Original articleValidation of the Insomnia Severity Index as an outcome measure for insomnia research
Introduction
Insomnia is a frequent complaint brought to the physician's attention. Its prevalence in the general population ranges from 9% for persistent sleep disturbances to 27% for occasional insomnia [1], [2]. Valid instruments are needed to assist health care practitioners in the assessment of insomnia complaints. Although polysomnography is the ‘gold standard’ for assessing sleep disorders such as sleep apnea, it is not recommended for routine use in the clinical assessment of insomnia [3]. Furthermore, it is not readily available to most clinicians. Structured and semi-structured clinical interviews [4], [5] are excellent methods to obtain systematic information on the nature, history, and severity of sleep difficulties. Although they are essential for a thorough examination of insomnia, clinical interviews are time-consuming and may not be practical for routine clinical use. In addition, clinical interviews require excellent knowledge about sleep disorders, which is not always the case for all general health-care practitioners.
Brief and efficient clinical instruments are needed for assessing the severity of insomnia. Although numerous self-report measures have been developed for the evaluation of insomnia [6], very few have been validated specifically as screening or outcome measure for insomnia. The Pittsburgh Sleep Quality Index (PSQI) [7] is a reliable and valid instrument assessing sleep quality and disturbances over a 1-month time interval; although it discriminates well poor from good sleepers, and is an excellent general screening measure of sleep disturbances, this instrument was not specifically designed for the assessment of insomnia. Completion of a daily sleep diary has become a standard assessment procedure in insomnia treatment research; it is a very practical and cost-effective method for assessing insomnia when repeated measurements are needed [8]. Daily morning estimates of sleep parameters yield a reliable and valid index of insomnia even though they do not reflect absolute values obtained from polysomnography [9], [10]. Although quite useful and easy to administer, neither of those instruments (PSQI and sleep diary) completely capture the diagnostic criteria for insomnia outlined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) [11] or in the International Classification of Sleep Disorders (ICSD) [12]. Although these measures provide subjective estimates of sleep disturbances, they do not target the degree of impairment and emotional distress associated with insomnia.
The Insomnia Severity Index (ISI) (Fig. 1) [4], is a brief self-report instrument measuring the patient's perception of his or her insomnia (see description in the Section 2.1). The ISI targets the subjective symptoms and consequences of insomnia as well as the degree of concerns or distress caused by those difficulties. Its content corresponds in part to the diagnostic criteria of insomnia [11]. The ISI comprises seven items assessing the severity of sleep-onset and sleep maintenance difficulties (both nocturnal and early morning awakenings), satisfaction with current sleep pattern, interference with daily functioning, noticeability of impairment attributed to the sleep problem, and degree of distress or concern caused by the sleep problem. Each item is rated on a 0–4 scale and the total score ranges from 0 to 28. A higher score suggests more severe insomnia. The ISI takes less than 5 min to complete and can be scored in less than 1 min. Two parallel versions are available for completion by a clinician and by a significant other (usually a spouse).
Although the ISI has been used in our clinical research program for 15 years, there has been no systematic evaluation of its psychometric properties. Preliminary studies have shown that its concurrent validity with daily sleep diaries is adequate [13] and that it is also sensitive to detect changes related to insomnia treatment [14], [15]. However, there have been no formal psychometric analyses of the reliability and validity of the ISI. The purpose of the present study was to document the psychometric properties of the ISI. In Study 1, the internal consistency and concurrent validity (with sleep diaries) were examined in a sample of patients evaluated at a sleep disorders centre. In Study 2, the concurrent and content validity were examined by comparing changes over time (with treatment) on the ISI measure with those obtained on daily sleep diaries and polysomongraphy, as well as with those obtained on a clinician's version of the ISI. A factor analysis was also conducted to examine the content validity of the ISI.
Section snippets
Participants
Data for Study 1 were compiled from an initial interview administered to 145 clinical patients presenting to a sleep disorder centre with a chief complaint of insomnia. The participants were 84 women and 61 men with a mean age of 41.4 years (SD=13.1, range 17–82) and a mean education level of 14.7 years (SD=3.3). They were community residents, and were predominantly single (44%) and employed (67%). Mean average insomnia duration was 10.0 years (SD=11.6) with a mean age of insomnia onset of 31.5
Participants
In Study 2, data collection was part of a larger study comparing the efficacy of cognitive-behavior therapy and pharmacotherapy for late-life insomnia [14]. The participants were 78 insomnia patients with a mean age of 65 (SD=6.7, range 55–84) and a mean education level of 14.4 years (SD=2.4). They were predominantly women (64%), married (68%), and retired (47%). The average sleep efficiency was 68.4% (SD=14.25). Mean average insomnia duration was 16.8 years (SD=16.9) and mean age of insomnia
Discussion
The Insomnia Severity Index is a brief instrument that was developed to assist in the clinical evaluation of patients with insomnia complaints and to measure outcome in treatment research. The two studies reported in this paper provide initial empirical support regarding the psychometric properties of the scale in clinical samples of young and older adults. Specifically, the internal consistency of the ISI was adequate, its concurrent validity was documented by significant correlations with an
Acknowledgements
Preparation of this article was supported in part by the National Institute of Mental Health Grant #MH55469.
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