Study objectives: The objective was to psychometrically evaluate the Insomnia Symptom Questionnaire (ISQ), a self-report instrument designed to establish a clinically relevant case definition of insomnia consistent with widely used insomnia classification criteria, using methods from classical test theory and item response theory (IRT).
Methods: The ISQ was evaluated using IRT algorithms in a cohort of 362 pre-, peri- and post-menopausal women recruited for the SWAN (Study of Women's Health Across the Nation) Sleep Study. This yielded a dichotomous outcome consistent with the presence/absence of insomnia. The internal consistency and criterion validity of the dichotomized ISQ were compared to traditional measures of sleep from sleep diaries, polysomnography, and the Pittsburgh Sleep Quality Index using kappa statistics, and indices of sensitivity, specificity, positive and negative predictive value (PPV), and likelihood ratio tests (LRs).
Results: The ISQ identified 9.8% of the sample as meeting insomnia, consistent with established diagnostic criteria. Reliability was established with Cronbach alpha (alpha = 0.89). The ISQ had high specificity (> 90%), but sensitivity, PPV, NPV, and LRs varied according to which sleep measure was used. Concurrent validity was not confirmed with any of the traditional sleep summary measures (kappas < 0.30).
Conclusions: The ISQ captures the multidimensionality of insomnia better than traditional sleep measures as it ascertains symptoms of insomnia that are based on DSM-IV and RDC criteria. The high specificities suggest that the ISQ has a high probability of correctly identifying those without insomnia and would be a cost-effective tool in large observational studies in which the prevalence of insomnia is likely to be about 10%. Further evaluation of the ISQ, including validation against clinical interviews, is warranted.