Pharmacist University of South Carolina College of Pharmacy
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Introduction: Bloodstream infections (BSIs) are associated with significant morbidity and mortality, especially in patients with co-morbidities that amplify the severity of illness. Time to positivity (TTP) has been investigated as a predictor for BSI outcomes. Assessment of local TTP data could be used as an antimicrobial stewardship tool. The purpose of this retrospective study is to determine the median TTP of monomicrobial blood cultures in critically ill and non-critically ill patients. The data will be used to calculate likelihood of growth at targeted time points from the index blood culture.
Methods: Hospitalized adults with a positive monomicrobial blood culture, between July 1, 2023 and December 31, 2023, were evaluated for inclusion. Adults with hemodynamic instability, defined as vasopressor use and/or systolic BP < 90mmHg, were grouped (critically ill) and compared to stable patients (non-critically ill). The primary objective was median TTP for gram-negative (GN) and gram-positive (GP) bacteremia with pathogen distribution stratified in 12- and 24-hour time increments to determine the threshold of growth. Further variables were compared between critically ill and non-critically ill populations.
Results: Among 672 unique patients, the average age was 65 years and 17% of patients had a Pitt bacteremia score of at least 4. The majority (86%) of BSIs were community onset. The sources were most commonly urine, line related, and intra-abdominal. Median TTP was 20 hours (14, 28). TTP was significantly shorter in patients with GNBSI. In critically ill patients, TTP among GNBSI was 18.4 hours compared to 22.08 hours in non-critically ill (p=0.0074). There was no difference in TTP of GPBSI. At the 48-hour time point, 96% of gram-negative and gram-positive pathogens had resulted positive. Approximately 90% of positive blood cultures had resulted by 36 hours.
Conclusions: The overwhelming majority (~90%) of GP and GN monomicrobial positive cultures resulted in 36 hours and nearly all (96%) by 48 hours with a shorter TTP among GNBSI in critically ill. These data will be used to establish a stewardship decision tool for streamlining antibiotics in the absence of positive cultures at specific time intervals following blood culture obtainment.
