This article considers the experiences of health consumers who have undergone testing for human immunodeficiency virus antibodies, T cells, and viral load. These HIV-related tests are deployed for the purposes of making definitive diagnoses; yet some test consumers experience ambiguous outcomes. Drawing on an analysis of differing end-user experiences of these tests, where consumers' knowledge reflected the multiplicity and heterogeneity in test design, the author explores how these experiences reflect particular knowledges about these tests. The article contributes to efforts analyzing (...) how health consumers are active end users co-constructing the social meaning of technologies in mutual relationship with other users. The author discusses how this new knowledge can be used to delineate a greater role for consumer evaluation of medical testing within a broader understanding of test design and performance. Relevant links are made to issues such as genetic testing and assessing claims about the efficacy of medical tests. (shrink)