Psychiatric diagnosis has become pervasive in modern culture, exerting an increasing influence on notions of personhood, identity practices and forms of self-governing. The broadening of diagnostic categories and increasing awareness regarding popular diagnostic categories has led to an increased demand for formal diagnosis within clinical encounters. However, there is continuing 'epistemological uncertainty' (Fox 2000) surrounding these entities, in part due to their lack of associated clinical biomarkers and their 'fuzzy boundaries'. Meanwhile, this diagnostic expansion has encountered resistance from those concerned with the alleged 'over-pathologisation' of emotional distress. Drawing upon the concepts of 'diagnostic cultures' (Brinkmann 2016) and the 'looping effects of human kinds' (Hacking 1995), this article considers some of the competing forces acting upon the contested boundaries of diagnostic categories as they play out within diagnostic interactions. The study involved ethnographic observations of diagnostic encounters within several UK-based mental health clinics. By focusing on interactions where diagnosis is negotiated, findings illustrate the role played by different kinds of diagnostic uncertainty in shaping these negotiations. It is argued that diagnostic reification plays a key role in the moral categorisation of patients, particularly where there is uncertainty regarding individual diagnostic status. © 2019 The Author. Sociology of Health & Illness published by John Wiley & Sons Ltd on behalf of Foundation for SHIL.