Objective: To assess the impact of initial specimen diversion device (ISDD) on inpatient and emergency department blood culture contamination (BCC), central-line–associated bloodstream infection (CLABSI) standardized infection ratios (SIRs), and antibiotic administration. Design: Single-center quasi-experimental prospective cohort study wherein phlebotomists used traditional venipuncture with or without the ISDD while registered nurses (RNs) used traditional venipuncture. Method: BCC events among phlebotomists and RNs were observed and compared from March 17, 2019, through January 21, 2020, defined by contaminant detection in 1 of 4 bottles for matched sets or 1 of 2 bottles in both subsets for coagulase negative staphylococci. CLABSIs throughout this period were recorded and SIRs were calculated. Enhanced oversight took place through July 21, 2019, with chart review assessing antibiotic use for patients with possible BCC. Results: Overall, 24% of blood cultures obtained were from patients in intensive care. Phlebotomists using traditional venipuncture (n = 4,759) had a 2.3% BCC rate; phlebotomists using the ISDD (n = 11,202) had a 0% BCC rate. RNs drew 7,411 BCs with a 0.8% BCC rate. The CLABSI SIR was decreased from 1.103 in 2017 and 0.658 in 2018 to 0.439 in 2019. The CLABSI incidence was 33%–64% of predicted value for each 2019 quarter. This range fell to 18%–37% after the exclusion of likely false-positive results. Among 42 patients with possible BCC under enhanced oversight, 2 patients were treated with prolonged antibiotic courses. Conclusions: ISDD use by phlebotomists was associated with BCC reduction and reduced false-positive CLABSI results. This patient-care quality improvement could constitute sustainable antibiotic stewardship expansion.