This article follows the introduction of COVID-19 polymerase chain reaction (PCR) diagnostic tests in France. It shows how, at the intersection of science, medicine, politics, and policy-making, the test, trace, and isolate (TTI) strategy played out during the first months of the pandemic against a backcloth of multiple shortages. In so doing, the authors move beyond trite explanations (such as ‘French public health’s backwardness’) to highlight how successive policy inflections affected the national response to the pandemic. The piece analyses the shifting French political discourse surrounding (scarce) COVID-19 tests while exploring ad-hoc regulations and guidelines as well as the intense ‘bricolage’ that they triggered in the field of clinical medicine. The authors contend that the limitations of the testing infrastructure in France during the first half of 2020 shaped the decision to resort to lockdown. The research article sheds light on two coexisting registers of professional uses of reverse transcription-polymerase chain reaction (RT-PCR) assays—a ‘public health use’ and a ‘clinical use’—and highlights the changing political and social relevance of these two registers, with scarcity as a major determinant of these changes. One of the striking aspects of the introduction of COVID-19 tests in France therefore lies in the enduring gap between the dynamics of the epidemic and the dynamics of testing. In this respect, the French situation is neither extreme nor unique, which makes this case study a relevant basis for the international comparison of testing practices in different phases of the COVID-19 pandemic.