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Outcomes and outcome measurement instruments1295these patient populations, using state of the art classifications. However, when conducting this review, we faced some challenges. We initially aimed to cluster the outcomes and outcome measurement instruments according to the Valderas & Alonso model.3 Domain assessment has rather been an indication, as instruments should ideally be assessed per subscale (unidimensional), which was not feasible due to the enormous amount of different outcome measurement instruments. Furthermore, the terminology used for the outcomes and outcome measurement instruments was often unclear, lacking, or inconsistently reported. We clustered reported outcomes based on frequency of used terminology, and thus do not refer to a standardized terminology. Finally, we cannot recommend specific outcome measurement instruments, since psychometric properties were not investigated in this review. ConclusionThis review provides insight into the large number of outcomes and outcome measurement instruments reported in clinical trials for GNDs and ID. The abundancy of available tools is problematic from an efficiency and generalizability perspective, highlighting the need for a more universal approach to the selection of outcomes and instruments. Moving forward, further collaborative efforts are recommended to achieve consensus on outcome selection. The output of this review may serve as a starting point for discussion about relevant outcomes and instruments in GNDs and ID, and to develop a core outcome set for these populations. Preferably, it will be applicable for care as well as research purposes with possible implications for market authorization and reimbursement of (orphan) drugs to improve patient-centered care by measuring what matters to affected individuals.FundingThis work was supported financially by Stichting Wetenschappelijk Onderzoek ‘s Heeren Loo. The funder had no role in the design and conduct of the study, preparation of the review, or approval of the manuscript.Annelieke Muller sHL.indd 129 14-11-2023 09:07