Respiratory and gastrointestinal complaints are among the most common reasons why adults are prescribed antibiotics in acute care. In recent years, multiplex molecular panels that rapidly test for 15-25 for pathogens simultaneously, including bacteria and viruses, have entered common use and dramatically changed clinicians’ basic approach to these conditions. The potential advantage of multiplex panels is that, if uses properly, rapid diagnosis may facilitate targeted antibiotic or antiviral therapy. But multiplex panels are “shotgun” tests that frequently produce false positives. When implemented without consideration of unintended consequences, their use can either have no impact or lead to inappropriate antibiotics. This study will address a gap in the literature by identifying when multiplex panel tests can change clinical management or benefit the patient and designing interventions to nudge testing in those situations. The proposed studies will promote patient safety and reduce misdiagnosis through diagnostic stewardship, an emerging model for infectious diseases care that uses “nudge” interventions to encourage evidence-based testing. Aim 1 will define high- and low-value use of multiplex panel testing by examining clinical outcomes after testing among patients seen in acute care at 200 hospitals contributing to the Premier Healthcare Database. Aim 2 will develop electronic health record-based interventions to nudge better testing practices in acute care based on qualitative input from frontline clinicians. Aim 3 will be a quasi-experimental trial of diagnostic stewardship interventions at two hospitals in the University of Maryland System. Other system hospitals will be concurrent controls. This research will lead to the first large-scale trial evaluating diagnostic stewardship of multiplex molecular panel tests in acute care. I am an infectious disease physician and hospital epidemiologist with a PhD in health services research. My long-term career goal is to become an independent investigator performing health services research to redesign systems of care with a focus on molecular diagnostics for patients with suspected infections. To achieve this goal, I will undergo formal training in molecular microbiology, healthcare informatics, and implementation science. I will be mentored by an interdisciplinary team including Dr. Daniel Morgan, an AHRQ-funded expert in diagnostic stewardship, Dr. J. Kristie Johnson, expert in clinical microbiology, Dr. Sarah Krein, an expert in mixed methods and user-centered design, and Dr. Anthony Harris, an expert in quasi- experimental trials. The long-term scientific goal of my research is to improve antibiotic use through high-quality and high-value diagnostic testing for infectious diseases.