Keywords ESSENCE, disease surveillance, system architecture Introduction VA ESSENCE analyzes ICD-9 diagnosis codes and demographic data from outpatient and emergency department (ED) visits using complex aberrancy-detection algorithms (1). In 2010, a new instance was stood up (VA Inpatient ESSENCE), which receives weekly feeds of inpatient data from all VA acute care hospitals starting October 1, 2009. Data include demographics, admission/ discharge data (including ICD-9 diagnosis codes), diagnosisrelated group (DRG), bedsection, procedure and surgery data.