What is the proper procedure for collecting and sending blood cultures when suspecting infective endocarditis?

Blood Culture Collection for Suspected Infective Endocarditis

Obtain at least 3 sets of blood cultures from separate peripheral venipunctures before starting antibiotics, with each set containing adequate blood volume (10 mL per bottle in adults, 1-7 mL total in children depending on age), and ensure both aerobic and anaerobic bottles are inoculated. 1

Number and Timing of Blood Cultures

For stable patients:

• Collect 3 blood culture sets by separate venipunctures on the first day 1
• Space collections at 30-minute intervals 1
• The European Society of Cardiology specifically recommends the first and last samples be drawn at least 1 hour apart 2
• If no growth by day 2 of incubation, obtain 2-3 additional sets 1

For critically ill/unstable patients:

• Obtain 3 separate venipunctures over a short period (1-2 hours) 1
• No interval required between sets when treatment cannot be delayed 1
• Initiate empirical antibiotics immediately after collection 1

The rationale for multiple sets is critical: bacteremia in IE is continuous (not intermittent), so timing relative to fever peaks is irrelevant 1. However, a single positive culture has contamination rates of 1-4.5%, making interpretation of potential contaminants impossible 1. With 3 sets, sensitivity exceeds 95-99% for detecting true bacteremia 1.

Blood Volume Requirements

Adults:

• Minimum 20 mL per venipuncture (10 mL per bottle) 1
• Volumes above 30 mL do not significantly improve yield 1
• Each set should include both aerobic and anaerobic bottles 1

Pediatric patients:

• Infants and young children: 1-3 mL total 1
• Older children: 5-7 mL total 1
• Infants: 0.5 mL or <1% of circulating blood volume is acceptable 1

The volume is crucial because bacteremia concentration in IE is often <1 organism/mL, and may be as low as 0.1 organisms/mL 1. Inadequate volume is a common pitfall leading to false-negative results.

Collection Technique

Critical technical points:

• Always draw from peripheral veins, never from central venous catheters (risk of contamination and misleading interpretation) 1
• Use meticulous sterile technique with skin preparation using isopropyl/ethyl alcohol or iodine-containing solution 1
• Sterilize blood culture bottle stoppers before inoculation 1
• If anaerobic bottles are unavailable or blood volume is insufficient, prioritize the aerobic bottle (anaerobes cause <5% of bacteremia) 1

Special Considerations

For patients with recent antibiotic exposure:

• There may be no value in obtaining >5 blood cultures over 2 days unless antibiotics were given within the past 2 weeks 1
• In stable patients with negative cultures, consider withholding antibiotics for ≥48 hours while obtaining additional cultures 1
• Prior antibiotic therapy is the most common cause of culture-negative IE 2

For patients with central venous catheters:

• The presence of a CVC may complicate diagnosis, as attempts to salvage the catheter can prolong bacteremia 1
• Some experts suggest S. aureus bacteremia should only be considered a major Duke criterion if cultures remain positive after CVC removal 1

Laboratory Communication

• Alert the microbiology laboratory that IE is suspected at the time of blood culture sampling 1
• This ensures appropriate incubation duration and consideration of fastidious organisms 1
• For suspected unusual/fastidious organisms, consult infectious disease or microbiology director for guidance on molecular diagnostics and serological testing 1, 2

Follow-up Cultures

• Repeat blood cultures 48-72 hours after starting treatment to verify effectiveness 1, 2
• Blood cultures should be negative for at least 72 hours before any cardiac device reimplantation 1

Common Pitfalls to Avoid

• Never delay collection for fever spikes - bacteremia in IE is continuous, not intermittent 1
• Never use arterial blood - it offers no advantage over venous blood 1
• Never collect from central lines - contamination risk invalidates results 1
• Never use inadequate blood volume - this is a leading cause of false negatives 1
• Never obtain only 1-2 sets - insufficient to distinguish contamination from true bacteremia 1