IL6 and Sepsis: In this study we characterize (1) how ISI changes following initiation of treatment for sepsis, (2) how ISI correlates with patient illness severity over time (as measured by the Sequential Organ Failure Assessment (SOFA) score and blinded retrospective physician assessment), and (3) how the performance of ISI changes as compared to other biomarkers (PCT, CRP, IL-6, and neutrophil elastase) in a prospectively recruited population of ED patients presenting with suspected sepsis.