Preliminary validation of the insomnia severity index in Danish outpatients with a medical condition

Abstract

Purpose: Insomnia is a frequent sleeping disorder in the general and clinical population. With an increasing proportion of health care services being provided as outpatient care, a short, valid and reliable tool is needed to identify insomnia in medical patients under outpatient care in Denmark. The Insomnia Severity Index (ISI) could be the needed tool if found valid and reliable. Hence, the aim of this study is to evaluate elements of the psychometric properties of the Danish version of ISI (ISI-DK). Methods: Outpatients from three hospital wards and one rehabilitation center were asked to complete the ISI-DK twice, 2 weeks apart. Internal consistency, discriminative validity, test-retest reliability, and measurement error was assessed.Results: The ISI-DK was completed by 249 (79.0%) participants the first time, and 163 (65.5%) the second time. Respondents had a mean age of 58.2 years (SD 13.5) and 63.5% were women. All but one of the discriminative hypotheses was accepted. Internal consistency was high in the global scale at 0.90 and good with Cronbach's alpha at 0.75-0.88 in the proposed subscales. The test-retest reliability was good, as the intraclass correlation was 0.90 (95% CI: 0.87; 0.93). Ceiling and floor effects were low < 4.4%. Standard error of measurement was 2.52 and smallest detectable change 6.99. Conclusion: This preliminary assessment showed encouraging results supporting the ISI-DK as a valid and reliable tool for screening insomnia severity in Danish outpatients with a medical condition, but further assessments are needed.

Measuring Subjective Sleep Quality: A Review

“…Importantly, the testretest reliability was significant in clinical and nonclinical populations [56]. In general, test-retest reliability after 2 weeks was satisfactory (mean ICC = 0.82) [47,56,61] and the ordinal alpha remained above the critical value of 0.70 after a CBT-I treatment [63]. The ISI exhibited significant correlations with several sleep questionnaires such as AIS and PSQI (but low correlation coefficients with ESS) and with different psychological, health, and psychopathological questionnaires.…”

“…In addition, Castronovo et al [63] reported how the first three items of the ISI were associated with quantitative estimates of sleep parameters (Table 2) obtained from the sleep diaries with moderate correlations, supporting the premise that the first three items have a diagnostic role. As regards validity, the studies demonstrated known-group differences on the basis of different criteria, such as PSQI or depression [47,58,59,61]. In one study [55] women had a higher ISI score than men, and in another study [54] sex was a predictive factor of ISI score, but this gender effect was not systematically confirmed [54,55,61].…”

“…This solution generally explained 61.80% of variance with mean factor loadings ranging from 0.50 to 0.90 for both factors and mean inter-factor correlation of 0.50 (Table 2). In medical patients, Dieperink et al [61] considered the 3-factor model, in which Severity of Nighttime Sleep Difficulties (items 1-3 with factor loadings > 0.59), Impact of Insomnia (items 5-7 with factor loadings > 0.72), and Sleep Dissatisfaction/Satisfaction (items 1, 4, 7 with factor loadings > 0.36) correlated with each other with values greater than 0.80. This solution was confirmed in other studies [56,62,63] (Table 2), even if Dieck et al [56] reported a different composition of factors (F1: items 2, 4, 7; F2: item 1; F3: items 5, 7) and a single correlation between F1 and F3 (0.794).…”

“…As regards validity, the studies demonstrated known-group differences on the basis of different criteria, such as PSQI or depression [47,58,59,61]. In one study [55] women had a higher ISI score than men, and in another study [54] sex was a predictive factor of ISI score, but this gender effect was not systematically confirmed [54,55,61]. Importantly, Castronovo et al [63] found that ISI was sensitive to change after a specific CBT-I treatment, with a reduction of the higher scores in each item.…”

Sleep Measurements in Women

“…Some studies find higher ISI scores in women, 87,88 but others have failed to find consistent gender differences. 89,90 Factorial invariance across genders have been found reporting solutions with different number of factors 91,92 (ie, the same factor structure is found in men and women, so items that tend to correlate strongly with each other in men also tend to correlate strongly in women and vice versa).…”