From the Guidelines
Blood cultures from two separate venipuncture sites are recommended when testing for sepsis to increase diagnostic accuracy and distinguish true pathogens from contaminants. Drawing blood from two different peripheral sites helps to confirm a true bloodstream infection when the same organism grows in both cultures, as this significantly reduces the likelihood that the positive result is due to skin contamination during collection 1. Skin contaminants like coagulase-negative staphylococci or diphtheroids may appear in a single culture but are unlikely to contaminate both samples.
Some key points to consider when obtaining blood cultures for sepsis include:
- Collecting adequate blood volume (typically 20-30 mL total, divided between bottles) increases the sensitivity of detecting bacteremia, as the concentration of bacteria in the blood during sepsis can be low (often less than 10 colony-forming units per milliliter) 1.
- The timing is also important - both sets should ideally be collected before starting antimicrobial therapy, but should not delay treatment beyond one hour in patients with suspected sepsis 1.
- Each set should include both aerobic and anaerobic culture bottles to detect the full spectrum of potential pathogens.
- This approach optimizes the chance of identifying the causative organism, which is crucial for targeted antimicrobial therapy and improved patient outcomes 1.
It's worth noting that the most recent and highest quality study, 1, recommends obtaining at least two sets of blood cultures (aerobic and anaerobic) before initiation of any new antimicrobial in all patients with suspected sepsis. Additionally, the study suggests that all necessary blood cultures may be drawn together on the same occasion, and that blood culture yield has not been shown to be improved with sequential draws or timing to temperature spikes.
From the Research
Rationale for Blood Cultures from Two Separate Venipuncture Sites
- The primary reason for collecting blood cultures from two separate venipuncture sites is to increase the likelihood of detecting pathogenic microorganisms in patients with sepsis 2, 3.
- Collecting blood cultures from two sites can help reduce contamination and increase the sensitivity of blood culture results 2.
- However, studies have shown that single-site sampling (SSS) may be non-inferior to multi-site sampling (MSS) in detecting pathogenic microorganisms 2, 3.
- SSS may also have advantages over MSS, such as higher positivity rates and higher sample volumes 3.
Importance of Blood Culture Timing
- Obtaining blood cultures before antibiotic administration is crucial, as antibiotic therapy can significantly reduce the likelihood of detecting pathogenic microorganisms 4.
- Delaying blood culture collection until after antibiotic therapy has started can result in a significant loss of pathogen detection 4.
- Gram-positive and Gram-negative pathogens are more frequently detected in blood cultures drawn before antibiotic therapy compared to those drawn during antibiotic therapy 4.
Clinical Implications
- The choice of sampling protocol (SSS or MSS) may not significantly impact the detection of pathogenic microorganisms in patients with sepsis 2, 3.
- However, SSS may be a more convenient and efficient method for collecting blood cultures, with potential advantages in terms of positivity rates and sample volumes 3.
- Clinicians should prioritize obtaining blood cultures before antibiotic administration to maximize the likelihood of detecting pathogenic microorganisms 4.