Abstract
Background: Adults with Down syndrome (DS) have a high dementia risk, highlighting the need for robust, DS-specific assessment tools. This study evaluated the psychometric properties of the Norwegian version of the CAMDEX-DS-II by examining reliability and validity of the informant interview and the CAMCOG-DS-II cognitive assessment.

Method: In this nationwide study, 108 adults with DS were assessed across 19 hospital units during 2021-2023. Participants underwent a standardised dementia assessment including the CAMDEX-DS-II battery. Reliability was assessed using Cronbach’s alpha, weighted kappa, and intraclass correlation coefficients (ICCs), while validity was evaluated using factor analysis, receiver operating characteristic (ROC) analyses, and external cognitive and functional measures.

Results: The CAMDEX-DS-II informant interview demonstrated good to excellent psychometric properties, with high internal consistency (α ≥ 0.83) in core cognitive-functional sections and strong inter-rater reliability, with most items showing excellent weighted kappa (κ ≥ 0.80). Scores aligned closely with clinician-determined diagnostic classifications. The CAMCOG-DS-II showed very good internal consistency (α = 0.84) and excellent inter-rater reliability (ICCs ≥ 0.90). CAMCOG-DS-II total and domain scores differed significantly across diagnostic groups, with moderate-to-large effect sizes. ROC analyses indicated good overall diagnostic accuracy, with areas under the curve (AUCs) > 0.80, and particularly strong discrimination in individuals with mild ID.

Conclusions: The Norwegian CAMDEX-DS-II provides reliable indicators of dementia-related change in adults with DS. The combined informant interview and cognitive assessment provided evidence based on relations to diagnostic classification and external measures, supporting their clinical utility in the specialist services and contributing to the international evidence base.

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