Insomnia Severity Index (ISI): Psychometric properties and diagnostic performance in Romanian population

Insomnia Severity Index (ISI) is one of the most widely used screening instruments for chronic insomnia, yet limitations in prior studies leave aspects of its psychometric properties incomplete. This study aimed to validate the ISI and investigate its screening utility in a clinical and nonclinical Romanian sample (N = 326; M_age = 25.84). Results indicate good internal consistency (α_T1 = .84, α_T2 = .86) and good test-rest reliability (r = .82). ISI significantly correlates with depression (r = .69), anxiety (r = .52), self-esteem (r = -.42), and quality of sleep (r = .75), supporting its concurrent and convergent validity. Regression analyses confirm its predictive validity, based on neurologists' (β = .39) and clinicians' (β = .43) diagnostic ratings of chronic insomnia. A cutoff score of 13 is optimal for a good balance between sensitivity (.79) and specificity (.68). Confirmatory factor analysis supports a two-factor model. Moreover, measurement invariance analyses show that ISI functions equivalently across the clinical and nonclinical groups. Overall, ISI scores provide reliable and valid information for screening chronic insomnia in patients with or without other psychiatric and/or medical conditions. (PsycInfo Database Record (c) 2026 APA, all rights reserved).