This article interrogates the relationship between the development of national diagnostic technologies and the exercise of sovereignty, by analysing a Brazilian project to produce a nucleic acid test (NAT) for the country’s blood screening programme. The concept of ‘molecular sovereignty’ is proposed to demonstrate that exercising sovereignty demands not only technological resources but also a sufficiently powerful and national imaginary to support local knowledge production as a means of advancing national healthcare priorities. First, this research article contextualises the political importance of blood safety for Brazil during its transition to democracy in the 1980s and the creation of its universal healthcare system. Then, it investigates how adopting the NAT led the state to invest in the production of a national technology. Third, the article unpacks the diagnostic test to consider how certain aspects of the project might ultimately strengthen the ability of global capital to cross national boundaries and create new markets. Lastly, it discusses how the project ended up creating a centralised and ‘closed’ system to avoid leaving the country vulnerable to the entry of global diagnostic companies. This case demonstrates how the molecularisation of blood, through the construction of a unified healthcare system driven by the constitutional right to health, can be deployed to construct imagined communities on the scale of a nation.