 Blood lactate is one of the markers that can predict the organ failures and mortality in emergency department ED patients with sepsis. Regarding its predictive role in the clinical deterioration in normotensive sepsis patients, a recent prospective observational study of the patients with moderately-high serum lactate 2.0-3.9 mmolL showed that one-forth of the patients eventually developed progressive organ dysfunctions or required vasopressor or mechanical ventilator. Unfortunately, low serum lactate 2.5 mmolL was still found up to 50 of overt septic shock patients, even in vasopressor-dependent cases. For the prognostic role on mortality, numbers of articles showed that blood lactate can also predict death in sepsis patients, especially in those who are elderly and critically-ill. However, no previous study was done in younger patients since lactate kinetics in the body may differ among the age groups. The primary aim of our study is to investigate the role of initial venous lactate levels in predicting the chance of severity progression to overt septic shock especially in non-elderly patients who suspected to have acute infections and without hemodynamic shock in ED. The secondary outcomes are to evaluate its prognostic role on hospital length of stay and 30-day mortality of this target population. 