The goal of this project is reduce unnecessary use of antibiotics in the ICU. The purpose of Phase I of the study is to identify the biomarker, or combination of biomarkers, that provides optimal test characteristics in identifying adults and children/neonates with presumed sepsis who have a very low likelihood of bacterial infection. Results of Phase I will result in development of a biomarker-based algorithm to inform need for antibiotic use in ICU patients. In Phase II, the impact of this biomarker-based algorithm on reducing antibiotic use in the ICU will be determined. Costs or savings associated with the algorithm will also be assessed.