["Sociology of Health & Illness, EarlyView. ", "\nAbstract\nDiagnosis is pivotal to medicine's epistemic system: it serves to explain individual symptoms, classify them into recognizable conditions and determine their prognosis and treatment. Medical tests, or investigative procedures for detecting and monitoring disease, play a central role in diagnosis. While testing promises diagnostic certainty or a definitive risk assessment, it often produces uncertainties and new questions which call for yet further tests. In short, testing, regardless of its specific application, is imbued with meaning and emotionally fraught. In this article, we explore individuals' ambivalent experiences of testing as they search for diagnostic certainty, and the anxieties and frustrations of those for whom it remains elusive. Combining insights from sociological work on ambivalence and the biopolitics of health, and drawing on qualitative interviews with Australian healthcare recipients who have undergone testing in the context of clinical practice, we argue that these experiences are explicable in light of the contradictory impulses and tensions associated with what we term ‘bio‐subjectification’. We consider the implications of our analysis in light of the development of new tests that produce ever finer delineations between healthy and diseased populations, concluding that their use will likely multiply uncertainties and heighten rather than lessen anxieties.\n"]