Background: Psychological distress is prevalent worldwide and can lead to the development of mental conditions such as major depression and anxiety disorders. It is essential to assess the severity of patient-reported outcomes to provide effective treatment for psychological distress. The Kessler Psychological Distress Scale (K6) is one of the most widely used psychological distress scales. However, the smallest detectable change (SDC) of the K6 score has not been elucidated. Therefore, the current study aimed to determine the SDC of the K6 score in a Japanese adult population. Methods: Participants aged 20-59 years who are native Japanese speakers were recruited from the panel list of a web research firm. The K6 score was assessed at baseline (T1) and at 2-week follow-up (T2). SDCs were calculated at the individual (SDCind) and group (SDCgroup) levels. Intraclass correlation coefficient agreement (ICCagreement) was calculated to assess test-retest reliability and Cronbach's alpha to evaluate internal consistency. Results: A total of 3254 (1627 [50%] female) responded at T1 and T2. The mean (standard deviation) K6 scores were 5.71 (5.84) at T1 and 5.65 (5.83) at T2. The SDCind and SDCgroup of the K6 score were 8.47 (35.31%) and 0.15 (0.63%), respectively. The ICCagreement was 0.73, and the Cronbach's alpha was 0.94. Conclusion: Our study provided evidence on the reliability and interpretation of the K6 score. Calculating the SDC of the K6 score can help identify the significance of changes in psychological distress over time and can determine the efficacy of interventions for psychological distress.
