Background: Depression has major negative consequences for individuals and society, and psychological assessment tools for early disease detection are needed. The aim of this study was to investigate the reliability and validity of an updated Japanese version of the Children's Depression Inventory (CDI-J) and set a cut-off score for the detection of depression. Methods: The participants consisted of 465 children and adolescents aged 7–17 years. The control (CON) groups consisted of students recruited from elementary and junior-high school (CONEJ) and children recruited from among hospital staff members (CONRE), while the outpatient clinical (OPC) groups consisted of pediatric psychosomatic outpatients (OPCPD) and adolescent psychiatric outpatients (OPCPS). The CON and OPC CDI-J scores underwent factor analysis using varimax rotation, followed by measurement invariance analysis. The Youth Self-Report (YSR) was administered to assess concurrent validity. The Mini-International Neuropsychiatric Interview was administered to the OPC group to diagnose current depressive symptoms. Receiver operating characteristics (ROC) analysis was conducted to evaluate case-finding performance and to set cut-off points for the detection of depression. Results: The CDI-J was reliable in terms of internal consistency (Cronbach α = 0.86; mean inter-item correlation, 0.16). Re-test reliability was substantial (mean interval 18 days: γ = 0.59, P < 0.05). The four-factor solution exhibited adequate internal consistency (range, 0.52–0.73) and correspondence (Pearson correlation of 0.65 with the YSR) for both the CON and OPC groups. On ROC analysis the optimal cut-off score was 23/24. Conclusion: The CDI-J can be used as a reliable and well-validated instrument alongside standard diagnostic procedures.
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