When should new blood cultures be obtained in a patient with a fever?

From the Guidelines

New blood cultures should be obtained when a patient develops a new fever or has persistent fever despite appropriate treatment, and at least two sets of blood cultures should be drawn before initiating antimicrobial therapy if possible, as recommended by the 2017 Surviving Sepsis Campaign guidelines 1. When deciding on the timing of new blood cultures, it's essential to consider the clinical suspicion of continuing or recurrent bacteremia or fungemia, as well as the need for a test of cure 48-96 hours after initiation of appropriate therapy for bacteremia/fungemia, as suggested by the 2008 American College of Critical Care Medicine and the Infectious Diseases Society of America guidelines 1. Some key points to consider when obtaining blood cultures include:

• Drawing blood cultures before starting antibiotics whenever possible, as antimicrobial therapy can rapidly sterilize the bloodstream and lead to false-negative results 1
• Collecting blood cultures just before the next antibiotic dose when bacterial levels in the bloodstream may be higher for patients already on antibiotics
• Obtaining at least two sets of blood cultures from different venipuncture sites to increase diagnostic yield and help distinguish true pathogens from contaminants, with each set including aerobic and anaerobic bottles with 8-10 mL of blood per bottle 1
• Considering the use of three sets of blood cultures for patients with suspected endocarditis or those with intravascular devices
• Labeling the blood culture with the exact time, date, and anatomic site from which it was taken, and drawing 20-30 mL of blood per culture, as recommended by the 2008 guidelines 1. It's also important to note that 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, and that paired blood cultures provide more useful information than single blood cultures, as stated in the 2008 guidelines 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, as recommended by the 2017 guidelines 1. Ultimately, the decision on when to obtain new blood cultures should be based on clinical judgment and the individual patient's circumstances, with the goal of balancing the need for timely antimicrobial therapy with the importance of obtaining accurate diagnostic information.

From the Research

Determining the Need for New Blood Cultures in Febrile Patients

• The decision to obtain new blood cultures in a patient with a fever should be guided by clinical judgment and the presence of signs and symptoms suggestive of sepsis 2.
• Blood cultures are essential for the evaluation of sepsis, but they may sometimes be obtained inappropriately, leading to high false-positive rates due to contamination 2.
• A study found that implementing clinician decision-support tools for evaluating patients with fever or signs and symptoms of sepsis resulted in a 46% decrease in blood culture obtainment 2.

Factors Influencing Blood Culture Positivity

• Antibiotic administration before blood culture sampling can significantly reduce the likelihood of pathogen detection 3.
• Blood culture positivity was higher among patients who did not receive antibiotics before sampling (50.6%) compared to those who were already receiving antibiotics (27.7%) 3.
• Gram-positive and Gram-negative pathogens were more frequently detected in blood culture sets drawn before antibiotic therapy compared to sets obtained during antibiotic therapy 3.

Clinical Outcomes and Blood Culture Results

• A study found no significant difference in length of stay or outcomes among patients with positive and negative blood cultures 4.
• However, patients admitted from the emergency department with positive blood cultures had a longer length of stay, although their overall prognosis was not worse 4.
• The presence of resistant pathogens can significantly impact clinical outcomes, with patients infected with resistant pathogens having a higher risk of hospital mortality 5.

Considerations for Obtaining New Blood Cultures

• New blood cultures should be obtained when there is a clinical suspicion of sepsis or a change in the patient's condition, such as the development of new signs or symptoms of infection 2, 3.
• The timing of blood culture sampling in relation to antibiotic administration should be considered, as obtaining cultures during antibiotic therapy can reduce the likelihood of pathogen detection 3.