In this article, we show how a particular biomarker comes into being in an emergency department in a hospital in Copenhagen, Denmark. We explore the contextual becoming of this biomarker, suPAR, through interviews with nurses and physicians and through relational ontology. We find that as a prognostic biomarker suPAR is challenged in it becoming as an object for clinical practice in the emergency department by the power of diagnostic practices and the desire for experience-based scripts that quickly enable the clinician to reach the right diagnosis. Although suPAR is enacted as a promising triage strategy suggesting a low or high risk of disease, the inability to rule out specific diagnoses and producing the notion of secure clinical actions make its non-specificity and prognostic character problematic in clinical practices. Specific diagnostic criteria versus prognostic interpretation and non-specificity risk profiling challenges the way healthcare workers in an emergency department understand the tasks they are set to solve and how to solve them. We discuss how the becoming of suPAR is strengthened through enactments of specificity and engagement in triage strategies and we reflect on it’s becoming through new diagnostic practices with the need to accommodate diagnostic ambiguity.