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Background: The Quick Dementia Rating System (QDRS) is a brief and rapid tool that can be administered by an informant without the need for a trained assessor., Objective: Our objective was to examine the validity, reliability, and cost-effectiveness of the informant QDRS in a Singapore memory clinic sample., Methods: We assessed a total of 177 older adults, among whom, 32 had no cognitive impairment (NCI), 61 had mild cognitive impairment (MCI), and 84 had dementia. Elderly underwent 1) the informant QDRS, 2) the Clinical Dementia Rating (CDR) as the gold standard diagnosis, 3) the Mini-Mental State Examination (MMSE), and 4) the Ascertain Dementia 8 (AD8) as comparisons to the QDRS. The extent to which the QDRS may reduce the recruitment cost (time) of clinical trials was also calculated., Results: The QDRS had excellent internal consistency (Cronbach alpha = 0.939). It correlated highly with the CDR-global (R = 0.897), CDR Sum-of-Boxes (R = 0.915), MMSE (R = -0.848), and the AD8 (R = 0.747), showing good concurrent validity. With an optimal cut-off of 1.5 for MCI (sensitivity 85.2%, specificity 96.3%) and 6 for dementia (sensitivity 90.1%, specificity 89.2%), the QDRS achieved a higher overall accuracy of 85.0%, as compared to MMSE (71.2%) and AD8 (73.4%). A simulated clinical trial recruitment scenario demonstrated that pre-screening with the QDRS followed by a confirmatory CDR would reduce the time needed to identify NCI subjects by 23.3% and MCI subjects by 75.3%., Conclusion: The QDRS is a reliable cognitive impairment screening tool which is suitable for informant-administration, especially for identification of MCI.