Validation of the Quick Dementia Rating System (QDRS): diagnostic properties and cutoff values.

UNLABELLED: Dementia underdiagnosis persists because lengthy staging tools hinder routine clinical use. The Quick Dementia Rating System (QDRS) offers a brief, informant-based alternative. OBJECTIVE: This study validates the Brazilian Portuguese QDRS and establishes optimal cutoff values for diagnostic accuracy. METHODS: Individuals from primary care and a secondary memory clinic in Southern Brazil were included. Informants completed the QDRS prior to clinical evaluation. All participants underwent comprehensive neurological and neuropsychological assessment, including the Clinical Dementia Rating (CDR), Mini-Mental State Examination (MMSE), and Montreal Cognitive Assessment (MoCa). Psychometric properties (internal consistency and concurrent validity), diagnostic accuracy, and staging agreement between different diagnostic groups and CDR categories were investigated. RESULTS: A total of 180 participants were included in this study (59 cognitively unimpaired, 63 with mild cognitive impairment [MCI], and 58 with dementia). The QDRS demonstrated excellent internal consistency (Cronbach's α=0.94) and strong concurrent validity with the CDR-Sum of Boxes (p=0.82, p<0.001), MMSE and MoCa scores. Agreement between corresponding QDRS and CDR domains ranged from moderate to good (Intraclass Correlation Coefficient - ICC=0.73-0.89). Diagnostic accuracy for dementia was excellent relative to both clinical diagnosis (cutoff 3.25; Area Under the Curve - AUC=0.98) and CDR staging (cutoff 4.25; AUC=0.97), with sensitivities and specificities exceeding 92%. Staging agreement with the CDR was substantial (weighted Kappa=0.64). CONCLUSION: The Brazilian Portuguese QDRS is a reliable and efficient instrument for dementia screening and staging. Large-scale testing in clinical practice and population-based research are warranted.