The Swedish version of the Insomnia Severity Index: Factor structure analysis and psychometric properties in chronic pain patients

Abstract

Objective Insomnia is the most commonly diagnosed comorbidity disorder among patients with chronic pain. This circumstance requests brief and valid instruments for screening insomnia in epidemiological studies. The main object of this study was to assess the psychometric properties and factor structure of the Swedish version of the Insomnia Severity Index (ISI). The ISI is a short instrument designed to measure clinical insomnia and one of the most common used scales both in clinical and research practice. However there is no study in Sweden that guarantees neither its factor structure nor its feasibility in chronic pain patients. We further examined the measurement invariance property of the ISI across the two sexes. Methods The ISI was administered to 836 (269 men and 567 women) chronic pain patients from the Swedish Quality Registry for Pain Rehabilitation. This study used demographic data, the Hospital Anxiety and Depression Scale (HADS), the Mental Summary Component (MSC) of the Health Survey (SF-36) and the item 7 from Multidimensional Pain Inventory (MPI). The sample was divided into two random halves: exploratory factor analysis (EFA) was performed in the first sample (N1 = 334, 40%) and confirmatory factor analysis (CFA) in the second half of the sample (N2 = 502, 60%). The measurement and structural invariance of the proposed structure (4-item version) between the two sexes as well as reliability and validity indexes were further assessed. Results Exploratory factor analysis using the principal axis factoring method generated one global factor structure for the ISI, explaining 63.1% of the total variance. The one factor solution was stable between the two sexes. Principal component analysis was also applied and indicated almost identical results. The structure was further assessed by CFA, resulting in an adequate fit only after omitting three items. The difference on structural and measurement invariance in the loadings by participants' sex was not significant (Δχ2 = 10.6; df = 3; p = .69 and Δχ2 = 2.86; df = 3; p = 41 respectively). The shorter version four-item Insomnia Severity Index (ISI-4) was analysed further. The Chronbach's alpha for the global ISI-4 score was 0.88. The construct validity of the ISI-4 was also supported by the, Hospital Anxiety and Depression Scale, the Mental Summary Component of quality of life and quality of sleep data. Pain intensity was significantly associated with the ISI-4 score (beta = .29, p < 001) whereas no significant correlation between four-item Insomnia Severity Index score and age was observed (p > 05). Conclusions and implications Although short, the four-item Insomnia Severity Index (ISI-4) version seemed to effectively assess insomnia in chronic pain patients. An important clinical implication is that the four-item Swedish Insomnia Severity Index can be used in chronic pain cohorts when screening for insomnia problems. Its measurement and structural invariance property across the two sexes shows that the ISI-4 is a valid measure of the insomnia across groups ...

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

“…Before conducting the CFA, data suitability was tested by the Kaiser-Meyer-Olkin (KMO) test with a limit of > 0.60 and by Bartlett's test of sphericity with a significance limit of p ≤ 0.05 [26]. The original article on ISI did not present a factor analysis, so the CFA was compared with previously tested models, with one [17,27], two [28], and three [10,13,29] factors. The goodness of fit of each model was assessed by the fit indices; Root Mean Square Error of Approximation (RMSEA) value < 0.08, and Comparative Fit Index (CFI) > 0.90, Tucker-Lewis fit Index (TLI) > 0.95, and Standardized Root Mean Square Residual (SRMR value < 0.08 [30].…”

“…The ISI is a brief seven item selfrated instrument, increasingly used to assess insomnia based on criteria from the International Classification of Sleep Disorders. The ISI has been translated into multiple languages, validated in 12 countries and as a webbased measurement [7][8][9][10][11][12][13][14][15][16][17][18]. In comparison to other PRO sleep measures, the ISI has diagnostic properties [19], and can be completed in a few minutes [8], diminishing the response burden.…”

Assessing insomnia in pain – Can short be good?

“…The suggested ISI-4 short sleep scale investigated in the study by Dragioti and co-authors seems to be a feasible and at least very promising tool in screening for insomnia in a clinical examination of chronic pain patients [1]. Moreover, it may prove to be a suitable instrument in assessing the severity of insomnia in epidemiological and clinical research.…”

“…The article by Dragioti and co-authors assessed the psychometric properties of the Swedish version of the ISI among 269 men and 567 women who were suffering from chronic pain and were referred to the pain and rehabilitation centre of Linköping University Hospital [1]. The factor structure was analyzed using explanatory factor analysis (EFA) and confirmatory factor analysis (CFA).…”

“…In the present issue of the Scandinavian Journal of Pain, Dragioti and co-authors report on the factor structure and psychometric properties of the 7-Item Insomnia Severity Index (ISI) with patients of a pain clinic [1]. They suggest that an even shorter 4-item version of the Insomnia Severity Index could effectively evaluate insomnia in chronic pain patients.…”