BACKGROUND: Overcrowding is a common problem in emergency departments. This is true for adult and pediatric emergency department (PED) and issues are potentially important (e.g. quality of care, financial, social, ethical). Optimum is one among several solutions implemented to fight this phenomenon. It is an electronic patient prioritization tool for PED devoted to non-vital emergencies. First usage assessments reported the tool was not used by the PED staffs despite their strong involvement during the development. OBJECTIVE: This paper aims at understanding why the PED staff did not use the Optimum system that has been designed with them and for them, through a user-centered design process. METHODS: PED staffs answered answer a short survey about their usage of Optimum. Depending on their answer (user vs. non-user), they either underwent an individual semi-structured interview or an unstructured one. Interviews were audio-recorded and transcribed and, from each interview, meaningful semantic units representing the reasons for using/non-using Optimum were extracted and organized iteratively following a grounded approach by three ergonomics experts till a consensus was reached. RESULTS: 12 interviews have been performed with 6 physicians, 5 nurses and 1 auxiliary nurse. Overall, the prioritization tool Optimum have received a mixed response from the PED staff: Optimum display is neither understood nor trusted by users. Moreover, it is mainly used to estimate the PED attendance rate and not to prioritize patients. CONCLUSIONS: This study shows how much it is difficult to implement new tool in wards despite a user-centered development and without being included in the daily used patient management tool.