Validity and Minimum Clinically Important Difference of the Danish Insomnia Severity Index for People with Persistent Spinal Pain

Objectives: Sleep difficulties are prevalent in the general population and affect approximately 60–80% of individuals with persistent spinal pain. Valid assessment tools are therefore essential for accurately measuring insomnia symptoms in this population. This study evaluated the validity and minimal clinically important difference (MCID) of the Danish version of the Insomnia Severity Index (ISI-DK) in individuals with persistent spinal pain.
Methods: Data were drawn from an ongoing cohort study of patients referred to the Department of Rheumatology, Aalborg University Hospital. Construct validity was assessed using confirmatory factor analysis (CFA), and internal consistency was evaluated with Cronbach’s alpha. Convergent validity was examined through correlations between ISI-DK scores and related constructs: pain intensity and pain-related disability (Brief Pain Inventory; BPI), pain interference with sleep (single BPI item), number of pain locations (standardized body chart), symptoms of anxiety (Generalized Anxiety Disorder-2), symptoms of depression (Patient Health Questionnaire-2), health-related quality of life (EuroQol 5-Domain 5-Level and EuroQol Visual Analogue Scale), and Work Ability Score. Results: A modified CFA model demonstrated acceptable fit, and internal consistency was high (Cronbach’s alpha = 0.87). Convergent validity was supported by expected positive correlations with pain intensity, sleep interference, number of pain locations, anxiety, and depression, and negative correlations with health-related quality of life and work ability. The MCID was estimated to range from 2.1 to 2.9 points.
Conclusion: These findings support the ISI-DK as a valid and reliable instrument for assessing insomnia in individuals with persistent spinal pain, suitable for detecting clinically meaningful changes in both clinical practice and research settings.