The ISI comprises seven items that evaluate difficulty falling asleep and staying asleep, problems waking up too early, satisfaction with current sleep patterns, interference with daily functions, noticeability of impairment attributed to sleep problems, and distress caused by the sleep problem. Each of the ISI items is rated on a scale of 0-4; the total score ranges from 0 to 28, with a higher score indicating greater insomnia severity. The total ISI scores are divided into four subcategories: 0-7, no clinically significant insomnia; 8-14, subthreshold insomnia; 15-21, moderate insomnia; and 22-28, severe insomnia. A cutoff score of 15 has been used as the threshold for clinically significant insomnia, and a score below 8 has been used to define remission after treatment (i.e., no longer meets the criteria for insomnia).28

Linguistic validation was achieved by having two sleep specialists translate the original ISI questionnaire into Korean; the Korean version was then translated back into English by one sleep specialist and one linguist, both of whom were fluent in Korean and English. Comparison of the original ISI with the final back-translated version was performed by individuals who were fluent in both languages and who were not involved in the research study. The final ISI-K was obtained after completion of these standard procedures.

A sample of patients was sequentially selected from those who visited a regional sleep center between August 2009 and May 2012; all participants were older than 18 years, and all met the criteria for one or more of the following disorders: primary insomnia, comorbid insomnia, or obstructive sleep apnea (OSA). The Diagnostic and Statistical Manual of Mental Disorders, fourth edition, text revision is used to diagnose primary insomniacs, the primary complaint of which is difficulty initiating or maintaining sleep, or nonrestorative sleep, for at least 1 month.29 Comorbid insomniacs meet the criteria of primary insomniacs, but are also diagnosed as simultaneously having another disease, such as a psychiatric disorder or medical disorder. Such comorbid disorders were diagnosed through an interview using sleep questionnaires. OSA was diagnosed for those with a Respiratory Disturbance Index greater than 5, based on the second edition of the International Classification of Sleep Disorders.30

All subjects completed the ISI-K, KESS,15 second edition of the Beck Depression Inventory,31 and PSQI-K,14 and submitted to overnight PSG. In order to examine test-retest reliability, 34 subjects were randomly selected to undergo a retest at a 4-week interval without treatment. The required sample size was calculated using the G-power 3.1.3 program based on an effect size of dz=0.5, an error probability of 0.05, and a statistical power of 0.80.

The PSQI-K and PSG were used as a subjective and an objective gold standard, respectively. The PSQI is a widely used measure of sleep quality32 and has been used in previous studies as a convergent measure with ISI in other language versions.24,26,27 PSG was used as the primary diagnosis tool for other sleep disorders, and as a complementary instrument for the evaluation of insomnia, although it is not indicated for the routine evaluation of this condition.33,34

The study and all procedures were approved by the institutional review board of a regional university hospital in Korea.

The data analyses were performed using SPSS version 18.0 (SPSS Inc., Chicago, IL, USA). The data are presented as mean±SD values, and p<0.05 was considered indicative of statistical significance. Descriptive statistics were used for examining the demographic data, test-retest reliability was examined with a paired-samples t-test, and internal consistency was examined with Cronbach's alpha coefficient. The correlations between ISI-K, KESS, and PSQI-K scores and various sleep parameters from PSG [sleep onset latency (SOL), wake after sleep onset (WASO), and sleep efficiency] were examined using Pearson's correlation coefficient. In addition, the receiver operating characteristic (ROC) curve was used to determine the optimal cutoff score based on its sensitivity and specificity; a perfect medical test would have 100% sensitivity and 100% specificity, corresponding to the upper left-hand corner (0, 1) on the ROC curve.35,36,37 The discriminative ability of the test across the full range of cutoffs was reflected by the area under the curve (AUC).35 A perfect test would have an AUC of 1.0; a commonly applied rule of thumb is that a test with an AUC greater than 0.9 has high accuracy, while 0.7-0.9 indicates moderate accuracy, 0.5-0.7 indicates low accuracy, and 0.5 is considered a chance result.38,39

The entire sample comprised 614 patients with sleep disorders, including 169 with primary insomnia, 133 with comorbid insomnia, and 312 with OSA. The age was 49.8±13.7 years. The gender ratios (i.e., percentage of women) in the primary insomnia, comorbid insomnia, and OSA groups were 61.2%, 64.7%, and 14.1%, respectively. The sleep efficiency, SOL, WASO, and KESS, PSQI-K, and ISI-K scores were 72.9±20.5%, 22.9±39.8 min, 99.6±76.0 min, 6.2±4.6, 11.1±4.9, and 16.4±7.9, respectively. The other demographic and clinical characteristics of the subjects are listed in Table 1.

Cronbach's alpha of the ISI-K was 0.92, which indicated a high internal consistency, and did not change substantially even after deleting any of the items on the scale. The item-total correlations ranged from 0.65 to 0.84, suggesting good homogeneity of the items for measuring the severity of insomnia (Table 2). The test-retest correlation was 0.86 for the total score (p<0.001), and ranged from 0.43 to 0.85 (Table 3) for the seven components.

There was a strong positive correlation between the total ISI-K score and other subjective sleep assessment tools, and the PSQI-K score (r=0.84, p<0.01), suggesting that patients with more severe perceived insomnia also reported worse sleep quality; and a weak negative correlation with the KESS score (r=-0.29, p<0.01), indicating that those with more severe insomnia reported less daytime sleepiness (Table 4).

The correlations between ISI-K and PSG parameters were as follows: there was a weak positive correlation between item 1 of the ISI-K (difficulty falling asleep) and SOL (r=0.22, p<0.01), a moderate positive correlation between item 2 of the ISI-K (difficulty staying asleep) and WASO (r=0.38, p<0.01), and a moderate negative correlation between the total score of the ISI-K and sleep efficiency (r=-0.46, p<0.01) (Table 4).

The ROC curve was used to evaluate the sensitivity and specificity of the ISI-K (Fig. 1). An AUC of 0.93 (95% confidence interval=0.92-0.95) was obtained. The sensitivity and specificity values corresponding to various ISI-K cutoff scores for discriminating between patients with and without insomnia are presented in Table 5. A cutoff score of 15.5 had a sensitivity of 0.92 and specificity of 0.82.