Journal of Medical Internet Research - International Scientific Journal for Medical Research, Information and Communication on the Internet #Preprint #PeerReviewMe: Warning: This is a unreviewed preprint. Readers are warned that the document has not been peer-reviewed by expert/patient reviewers or an academic editor, may contain misleading claims, and is likely to undergo changes before final publication, if accepted, or may have been rejected/withdrawn. Readers with interest and expertise are encouraged to sign up as peer-reviewer, if the paper is within an open peer-review period. Please cite this preprint only for review purposes or for grant applications and CVs (if you are the author). Background: As a result of advances in diagnostic testing in the field of Alzheimer’s Disease (AD), patients are diagnosed in earlier stages of the disease; for example in the stage of mild cognitive impairment (MCI). This poses novel challenges for the clinician during the diagnostic work-up with regard to diagnostic testing itself (which tests), but also how to engage patients in this decision and how to communicate test results. To this background, tools to support decision making and improve risk communication could be valuable for clinicians and patients. Objective: To describe the development of an online tool for clinicians in a memory clinic setting, that: 1) facilitates the interpretation of biomarker results in individual patients with MCI regarding their risk of progression to dementia, 2) supports clinicians in communicating biomarker test results and risks to MCI patients and their caregivers, and 3) supports clinicians in a process of shared decision-making regarding the diagnostic work-up of AD. Methods: A multi-phase, mixed-methods approach was used: phase 1) qualitative needs assessment among professionals, patients and caregivers, phase 2) iterative process of development and design of the tool (ADappt), and phase 3) quantitative and qualitative assessment of usability and acceptability of ADappt. Across these phases, co-creation was realized via a user-centered, qualitative approach with clinicians, patients and caregivers. Results: In phase 1, clinicians indicated the need for risk calculation tools and visual aids to communicate test results to patients. Patients and caregivers expressed their needs for more specific information on their risk for developing AD and related consequences. In phase 2, we developed the content and graphical design of ADappt encompassing three modules: a risk calculation tool, a risk communication tool including a summary sheet for patients and caregivers, and a conversation starter to support shared decision making regarding the diagnostic work-up. In phase 3, ADappt was considered to be clear and user friendly. Conclusions: This study presents ADappt, an online tool, developed in a multi-phase design characterized by co-creation, that supports clinicians in a memory clinic setting with individually tailored interpretation of biomarker test results, communication of test results and risks to patients and their caregivers, and shared decision making on diagnostic testing.