Why is taking blood cultures necessary in sepsis?

Why Blood Cultures Are Necessary in Sepsis

Blood cultures must be obtained before starting antibiotics in sepsis because they enable pathogen identification and antimicrobial susceptibility testing, which allows for targeted antibiotic de-escalation—a practice associated with improved survival, reduced antimicrobial resistance, fewer side effects, and lower costs. 1

Primary Reasons for Obtaining Blood Cultures

Pathogen Identification and Targeted Therapy

• Obtaining cultures before antimicrobials significantly increases the yield of pathogen detection, making identification of the causative organism far more likely 1
• Isolation of the infecting organism allows for de-escalation of antimicrobial therapy first at the point of identification and then again when susceptibilities are obtained 1
• De-escalation has been associated with improved survival in several observational studies and is a cornerstone of antibiotic stewardship programs 1

Impact on Mortality and Outcomes

• Several retrospective studies have demonstrated that obtaining cultures prior to antimicrobial therapy is associated with improved outcomes 1
• Blood cultures remain the most sensitive method for detecting bacteremia and should be obtained when there are signs of systemic infection including leukocytosis, even without fever 2

Critical Timing Considerations

The Window of Opportunity

• Sterilization of cultures can occur within minutes to hours after the first dose of an appropriate antimicrobial 1, 2
• Research demonstrates that blood culture positivity drops from 50.6% when obtained before antibiotics to only 27.7% when obtained during antibiotic therapy (p <0.001) 3
• In patients with severe sepsis manifestations, the sensitivity of blood cultures obtained shortly after antimicrobial initiation drops to only 52.9%, representing an absolute difference of 12.0% in positive culture rates 4

Balancing Speed with Diagnostic Yield

• The Surviving Sepsis Campaign recommends obtaining blood cultures before antimicrobials if this results in no substantial delay—defined as approximately 45 minutes or less 1
• The risk/benefit ratio favors rapid administration of antimicrobials if it is not logistically possible to obtain cultures promptly 1
• Antimicrobials should be administered within 1 hour of recognition of sepsis or septic shock, so cultures must be obtained efficiently 1

Proper Blood Culture Technique

Number and Source of Cultures

• At least two sets of blood cultures (aerobic and anaerobic bottles) are recommended before initiation of any new antimicrobial in all patients with suspected sepsis 1
• At least one culture should be drawn percutaneously (peripherally via venipuncture) 1
• Each blood culture should contain 20-30 mL of blood to optimize pathogen detection 2

Special Circumstances with Vascular Catheters

• In patients with an intravascular catheter in place >48 hours where catheter-associated infection is suspected, at least one blood culture set should be obtained from the catheter along with simultaneous peripheral blood cultures 1
• Drawing blood cultures from an intravascular catheter does not eliminate the option of removing the catheter (particularly non-tunneled catheters) immediately afterward 1

Timing of Collection

• All necessary blood cultures may be drawn together on the same occasion—blood culture yield has not been shown to be improved with sequential draws or timing to temperature spikes 1
• In patients already receiving antibiotics, blood cultures should be obtained before the next dose when blood levels are at their lowest to maximize yield 2

Common Pitfalls to Avoid

Inappropriate Culture Practices

• "Pan culture" of all sites should be discouraged unless the source of sepsis is not clinically apparent, because this practice can lead to inappropriate antimicrobial use 1
• If history or clinical examination clearly indicates a specific anatomic site of infection, cultures of other sites (apart from blood) are generally unnecessary 1

Technical Errors

• Delaying collection of blood cultures until after the next antibiotic dose significantly reduces yield 2
• Collecting insufficient blood volume decreases sensitivity 2
• Obtaining cultures from existing lines rather than fresh venipuncture sites when possible increases contamination risk 2

Clinical Scenarios Where Blood Cultures Are Strongly Indicated

• Patients with suspected sepsis or septic shock, even without fever 2
• Immunocompromised patients with signs of infection 2
• Patients with rising WBC count despite ongoing antibiotics, as this suggests either inadequate antimicrobial coverage, development of resistance, or a new/secondary infection 2
• Patients with indwelling vascular catheters and rising inflammatory markers 2
• Patients with valvular heart disease and unexplained inflammatory response 2

Impact on Antibiotic Stewardship

• Positive blood cultures in patients on antibiotics strongly suggest inadequate antimicrobial coverage and may necessitate adjustment of therapy 2
• De-escalation based on culture results is associated with less resistant microorganisms, fewer side effects, and lower costs 1
• Multiple positive blood cultures (versus a single positive) help distinguish true bacteremia from contamination 2