This article draws on ethnographic data from a Norwegian emergency primary care clinic (EPCC) to explore nurses’ discretionary application of guidelines. Specifically, it analyses nurses’ discretionary use of the Manchester Triage System (MTS) when performing face‐to‐face triage, that is, assessing the urgency of patients’ complaints. The analysis shows how nurses assessed patients at odds with MTS prescriptions by collecting supplementary data, engaging in differential diagnostic and holistic reasoning, relying on emotion and intuition, and allowing colleagues and patients to influence their reasoning. The findings also show how nurses’ reasoning led them to override guidelines both overtly and covertly. Based on this evidence, it is argued that nurses’ assessments relied more on internalised ‘triage mindlines’ than on codified triage guidelines, although the MTS did function as a support system, checklist and system for supervisory control. The study complements existing research on standardisation in nursing by providing an in‐depth analysis of nurses’ methods for navigating guidelines and by detailing how deviations from those guidelines spring from their clinical reasoning. The challenges of imposing a managerial logic on professional labour are also highlighted, which is of particular relevance in light of the drive towards standardisation in modern healthcare.