Should I obtain blood cultures (Blood cx) in a patient with no fever, ongoing antibiotics, and a rising white blood cell (WBC) count?

Blood Cultures in Patients with No Fever, Ongoing Antibiotics, and Rising WBC

Blood cultures should be obtained in patients with rising WBC count and ongoing antibiotics, even in the absence of fever, as this may indicate persistent or evolving bacteremia that requires adjustment of antimicrobial therapy. 1

Rationale for Obtaining Blood Cultures

• Rising WBC count is a significant indicator of potential bacteremia, even when fever is absent, as approximately 21.6% of bacteremic patients may not present with fever 2
• 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 3
• Systemic Inflammatory Response Syndrome (SIRS) and increasing C-reactive protein (CRP) are powerful predictors of positive blood cultures, independent of fever 4

Timing and Technique for Blood Cultures

• Blood cultures should be obtained before the next dose of antibiotics when blood levels are at their lowest to maximize yield 3
• At least two sets of blood cultures (aerobic and anaerobic) should be collected from separate peripheral venipuncture sites 1
• Each blood culture should contain 20-30 mL of blood to optimize pathogen detection 1
• Proper aseptic technique is essential to minimize contamination, including thorough skin preparation with alcohol or iodine-containing solution 5

Special Considerations with Ongoing Antibiotics

• The sterilization of blood cultures can occur within minutes to hours after antimicrobial administration, making collection timing critical 1
• In patients already receiving antibiotics, the yield of blood cultures decreases but remains clinically valuable for detecting persistent bacteremia or inadequate antimicrobial coverage 1
• Rising WBC count despite ongoing antibiotics suggests either inadequate antimicrobial coverage, development of resistance, or a new/secondary infection 3

Clinical Scenarios Where Blood Cultures Are Strongly Indicated

• Patients with suspected sepsis or septic shock, even without fever 1
• Immunocompromised patients with signs of infection 3
• Patients with indwelling vascular catheters and rising inflammatory markers 1
• Patients with valvular heart disease and unexplained inflammatory response 1
• Patients with severe infections requiring ICU admission 1

Interpretation and Clinical Application

• Positive blood cultures in patients on antibiotics strongly suggest inadequate antimicrobial coverage and may necessitate adjustment of therapy 1
• De-escalation of antimicrobial therapy based on culture results is a cornerstone of antibiotic stewardship and is associated with improved outcomes 1
• Multiple positive blood cultures (versus a single positive) help distinguish true bacteremia from contamination 1

Common Pitfalls to Avoid

• Delaying collection of blood cultures until after the next antibiotic dose, which significantly reduces yield 5
• Collecting insufficient blood volume, which decreases sensitivity 1
• Obtaining cultures from existing lines rather than fresh venipuncture sites when possible, which increases contamination risk 5
• Waiting for fever to develop before obtaining cultures, as bacteremia can exist without fever, particularly in elderly or immunocompromised patients 2