Feasibility and acceptability research for HIV self-testing (HIVST) often emphasises the importance of good test conduct and correct test interpretation for knowing one’s HIV result while overlooking the ways in which different uncertainties and meanings emerge around testing. Using empirical examples from a quantitative study assessing an app-based strategy in Cape Town, South Africa, this research article explores the practice of HIVST and how people deal with uncertainties while using the app in question, named ‘HIVSmart!’. We use the concept of ‘living under’ to explore the practices of HIV testing for those who fit the definition of being ‘at risk’ of HIV (note that an individual’s HIV status must be unknown in order for them to fit this definition) and to understand how an app-based HIVST strategy fits within these practices. We show how the app and oral self-test—as well as knowledge around HIV risk behaviours, comparisons between different testing methods, and the guidance and presence of healthcare staff—alleviate as well as generate uncertainty and constitute HIV status as an ongoing process. The effective implementation of new strategies for HIVST requires consideration of multiple aspects of the testing process, including local understandings of HIV risk, access to healthcare staff, and the meaning of certain test methods within a particular context.