The transfer of a patient's medical data between healthcare professionals is sometimes needed to take relevant aspects of the patient's health into account to offer fitting care for the specific patient. A patient's medical data is stored in the systems of the different healthcare professionals that treat or have treated the patient, thus explaining the need for transfer of medical data. In the Netherlands, it is required by law that the patient gives permission for the transfer both to the healthcare professional requesting the medical data and the one providing it before transfer can take place. The law went into effect partially on July 1st, 2017, and has to be fully implemented in three years' time. However, the transfer infrastructure is still in development, resulting in varying and even conflicting communication about the system. This makes it difficult for patients to understand and make the decision about access to their medical data. The aim of this thesis is to facilitate and support patients in making this decision. The patients' decision is influenced by numerous factors, such as their health, their previous experiences with healthcare, their values and their preferences regarding their medical data. The patients' perspective is therefore the focus of this thesis. The research question answered in this thesis is 'How can patients make an informed decision that fits their wishes regarding the accessibility of their medical data?' Contextmapping techniques such as sensitizing packages and personas helped revealing the patients' wishes regarding their medical data and their current experiences with (the transfer of) medical data. Through literature research, decision-making in healthcare was explored, leading to the identification of a combination of informed consent and shared decision making as a fitting approach for communication about the patient's decision.

Following the analysis, a solution was designed, consisting of necessary steps for creating the right context to enable patients to make a decision that fits their wishes about access to their medical data. This included a patient decision aid to concretely facilitate and support the patient during the decision-making process. Previous to consultation, the decision aid prepares patients for a conversation with the healthcare professional. It informs patients about the subject, makes them aware of the fact that the choice is theirs, and encourages them to think about their personal values and preferences. The decision aid enables patients to fit the decision about access to their medical data to their health situations and personal preferences. The design was then evaluated by discussing the patient decision aid with experts from practice, by testing the tool with patients and by using patient decision aid criteria identified in literature.