What is the recommended time interval between taking two blood cultures?

Blood Culture Timing Recommendations

In critically ill patients, blood cultures can be obtained sequentially over a short time interval (minutes) with no specific timing requirement between cultures. 1

Key Recommendations for Blood Culture Collection

Timing Between Blood Cultures

• For critically ill patients requiring urgent antimicrobial therapy: No specific time interval is required between blood cultures 1
• For non-critically ill patients: While older expert opinion suggested 30-60 minute intervals, research has shown that drawing blood cultures simultaneously or at intervals does not affect yield 1
• The timing should be dictated by patient acuity rather than arbitrary time intervals 1

Number of Blood Cultures

• Obtain a minimum of 2 and maximum of 3 sets of blood cultures per septic episode 1
• 2 blood culture sets detect approximately 90% of bloodstream infections 2
• 3 blood culture sets detect 96-98% of bloodstream infections 2
• 4 blood culture sets may be needed to achieve >99% detection rate 2

Volume of Blood

• For adults: 20-30 mL of blood per culture set (depending on manufacturer) 1
• This volume is typically split between aerobic and anaerobic bottles
• Blood volume is the most important variable for detecting bacteremia 1

Special Considerations

Catheter-Related Infections

• For patients with intravascular catheters: Draw one blood culture through the catheter and at least one from a peripheral venipuncture 1, 3
• Label each blood culture with exact time, date, and anatomic site 1

Suspected Endocarditis

• For suspected endocarditis: Obtain follow-up blood cultures every 24-48 hours until bloodstream infection has cleared 3
• Persistent positive cultures may indicate need for surgical intervention 3

Pediatric Patients

• Blood volume requirements differ for pediatric patients based on weight 1
• For neonates, studies suggest that two-site blood cultures improve pathogen detection compared to single-site cultures 4

Common Pitfalls to Avoid

1. Delaying antimicrobial therapy: In sepsis or septic shock, do not delay antimicrobial therapy to obtain blood cultures if it would cause significant delay 1

2. Inadequate blood volume: Collecting insufficient blood volume significantly reduces culture yield 1

3. Contamination: Use proper skin antisepsis (2% chlorhexidine gluconate in 70% isopropyl alcohol is preferred) and allow appropriate drying time (30 seconds) 1

4. Improper labeling: Always label cultures with exact time, date, and anatomic site 1

5. Obtaining cultures after starting antibiotics: This significantly reduces culture yield; obtain cultures before antimicrobial therapy whenever possible 1

The most recent evidence indicates that the timing between blood cultures is less important than previously thought, with patient acuity, proper technique, adequate blood volume, and obtaining cultures before antibiotics being the critical factors for optimal diagnostic yield.