OBJECTIVES: Currently, diagnosis of a bloodstream infection relies on culture-based methods and takes 2-3 days. The development of a rapid diagnostic tool has been deemed a high priority by the Infectious Disease Society of America. Available rapid tests are limited by the need to target specific pathogens. Rapid tests for universal detection of bacterial or fungal pathogens are in development. Commercialization of such a test will allow physicians to make rapid decisions, avoid unnecessary hospitalization, and decrease morbidity. No economic analyses have yet assessed the potential medical cost impacts of such a test.   METHODS: We performed a decision tree analysis to estimate the potential medical cost consequences of a rapid test to detect bloodstream infection, using data from published literature and expert opinion. We considered potential impacts in the areas of diagnostic testing, prevention of diseases, and reduction in hospital admissions. Medicare pricing schedules were used, analyses were conducted from the healthcare payer perspective, and a base case cost of $20/test was used. RESULTS: Our model demonstrates that with population-wide implementation, the annual U.S. medical cost savings associated with decreased clinical diagnostic testing, reduced antimicrobial use, disease prevention, avoidance of hospitalization, and reduction in hospital length of stay could be $0.2, $0.3, $1.2, $1.1 and $1.6 billion, respectively. An increase in diagnostic testing cost to $40, $50 and $60/test lowers net savings by 3.6%, 5.4% and 7.2%, respectively. A tripling of the testing cost still allows for net savings of >$3.6 billion to the healthcare system. CONCLUSIONS: Our analysis suggests that routine hospital-based use of a rapid diagnostic test for bacterial and fungal bloodstream infections would result in substantial savings to U.S. healthcare payers. If such a test were available, and its negative predictive value for bloodstream infection were very high, it would likely become a dominant diagnostic strategy.