Psychometric Evaluation of the Insomnia Symptom Questionnaire: a Self-report Measure to Identify Chronic Insomnia

Insomnia is a major public health problem affecting millions of individuals, along with their families and communities.1 Insomnia is defined as difficulty initiating or maintaining sleep, or unrefreshing sleep despite adequate opportunity for sleep. It is the most commonly reported sleep disturbance, affecting up to 30% of the adult population.2–4 Women are 1.4 times as likely as men to complain of insomnia symptoms,5 and this number significantly increases with the transition into the menopausal period.3,6–8 Recent epidemiological data indicate that approximately 40% of women between the ages of 40 and 55 years report recent sleep difficulties that resemble symptoms of insomnia.6 Diagnostic criteria for insomnia from the American Academy of Sleep Medicine and the American Psychiatric Association include: (1) a complaint of problems initiating or maintaining sleep, and/or that sleep is nonrestorative or poor in quality; (2) the duration of the sleep complaint is at least 1 month; and (3) complaints of significant impairment in social, occupational or daily functioning.9,10 Although these criteria do not include frequency of occurrence, a frequency criterion of ≥ 3 nights per week is commonly used.11 Clinical interviews conducted by trained clinicians are used to establish a clinical diagnosis of insomnia, but the use of a fully structured interview12 is often impractical in large epidemiological studies because it is time-consuming and costly.13,14 Cost-effective metrics for assessing insomnia are needed. Various symptom severity questionnaires have incorporated diagnostic criteria for chronic insomnia.15 Several of these correspond directly to specific established diagnostic criteria by assessing sleep complaints, duration and/or daytime consequences. These include the Insomnia Severity Index (ISI),16 the Medical Outcomes Study (MOS) Sleep measure,17 the Women's Health Initiative Insomnia Rating Scale (WHIIRS),18 the Athens Insomnia Scale,19 and the Short Insomnia Questionnaire (SDQ).20 However, none of these instruments establishes a case definition of insomnia that would be an appropriate analog of clinical interviews for epidemiological studies, and none provides a dichotomous outcome based on a case definition that would allow a clinician or researcher to confidently categorize participants.15 Other sleep instruments assess specific sleep characteristics or quality, but were not designed to identify insomnia per se. For instance, the Pittsburgh Sleep Quality Index (PSQI)21 is a questionnaire with established reliability and validity that is frequently used for measuring sleep quality and identifying good and bad sleepers. However, it was not designed to assess insomnia based on diagnostic criteria. The PSQI's reliability and validity for identifying people who have difficulty initiating or maintaining sleep, a necessary symptom of insomnia, may be improved by using a more stringent cutoff score (> 10).20–22 Nevertheless, the PSQI does not ascertain frequency of sleep symptoms nor specific information on daytime consequences. Based on the above, we identified the need for a self-report questionnaire guided by established diagnostic criteria that could be used to identify insomnia in large epidemiological or health services research studies. We administered the 13-item Insomnia Symptom Questionnaire (ISQ) (Appendix 1) to pre-, peri-, and post-menopausal women in a large observational study and compared the reports of symptoms from this instrument to other measures of sleep including the PSQI, sleep diaries, and nocturnal polysomnography (PSG) data. There was no true “gold standard” for diagnosing insomnia in our sample as we had no clinical interview. Rather, the available sleep measures were used to suggest the presence of a sleep disorder. Therefore, the analyses focused on the internal consistency and criterion (i.e., concurrent) validity of the ISQ. The aims of the present analyses were (1) to describe the development of a scoring algorithm using item response theory (IRT) that yielded a clinically relevant case definition of insomnia, and (2) to present the psychometric properties of the ISQ.