Background
Physiologic instability (PI) is a common, critical problem in the emergency department (ED) [1, 2], and can have different underlying causes. The ability to determine the underlying cause of instability is paramount for early treatment and risk stratification [3]. Lactate has been shown to have prognostic value in some categories of unstable patients [4, 5]. The objective of this study was to investigate how serum lactate concentrations differ across categories of PI and the association of lactate concentrations with clinical deterioration for each category.