Laboratory analysis of blood cultures is the gold standard for diagnosing bloodstream infections, especially for patients with suspected sepsis or septic shock. But contamination during collection poses major challenges. Blood culture contamination can compromise quality of care and lead to unnecessary antibiotic use and prolonged length of hospitalization. Clinical and Laboratory Standards Institute (CLSI) guidelines recommend contamination rates of ≤ 3%, with ≤ 1% considered optimal.
The CDC’s Blood Culture Contamination factsheet1 outlines both the harms of contamination—false-positive sepsis diagnoses, unnecessary antibiotics, C. difficile infections, extended hospital stays, and increased costs—and the strategies to prevent contamination. Recommended strategies include the following:
- Obtaining blood cultures for the right patients, in the right settings, and at the right time (diagnostic stewardship).
- Prioritizing peripheral venipuncture over line draws.
- Employing dedicated phlebotomy teams.
- Using skin antisepsis, bottle disinfection, and diversion devices.
- Monitoring rates monthly, stratified by unit and collector, and sharing data across stewardship and infection prevention teams.
In this issue of MLO, we have articles on both antimicrobial stewardship (“Antimicrobial stewardship: Empowering labs to drive clinical impact through diagnostics,” page 18) and sepsis (“The need for earlier detection and reliable intervention monitoring for managing sepsis,” page 22). This week I started thinking about blood culture contamination after reading an interesting study published in the Journal of Microbiology that analyzed over 362,000 blood cultures from 52 hospitals across 19 states (2019–2021).2 Contamination rates averaged 1.38% in ICUs and 0.96% on wards when defined by College of American Pathologists (CAP) criteria. Rates were slightly higher when broader CDC/NHSN definitions were applied, underscoring how benchmarks shift with definitions.
The study also found wide variation in practice. Nearly all hospitals tracked contamination rates, yet only 21% monitored single-draw cultures and 39% tracked positivity rates. Few shared data outside the laboratory, limiting quality improvement efforts. Facilities that avoided central line draws, used electronic prompts, or engaged in stewardship interventions reported lower contamination rates. Competency training for blood cultures for nonphlebotomy staff was provided in 40 of the 52 hospitals.
An important element stressed in this study was the variability in how blood culture contamination was defined. Hospitals used CAP (65%), CLSI (17%), and NHSN (17%) criteria to define BCC. However, there is no nationally standardized definition for blood culture contamination. Without it, hospitals may underestimate their true rates or fail to meet best practice thresholds.
With standardization and adoption of proven practices, hospitals can meaningfully reduce contamination, avoid unnecessary antibiotic exposure, cut costs, and improve outcomes for patients with suspected bloodstream infections.
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REFERENCES
- Centers for Disease Control and Prevention. Blood Culture Contamination: An Overview for Infection Control and Antibiotic Stewardship Programs Working with the Clinical Laboratory. Accessed September 6, 2025. Available at: https://www.cdc.gov/antibiotic-use/core-elements/pdfs/fs-bloodculture-508.pdf.
- Fabre V, Hsu YJ, Carroll KC, et al. Multicenter evaluation of blood culture contamination and blood cultures practices in US acute care hospitals: time for standardization. J Clin Microbiol. 2025;63(8):e0053025. doi:10.1128/jcm.00530-25.
