In prevention and health promotion interventions, screening methods and risk profile assessments are often used as tools for establishing the interventions’ effectiveness, for the selection and determination of the health status of participants. The role these instruments fulfil in the creation of effectiveness and the effects these instruments have themselves remain unexplored. In this paper, we have analysed the role screening methods and risk profile assessments fulfil as part of prevention and health promotion programmes in the selection, enrolment and participation of participants. Our analysis showed, that screening methods and health risk assessments create effects as they objectify health risks and/or the health status of individuals, i.e., they select the individuals ‘at risk’ and indicate the lifestyle modifications these people are required to make in order to improve their health. Yet, these instruments also reduce the group of participants thereby decreasing the possible effect of interventions, as they provide the legitimisation for people to make choices to whether they enrol or not and what lifestyle changes they incorporate into their lives. In other words, they present a space of interaction, in which agency is distributed across the practice nurses, the participants and the instruments. Decisions were not just made upon the projection of the outcomes of these instruments; decisions that were made by both the patients and practice nurses were the resultant of their opinions on these outcomes that were formed in interaction with the instruments.