Understanding Blood Cultures: Importance, Contamination, and Implications
Blood cultures are a critical laboratory test used in acute care hospitals to detect the presence of bacteria in a patient’s bloodstream. This test plays a vital role in identifying infections early, determining their cause, and guiding appropriate treatment, especially with antibiotics. However, the accuracy of blood cultures can be compromised by contamination, leading to false positives and potentially unnecessary antibiotic exposure, which can prolong hospitalization.
The Challenge of Contamination
Traditionally, U.S. healthcare facilities aim to limit contamination rates in blood cultures to less than 3%, with an optimal goal of 1% or lower. These metrics are crucial for ensuring patient safety and effective treatment. However, a recent study led by researchers from Johns Hopkins Medicine highlights a significant issue: there is no standardized definition of what constitutes contamination in blood cultures. This lack of uniformity can lead to discrepancies in reported contamination rates, raising concerns about the reliability of these metrics.
The Study: A Closer Look at Definitions
Funded by the U.S. Centers for Disease Control and Prevention (CDC), the study surveyed 52 acute care hospitals across 19 states and the District of Columbia. The researchers analyzed over 360,000 blood cultures collected over a two-year period, from September 2019 to August 2021. The aim was to assess how hospitals define blood culture contamination (BCC) and how these varying definitions affect the reported rates of contamination.
Findings on Definitions and Rates
The study revealed that 65.4% of the surveyed hospitals used criteria from the Clinical and Laboratory Standards Institute (CLSI) or the College of American Pathologists (CAP) to define BCC. In contrast, 17.3% used locally defined criteria or a combination of CAP/CLSI criteria with a comprehensive list of nonpathogenic skin surface microorganisms from the National Healthcare Safety Network (NHSN). Approximately half of the hospitals targeted a BCC threshold of less than 3%.
The researchers found that the variation in definitions significantly impacted the assessment of BCC rates. For instance, using the CAP criteria, the overall BCC rate was reported at 1.38% for cultures from intensive care unit (ICU) patients and 0.96% for those from hospital wards. However, when considering the NHSN list of skin commensals, the BCC rates increased to 1.49% for ICUs and 1.09% for wards, indicating that the inclusion of skin commensals provides a more accurate reflection of actual contamination rates.
Quality Indicators and Their Importance
In addition to contamination rates, hospitals often track other quality indicators related to blood cultures. These include the number of blood culture bottles collected—four bottles are recommended for optimal bacterial detection—and blood culture positivity, which measures the percentage of cultures that contain actual bacterial pathogens. Low positivity rates can result from insufficient blood volume per culture bottle, an inadequate number of bottles, or overtesting patients who may not need a culture.
Interestingly, the study found that only a minority of hospitals monitored these additional metrics, highlighting a significant opportunity for improvement in diagnosing bloodstream infections.
Patient Outcomes and BCC Correlation
The researchers also explored the relationship between BCC and specific patient outcomes, such as central-line associated bloodstream infections (CLABSIs) and antibiotic usage. They discovered a concerning correlation: for every 1% increase in the BCC rate, there was a 9% increase in CLABSIs. This finding underscores the potential negative impact of contamination on patient safety and treatment efficacy.
Implications for Quality Improvement
The study’s findings emphasize the critical need for standardization in defining blood culture contamination. The variation in definitions can lead to misleadingly low contamination rates, which may mask underlying issues and compromise patient safety. As Dr. Valeria Fabre, the study’s lead author, notes, hospitals must develop effective strategies to address the problem of contamination and improve the accuracy of blood culture diagnostics.
Collaborative Efforts in Research
The research team comprised experts from various institutions, including the Johns Hopkins Bloomberg School of Public Health and affiliated hospitals. Their collaborative efforts aim to shed light on the complexities of blood culture contamination and its implications for patient care. The study was supported by the CDC Prevention Epicenters Program, highlighting the importance of ongoing research in this area.
By addressing the challenges associated with blood culture contamination, healthcare facilities can enhance their diagnostic capabilities, improve patient outcomes, and ultimately ensure safer treatment practices.
