In the paper, the authors describe the evaluation and design phases that led to the definition of a novel GUI (Graphic User Interface) designed for a medical device in biology laboratories, i.e. an analytical device for diagnostic investigations that use the serum agglutination technique on a microplate. In particular, some methods of the Human-Centred Design (HCD) approach were used, which made it possible to evaluate the existing state of the interface to be redesigned (phase 1), define the design requirements of the new GUI (phase 2), and finally evaluate the usability levels of the new interface thanks to a usability test session which involved 17 participants with specific training in the biological field (phase 3). The results highlighted the methodology’s effectiveness and possible replicability for other products and graphic interfaces to be used in the medical field and beyond. Furthermore, thanks to the methodological approach adopted, it was possible to identify the main criticalities of the end users, the areas of intervention, and the relevant present and future design solutions. Fundamentally, the project of medical devices implies the knowledge of human factors, which focus on the interaction between human beings and the system, as an element to minimize the risks of use and ensure a safer and usable medical device.