Development and Validation of the Insomnia Daytime Symptoms and Impacts Questionnaire (IDSIQ)

Background and Objective Chronic insomnia has major consequences for daytime functioning, yet no fully validated patient-reported outcome instrument for once-daily assessments is available to measure these consequences. This study describes the development and psychometric evaluation of the Insomnia Daytime Symptoms and Impacts Questionnaire (IDSIQ). Methods The Daytime Insomnia Symptom Scale (DISS), an existing 20-item instrument for assessing daytime functioning, was modified to give an 18-item version of the IDSIQ (IDSIQ-18) based on iterative qualitative interviews with 54 subjects with insomnia and expert input. The construct validity and other psychometric properties of the IDSIQ-18 were analyzed based on an interventional study (NCT03056053) in which subjects with insomnia received zolpidem (5 or 10 mg) daily for 2 weeks and an observational study among subjects with no diagnosis of insomnia (good sleepers). Participants in both studies completed the IDSIQ-18 daily for 2 weeks. Exit interviews were conducted with a sample of subjects who completed the interventional study to elicit concepts defining the experience of insomnia, to assess understanding of the response scales, and to determine meaningful change thresholds. Exploratory factor analysis and Rasch analysis were conducted to further assess the structure and latent model for the scoring of the final IDSIQ instrument. Further psychometric evaluation of the final IDSIQ was then conducted. Results Subjects in both the interventional study (N = 114) and observational study (N = 103) were predominantly female (65% for subjects with insomnia and 60% for good sleepers). Mean age was 51 years for subjects with insomnia and 45 years for good sleepers. Subjects in the exit interviews (N = 41) demonstrated a good understanding of the IDSIQ-18 response scales. Day 1 mean scores were higher (worse) in subjects with insomnia compared with good sleepers. Based on inter-item correlation, exploratory factor, and Rasch analyses and review of the qualitative data, four items were removed. This yielded the final IDSIQ, with 14 items comprising three domains: Alert/Cognition, Mood, and Sleepiness. The domain structure was determined in a confirmatory factor analysis. Evidence of internal consistency reliability was strong: day 1 Cronbach’s alpha was 0.917 for IDSIQ total score and 0.806–0.918 for the domains. Test-retest reliability, assessed for subjects with insomnia with no change on the Patient Global Assessment of Disease Severity scale between day 1 and day 8, was also good (intra-class correlation coefficient 0.856–0.911). Meaningful change thresholds derived for this sample using anchor-based approaches were 20 for IDSIQ total score, 9 for the Alert/Cognition domain, 4 for the Mood domain, and 4 for the Sleepiness domain. Conclusions These studies, which closely followed Food and Drug Administration Guidance for Industry on patient-reported outcome measures, support use of the IDSIQ as a fit-for-purpose measure for deriving valid and reliable endpoints in insomnia clinical research trials and real-world studies.