When should blood cultures be drawn?

From the Guidelines

Blood cultures should be drawn before starting antimicrobial therapy in patients with suspected sepsis or septic shock, if doing so results in no substantial delay in the start of antimicrobials 1.

Key Considerations

• The goal is to obtain cultures prior to the administration of antimicrobials to increase the yield of cultures and make identification of a pathogen more likely.
• At least two sets of blood cultures (aerobic and anaerobic) are recommended before initiation of any new antimicrobial in all patients with suspected sepsis 1.
• Blood cultures can be drawn together on the same occasion, and there is no need to time them to temperature spikes or draw them sequentially 1.
• In patients with an intravascular catheter, at least one blood culture set should be obtained from the catheter, along with simultaneous peripheral blood cultures, to assist in the diagnosis of a potential catheter-related bloodstream infection 1.

Best Practices

• Proper skin antisepsis is crucial to prevent contamination, and 2% chlorhexidine gluconate in 70% isopropyl alcohol is the preferred skin antiseptic 1.
• The injection port of the blood culture bottles should be wiped with 70-90% alcohol before injecting the blood sample into the bottle to reduce the risk of introduced contamination 1.
• Paired blood cultures provide more useful information than single blood cultures, and single blood cultures are not recommended, except in neonates 1.

Timing and Volume

• Blood cultures should be drawn before antibiotic administration, as antibiotics can rapidly suppress bacterial growth, potentially leading to false-negative results and complicating diagnosis and treatment 1.
• For adults, 8-10 mL of blood per bottle is ideal, while pediatric volumes are weight-based 1.

Special Considerations

• In patients without a suspicion of catheter-associated infection, at least one blood culture should be obtained peripherally, but no recommendation can be made as to where additional blood cultures should be drawn 1.
• Molecular diagnostic methods may offer the potential to diagnose infections more quickly and more accurately than current techniques, but additional validation is needed before recommending these methods as an adjunct to or replacement for standard blood culture techniques 1.

From the Research

Timing of Blood Cultures

• Blood cultures should be obtained before antibiotic therapy begins, as administering antibiotics before obtaining blood cultures can decrease the sensitivity of blood cultures 2, 3, 4
• Obtaining blood cultures during antibiotic therapy is associated with a significant loss of pathogen detection 3
• The sensitivity of blood cultures obtained after initiation of antimicrobial therapy is lower than that of preantimicrobial blood cultures 4

Indications for Blood Cultures

• Blood cultures are recommended for patients with sepsis, meningitis, complicated pyelonephritis, endocarditis, and health care-associated pneumonia 2
• Blood cultures are not recommended for patients with cellulitis, simple pyelonephritis, and community-acquired pneumonia, as the chance of a false-positive culture is greater than the prevalence of true positive cultures 2
• A clinical decision rule can be used to predict the need for blood cultures, with major criteria including temperature > 39.5 degrees C, indwelling vascular catheter, or clinical suspicion of endocarditis, and minor criteria including temperature 38.3-39.4 degrees C, age > 65 years, chills, vomiting, hypotension, and laboratory abnormalities 5

Procedure for Obtaining Blood Cultures

• At least two blood cultures should be obtained from separate peripheral sites 2
• Two bottles of ≥7 mL should be obtained from each site 2
• Blood cultures should be obtained before antibiotic therapy begins, and the results should be interpreted in the context of the clinical picture 2, 3, 4