When should blood cultures be repeated in a patient with a positive initial blood culture?

From the Guidelines

Blood cultures should be repeated 2-4 days after the initial positive culture and as needed thereafter to document clearance of bacteremia. This approach is supported by the Infectious Diseases Society of America guidelines 1, which recommend follow-up blood cultures to assess the effectiveness of treatment and identify potential complications. The frequency of repeat cultures may vary depending on the patient's clinical condition, with some experts suggesting daily cultures for patients with Staphylococcus aureus bacteremia until documented clearance 1. However, the most recent study on the topic suggests that even a single positive culture after starting therapy is a poor prognostic sign, and failure to immediately clear cultures should prompt further investigation 1.

Key considerations for repeat blood cultures include:

• Clinical deterioration or persistent fever despite treatment
• Changes to antimicrobial therapy to assess efficacy
• Suspected contamination with common skin contaminants
• Endovascular infections, which may require more frequent cultures (every 24-48 hours) until negative
• The "skip phenomenon," where patients may have intermittently negative blood cultures prior to complete clearance, which may have implications for the timing of reimplantation of cardiac devices or intravascular catheters 1.

Overall, the goal of repeat blood cultures is to document microbiological clearance, identify persistent infections requiring extended treatment or source control, and guide the overall duration of antimicrobial therapy.

From the FDA Drug Label

5.9 Persisting or Relapsing S. aureus Bacteremia/Endocarditis Patients with persisting or relapsing S. aureus bacteremia/endocarditis or poor clinical response should have repeat blood cultures.

Repeat blood cultures should be performed in patients with:

• Persisting or relapsing S. aureus bacteremia/endocarditis
• Poor clinical response 2

From the Research

Repeating Blood Cultures in Patients with Positive Initial Blood Culture

• The decision to repeat blood cultures in a patient with a positive initial blood culture depends on various factors, including the type of infection, the presence of persistent bacteremia, and the clinical context 3, 4.
• In patients with endocarditis, persistent bacteremia and fever may be present in the first 3 days of treatment, despite appropriate antibiotic therapy, and are more common in Staphylococcus and Enterococcus species 3.
• Studies have shown that persistent positive blood cultures after 48-72 hours of antibiotic therapy are associated with a worse prognosis in patients with left-sided infective endocarditis 4.
• However, other studies suggest that repeating blood cultures may not be necessary in all cases, as concurrent antimicrobial administration does not alter blood culture yield, and little new information is gained from repetitive culturing 5.
• The timing of valve culture negativization is also an important consideration, as valve cultures at surgery become negative in most cases after 14-21 days of antibiotic therapy, with no calculated benefit for prolonging therapy after 21 days 3.

Specific Considerations

• In patients with methicillin-resistant Staphylococcus aureus (MRSA) bacteremia, treatment with daptomycin, linezolid, or vancomycin may be effective, but the choice of antibiotic should be guided by clinical and microbiologic factors 6.
• The role of infectious disease specialists in recommending antibiotic therapy is crucial, as their recommendations can improve the appropriateness of antibiotherapy and patient outcomes 7.
• The notification method of positive blood cultures and the timing of antibiotic recommendations can also impact patient care and outcomes 7.

Key Findings

• Persistent positive blood cultures after 48-72 hours of antibiotic therapy are associated with a worse prognosis in patients with left-sided infective endocarditis 4.
• Repeating blood cultures may not be necessary in all cases, as concurrent antimicrobial administration does not alter blood culture yield 5.
• The timing of valve culture negativization is an important consideration in patients with endocarditis, as valve cultures at surgery become negative in most cases after 14-21 days of antibiotic therapy 3.