This article is concerned with the in situ negotiation of epistemic authority through the mobilisation of summaries in the context of medical emergencies. It focuses on the use of the discourse marker so in prefacing summaries as a strategy for claiming epistemic primacy, with particular attention being paid to so's multimodal accomplishment. Taking an interactional sociolinguistic approach, we bring together simulated and real-life trauma emergencies and zoom in on the role of the institutionally defined team leader.Our findings illustrate that summarising in this context is almost exclusively reserved for team leaders, with so being an integral part in this process, consistently prefacing team leaders’ summaries. These summary acts, in turn, contribute to the summarisation process, which is part and parcel of role performance and doing leadership. We unpack the spatiotemporal dimensions of so, highlighting the systematicity of its use in the material space of the emergency room, and make a case for the need to capture discourse markers in the situated spatiolinguistic context.