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Objective: Apathy is a prominent syndrome across neurodegenerative diseases. The Dimensional Apathy Scale (DAS) assesses three apathy subtypes—executive, emotional, and initiation—and is sensitive and valid in amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and Parkinson's disease. This study describes the development of the brief DAS (b-DAS), which will enable apathy to be swiftly detected in the clinic. Method: 102 ALS and 102 AD patients' previously collected data were used. Mokken analyses were performed on item-level data of each informant/carer-rated DAS subscale (executive, emotional, and initiation) for the initial scale reduction. Item-total correlational analyses against standard apathy (convergent validity criteria) and depression (divergent validity criteria) measures and qualitative examination of items aided final item selection. Receiver operating curve analysis determined optimal cutoffs for the reduced subscales. Results: Mokken analyses suggested unidimensionality of each DAS subscale. Three items were removed that failed to satisfy monotone homogeneity model requirements, three items were removed due to validity criteria not being met, and six items were removed due to a combination of lower item scalability and item-total correlations. Item-theme examination further reduced the b-DAS to nine items, three per subscale, with a supplemental awareness deficit assessment being added. Sensitivity- and specificity-based optimal cutoffs were calculated for each b-DAS subscale. Conclusions: This study presents the b-DAS, an informant/carer-based robust yet short multidimensional apathy instrument with good convergent and divergent validity, with recommended clinical cutoffs. The b-DAS is appropriate for use in the clinic and for research to quickly and comprehensively screen for apathy subtype impairments. [ABSTRACT FROM AUTHOR]