Objectives: Chronic insomnia and depression are often associated. Measuring the impact on quality of life associated with changes in sleep in co-treatment of insomnia and depression requires a valid and reliable patient reported outcome (PRO) instrument. This study aimed to assess the validity of the Sleep Impact Scale (SIS), a sleep-specific PRO instrument, in a population comorbid with Major Depressive Disorder (MDD) and insomnia to support its use in clinical or clinical trial applications.
Research design and methods: Data from 379 subjects enrolled in a 27 week US, multi-center, phase IV, randomized, double-blind, parallel group, placebo-controlled trial of zolpidem tartrate extended-release taken in combination with escitalopram vs. placebo combined with escitalopram were pooled across treatment groups. Results from multi-trait analyses, tests of internal consistency and test-retest reliability, concurrent validity, known-groups validity, responsiveness, and thresholds for minimal important difference (MID) were examined.
Results: Mean baseline scores on the SIS ranged from 22.85 (+/-13.41) on Satisfaction with Sleep to 43.49 (+/-21.12) on Mental Fatigue, reflecting impairments due to sleep problems. The SIS was found to be internally consistent (alpha > or = 0.70 for all domains) and have good construct validity. The item-domain correlations were > or = 0.52 with no instance of an item correlating more highly with a domain other than its own. There were some floor and no ceiling effects. The test-retest reliability of the SIS domains ranged between 0.68 and 0.83. Clinical validity assessed through known groups methods was supported. The SIS was responsive to changes on all domains. Preliminary estimates of minimum important difference (MID) were obtained to interpret changes in SIS domains.
Limitations: Limitations include the need for further qualitative research on content validity and the lack of a patient global assessment of change.
Conclusions: This study yielded adequate evidence of the validity of the SIS for use in clinical trials and research on MDD patients with comorbid insomnia.