it impact management?

When to Repeat Blood Cultures and How It Changes Management

Blood cultures should be repeated only when there is clinical suspicion of continuing or recurrent bacteremia, for test of cure after 48-96 hours of appropriate therapy, or in specific high-risk scenarios such as S. aureus bacteremia, endovascular infections, or immunocompromised patients. 1

Indications for Repeating Blood Cultures

Recommended scenarios for repeat blood cultures:

1. Persistent fever or clinical deterioration despite appropriate antimicrobial therapy 1
2. Test of cure after 48-96 hours of appropriate antimicrobial therapy 1, 2
3. S. aureus bacteremia - repeat every 24-48 hours until clearance is documented 2
4. Endovascular infections (including infective endocarditis) 2
5. Intravascular catheter-related infections 2
6. Immunocompromised patients 2

NOT recommended for routine repetition:

• Uncomplicated cellulitis/erysipelas 1, 3
• Simple pyelonephritis 1, 3
• Community-acquired pneumonia 1, 3
• After initial negative cultures in the absence of new clinical concerns

How Repeat Blood Cultures Change Management

1. Documentation of Clearance

• Confirms effectiveness of current antimicrobial therapy 2
• Negative follow-up cultures in bacteremia support continuing current treatment 2

2. Persistent Bacteremia Detection

• Persistent positive cultures (especially after 48-72 hours of appropriate therapy) are associated with:
  • Doubled mortality risk 4
  • Need for treatment modification 2, 4
  • Possible uncontrolled source of infection requiring intervention 2, 5

3. Source Control Assessment

• Persistent bacteremia may indicate inadequate source control 5
• May prompt surgical intervention, catheter removal, or drainage procedures 2

4. Treatment Duration Decisions

• Guides duration of antimicrobial therapy 2
• For S. aureus bacteremia, treatment typically continues for at least 14 days after first negative blood culture 2

5. Detection of New Pathogens

• May identify superinfection or new pathogens requiring treatment modification 6
• Note: This is relatively uncommon, occurring in only about 2.5% of repeat cultures 6

Proper Technique for Blood Cultures

• Obtain paired blood cultures (two sets) 1
• Each set should contain 20-30 mL of blood 1
• Use 2% chlorhexidine gluconate in 70% isopropyl alcohol for skin antisepsis 1
• For patients with intravascular catheters, obtain at least one culture peripherally and one through the catheter 1
• Label cultures with exact time, date, and anatomic site 1

Timing of Repeat Cultures

• For test of cure: 48-96 hours after initiation of appropriate therapy 1, 2
• For S. aureus bacteremia: every 24-48 hours until clearance 2
• For suspected persistent infection: when clinically indicated by persistent fever or clinical deterioration 1

Cautions and Pitfalls

• Avoid unnecessary repeat cultures - they account for approximately one-third of all blood cultures with minimal additional yield 6
• Don't delay antimicrobial therapy to obtain cultures in critically ill patients with suspected sepsis 1
• False positives can lead to unnecessary treatment, increased costs, and prolonged hospitalization 3
• Persistent bacteremia is more common with certain organisms (S. aureus, especially MRSA, and Enterococcus) 7, 5
• Source control within 48 hours significantly improves outcomes in bacteremia 5

By following these evidence-based recommendations for repeating blood cultures, clinicians can optimize patient management while avoiding unnecessary testing and costs.