Evaluation of the reliability and validity of the Medical Outcomes Study sleep scale in patients with painful diabetic peripheral neuropathy during an international clinical trial
© Viala-Danten et al; licensee BioMed Central Ltd. 2008
Received: 19 May 2008
Accepted: 17 December 2008
Published: 17 December 2008
Sleep is an important element of functioning and well-being. The Medical Outcomes Study Sleep Scale (MOS-Sleep) includes 12 items assessing sleep disturbance, sleep adequacy, somnolence, quantity of sleep, snoring, and awakening short of breath or with a headache. A sleep problems index, grouping items from each of the former domains, is also available. This study evaluates the psychometric properties of MOS-Sleep Scale in a painful diabetic peripheral neuropathic population based on a clinical trial conducted in six countries.
Clinical data and health-related quality of life data were collected at baseline and after 12 weeks of follow-up. Overall, 396 patients were included in the analysis. Psychometric properties of the MOS-Sleep were assessed in the overall population and per country when the sample size was sufficient. Internal consistency reliability was assessed by Cronbach's alpha; the structure of the instrument was assessed by verifying item convergent and discriminant criteria; construct validity was evaluated by examining the relationships between MOS-Sleep scores and sleep interference and pain scores, and SF-36 scores; effect-sizes were used to assess the MOS-Sleep responsiveness. The study was conducted in compliance with United States Food and Drug Administration regulations for informed consent and protection of patient rights.
Cronbach's alpha ranged from 0.71 to 0.81 for the multi-item dimensions and the sleep problems index. Item convergent and discriminant criteria were satisfied with item-scale correlations for hypothesized dimensions higher than 0.40 and tending to exceed the correlations of items with other dimensions, respectively. Taken individually, German, Polish and English language versions had good internal consistency reliability and dimension structure. Construct validity was supported with lower sleep adequacy score and greater sleep problems index scores associated with measures of sleep interference and pain scores. In addition, correlations between the SF-36 scores and the MOS-Sleep scores were low to moderate, ranging from -0.28 to -0.53. Responsiveness was supported by effect sizes > 0.80 for patients who improved according to the mean sleep interference and pain scores and clinician and patient global impression of change (p < 0.0001).
The MOS-Sleep had good psychometric properties in this painful diabetic peripheral neuropathic population.
As this study was conducted from 2000 to 2002 (i.e., before the filing requirement came out), no trial registration number is available.
Sleep is an important element of functioning and well-being and is associated with clinical status and general health. Indeed, sleep problems have been found to be associated with depression, anxiety, impaired social functioning, hospitalizations, chronic medical conditions and mortality [1–3]. A number of sleep questionnaires exist that are designed either to measure different aspects of sleep such as quality of sleep, to identify the impact of sleep problems on daily functioning, or to evaluate the impact of sleep disorders on patients' life [4–13].
Painful diabetic peripheral neuropathy (DPN) is one of the most common complications of type I and type II diabetes. Painful DPN may affect sleep, work, social activities and relations, physical mobility, levels of anxiety and depression and energy [14–16], thus leading to the substantial impairments in patients' health-related quality of life (HRQoL) [17, 18]. Sleep disturbance is common in chronic pain and is of particular concern in painful DPN as it may influence the progression of type II diabetes . A recent study confirmed the association of painful DPN with sleep impairment .
The present study examines perceptions of sleep in an international clinical trial aimed at evaluating the efficacy and safety of the pregabalin, a treatment for pain relief in patients with painful DPN. The Medical Outcomes Study-Sleep Scale (MOS-Sleep) was administered to patients in order to assess the impact of the pregabalin treatment on patients' quality of sleep. We used the data from the international clinical trial to provide information on the psychometric properties of the MOS-Sleep in patients with DPN.
A total of 512 patients in Germany, Hungary, Poland, Australia, the United Kingdom and South Africa were screened for the clinical trial. Selected patients had to have been diagnosed with type I or II diabetes and with painful, distal, symmetrical, sensorimotor polyneuropathy at least one year prior to inclusion. Three hundred and ninety-seven patients were randomized to medication.
The analysis of the psychometric properties was performed on all subjects who completed at least 50% of the items of the MOS-Sleep at the baseline visit (more than 99% of the sample). The analysis of change over time (responsiveness) of the MOS-Sleep was performed on all subjects who completed at least 50% of the items of the MOS-Sleep at baseline and at termination visits (more than 90% of the sample).
The study was a double-blind, randomized, placebo-controlled, multicenter, phase III clinical trial, conducted to evaluate the efficacy and safety of 3 regimens of pregabalin (150, 300, or 600 mg/day) compared to placebo in DPN patients. It was conducted in accordance with the International Conference on Harmonisation Guidelines for Good Clinical Practice (GCPs), the Declaration of Helsinki, and in compliance with United States (US) Food and Drug Administration (FDA) regulations for informed consent and protection of patient rights. The study consisted of a 1-week baseline phase, a 12-week double-blind treatment phase and a 1-week follow-up period. Six visits plus 1 follow-up visit were scheduled during this study .
Eligible patients were given a daily pain diary at visit 1 (V1) and had to record pain during the next 7 days of the baseline phase. The diary consists of a single item with an 11-point numeric self-administered rating scale ranging from 0 (no pain) to 10 (worst possible pain). At the end of the baseline phase, a mean of the daily pain scores was calculated; patients with score of 4 or higher were randomized at visit 2 (V2) and started the treatment phase. During the 7 days prior to randomization, patients had to complete the sleep interference diary, in which patients described how their neuropathic pain had interfered with their sleep. It consists of a single-item with an 11-point numerical rating scale, ranging from 0 (pain does not interfere with sleep) to 10 (pain completely interferes with sleep) that allows the calculation of a weekly mean sleep interference score [21–23]. At V2 and at termination visit (V6; 12 weeks after starting medication), patients were asked to complete the MOS-Sleep and the SF-36 questionnaire.
The MOS-Sleep is a 12-item measure developed using patients with chronic illness; it is divided into 6 dimensions evaluating "sleep disturbance," "snoring," "sleep awakening short of breath or with headache," "sleep adequacy," "somnolence," and "quantity of sleep/optimal sleep" . A sleep problems index summarizing information across 9 items of the MOS-Sleep can also be scored. Support for the reliability and validity of the US English version has been reported in the general population, patients with overactive bladder, patients with post-herpetic neuralgia [24, 25]; Spanish version properties have been assessed in patients with neuropathic pain . Several language versions (Polish, German and Hungarian) of the instrument have been subsequently developed following rigorous and standardized methodology including 2 forward translations, 1 backward translation, cognitive debriefings and international harmonization ; Australian, South African and UK English versions have undergone an adaptation from the original US English version of the MOS-Sleep. The SF-36 v.1 is a generic health survey that includes 36 items measuring 8 multi-item domains : "bodily pain," "general health perception," "mental health," "physical functioning," "role limitation due to emotional problems," "role limitation due to physical health problems," "social functioning" and "vitality." Scores for each of these 8 concepts are transformed linearly to have scores ranging from 0 to 100, higher scores indicating better HRQoL.
Daily pain and sleep interference diaries were collected again at V6. A Clinical Global Impression of Change (CGIC) and a Patient Global Impression of Change (PGIC) were also collected. The CGIC is a clinician-rated instrument that measures change in patient's overall status on a 7-point scale ranging from 1 ("very much improved") to 7 ("very much worse"); the PGIC is a patient-rated instrument that measures change in patient's overall status utilizing the same 7-point scale as above.
Description and scoring rules of the MOS-Sleep
Item content of the MOS-Sleep
Dimensions and Sleep problems index
Trouble falling asleep
Awaken during sleep
Time to fall asleep
Trouble staying awake
Enough sleep, feel rested
Amount sleep needed
Snore during sleep
Awaken short of breath or headache
Awaken short of breath or headache
Quantity of sleep/Optimal sleep
Quantity of sleep
Sleep problems index
Item # 01, 03, 04, 05, 06, 07, 08, 09, 12
Psychometric analysis of the MOS-Sleep
Internal consistency reliability, estimated by Cronbach's alpha coefficient, reflects the extent to which multiple items in a dimension are inter-correlated and form a dimension measuring a same underlying concept . An alpha coefficient of 0.70 or higher is considered a satisfactory level of reliability for group comparisons .
Multitrait scaling analysis  was used to evaluate the structure of the multi-item dimensions (i.e. "sleep disturbance," "adequacy of sleep" and "somnolence" dimensions and sleep problems index) in order to verify that items measured the concept of their hypothesized dimension. Two criteria were assessed: item convergent criterion (correlation between each item and its own dimension) is met when value is greater than 0.40; item discriminant criterion (extent to which item correlates more highly with the dimension it represents than with other dimensions) states that each item should have a higher correlation with its own dimension than with any of the others.
Construct validity was tested with the following two analyses. The ability of the MOS-Sleep scores to discriminate between groups of subjects according to the severity of the disease was evaluated . The weekly mean sleep interference score and the weekly mean pain score at baseline were used to define groups of patients differing in severity. Since patients had to have pain at baseline to qualify for the present study, the majority of the population tended to be in the most severe of the groups suggested by Zelman et al. . Thus, in the present study, pain groups were defined according to both the cut-points suggested by Zelman and the mean pain score distribution determined from the pain interference diary. As a result, four groupings were defined that contained a balanced distribution of the study population: moderate pain was defined as patients with a score of 3.01 through 6.00, and three categories of severe pain were defined as patients with a score of 6.01 through 7.00, patients with a score of 7.01 through 8.00, and patients with a score of 8.01 through 10.00. As no cut-points were published for the sleep interference score, groups similar to those used for the pain score were defined: no or mild sleep interference was 0 through 3.00; moderate sleep interference was 3.01 through 6.00; and two severe sleep categories were defined as those with a score of 6.01 through 7.00, and those with a score of 7.01 through 10.00. As HRQoL data tend to be non-normally distributed, Kruskal-Wallis and Chi-square non-parametric tests were computed to compare MOS-Sleep scores by the different severity groups.
Validity was also assessed by examining Spearman rank-order correlations between the MOS-Sleep scores and the SF-36 scores. Based on previous work , we hypothesized that the highest correlations would be between sleep and mental health dimensions of the SF-36; moderate correlations were expected between all SF-36 scores and the MOS-Sleep scores, except for the MOS-Sleep "snoring" score for which low correlations were expected.
Responsiveness to change was evaluated for the MOS-Sleep scores for groups of patients based on their change in health status over 12 weeks (between V2 and V6). Using the CGIC and PGIC, subgroups of 'much improved,' 'improved,' 'stable' and 'worsened' patients were defined. In the absence of published thresholds, the following groups of patients were defined based on the distribution of the changes in both the weekly sleep interference and pain diaries mean scores: 'much improved' (-10 ≤ mean sleep/pain score ≤ -4); 'improved' (-4 < mean sleep/pain score ≤ -1); 'stable' (-1 < mean sleep/pain score < 1); and 'worsened' (1 ≤ mean sleep/pain score ≤ 10). Effect Size (ES) and the Standardized Response Mean (SRM) were calculated, with the following values used for the interpretation of ES and SRM: ES and SRM = 0.20, small change; ES and SRM = 0.50, moderate change; ES and SRM = 0.80, large change [35, 36]. The responsiveness analysis was performed on the subjects for whom the MOS-Sleep was completed at baseline and at termination visit and is considered assessable (i.e. at least 50% of the items were completed).
Statistical analyses were performed on the overall population and for the five countries of the study separately.
Main analyses were performed using SAS software (Statistical Analysis System, version 8.02) for Windows. Multitrait analyses were performed using the MAP-R program. The threshold for statistical significance was set up at 5%.
Description of the population at baseline
Description of the population at baseline (N = 396)
Age (years; mean ± STD)
59 ± 11
Asian or Pacific
Mean score ± STD
Mean sleep interference score
5.6 ± 2.2
Mean pain score
6.4 ± 1.4
MOS-Sleep dimension scores
Awaken Short of Breath
24.3 ± 27.2
54.9 ± 26.3
Quantity of sleep
6.0 ± 1.6
40.2 ± 33.3
42.0 ± 24.2
Sleep Problems Index
48.8 ± 19.6
43.5 ± 28.7
Optimal sleep (yes; %)
Mean sleep interference scores ranged from 5.37 (STD = 2.16) in Poland to 6.49 (STD = 2.45) in South Africa at baseline. The mean sleep interference score across countries was 5.59 (STD = 2.17). Mean pain scores ranged from 6.14 (STD = 1.44) in Germany to 7.48 (STD = 1.52) in South Africa. The mean pain score across countries was 6.42 (STD = 1.44).
Item missing data for the MOS-Sleep
Percentage of item missing data ranged from 0.0% in South Africa and the United Kingdom to 1.5% in Germany at baseline (V1) and from 0.0% in Hungary to 1.6% in the United Kingdom at termination visit (V6).
Psychometric properties of the MOS-Sleep
Internal consistency reliability
Cronbach's alpha and item convergent and discriminant criteria of the MOS-Sleep multi-item scores (N = 381)
No of items
Range of item-scale correlations
% of items meeting the convergent criterion
% of items meeting the discriminant criterion
Sleep problems index
78% (all except items 5 and 9)
Multitrait Scaling Analysis
Items in the "sleep disturbance," "somnolence," and "sleep adequacy" dimensions of the MOS-Sleep had item-scale correlations ranging from 0.47 to 0.76 (Table 3). Items of the sleep problems index had correlations ranging from 0.34 to 0.62 (only items 5 and 9 did not reach the threshold of 0.40). All items had a higher correlation with their own dimensions than with the others. Per language version, the majority of items had item-scale correlations higher than 0.40 for their hypothesized dimension, except for items 4 and 12 of the "sleep adequacy" dimension in the Australian and Hungarian language versions (correlation = 0.37 and 0.34, respectively), item 11 of the "somnolence" dimension in the German, Hungarian and South African language versions (correlation = 0.28, 0.38, 0.34, respectively), and item 3 of the "disturbance" dimension in the Hungarian language version (correlation = 0.34). Item discrimination criterion across dimensions was satisfied, except for item 12 of the "sleep adequacy" dimension of the Australian, Hungarian and UK English versions, and item 4 of the South African, item 6 of the "somnolence" dimension of the German and South African and item 3 of the "sleep disturbance" dimension of the Hungarian version.
Spearman correlation coefficients between MOS-Sleep scores and SF-36 scores at baseline (N = 396)
Short of breath or headache
Quantity of sleep
Sleep problems index
Distribution of the change in the MOS sleep problems index according to the change in health status (N = 356)
Change in scores of the
MOS sleep problems index
p(change = 0)
Change in mean sleep interference score
p(KW) < 0.0001
Much improved (-10 to -4)
Minimally improved (-4 to -1)
No change (-1 to +1)
Worse (+1 to +10)
Change in mean pain score
p(KW) < 0.0001
Much improved (-10 to -4)
Minimally improved (-4 to -1)
No change (-1 to +1)
Worse (+1 to +10)
Clinician global impression of change
p(KW) < 0.0001
Very much improved
Patient global impression of change
p(KW) < 0.0001
Very much improved
ES and SRM were > 0.80 for the 'much improved' group of patients, between 0.50 and 0.80 for the 'minimally improved' group of patients, and between 0.20 and 0.50 for the 'no change' and 'worse' groups whatever the criterion used to define these groups. Changes in the sleep problems index score were highly statistically different from 0 for all the groups of improved ('very much', 'much' and 'minimally') patients based on the mean sleep interference score, the pain score, and the clinician and patient global impression of change (p < 0.0001) (Table 5). Stable patients, as defined by no change in the mean sleep interference score or no change in the mean pain score, also showed a significant (but lower than for the 'improved' group) change in MOS sleep problems index score (p = 0.0005 and 0.0003, respectively), whereas stable patients as defined by the CGIC and the PGIC showed no statistically significant change in MOS sleep problems index score. Changes in the sleep problems index score of the 'worsened' patients defined on the mean sleep interference score, the CGIC and the PGIC were not statistically different from 0 (Table 5).
The objective of this study was to provide information on the psychometric properties of the MOS-Sleep in a DPN population. The instrument is a 12-item questionnaire developed to evaluate patient reported sleep outcomes in terms of "sleep disturbance," "sleep adequacy," "somnolence," "quantity of sleep," "snoring" and "awaken short of breath or with a headache."
The MOS-Sleep was included in an international clinical trial conducted in Germany, Hungary, Poland, Australia, United Kingdom and South Africa; the respective language version of the instrument was administered in each of the countries. Psychometric properties were assessed on the pooled data across the 6 countries. When the sample sizes permitted it, the psychometric properties of the MOS-Sleep were evaluated for each of the translated versions in each country.
A total of 396 patients with DPN participated in the study, representative of differing socio-demographics (ethnicity, gender, age) and clinical characteristics. Item completion rates were similar for each language version of the MOS-Sleep, suggesting good acceptability of the questionnaire by patients across the 6 countries.
Based on the results of the psychometric evaluation, the different language versions of the MOS-Sleep displayed adequate and generally comparable reliability and validity. The responsiveness of the tool was also demonstrated on the pooled population.
The robustness of the structure of the "sleep disturbance" and "somnolence" multi-item dimensions and the sleep problems index of the MOS-Sleep versions was demonstrated, whether tested on the pooled population or on the country-based populations. Dimensions showed satisfactory to excellent item convergent validity in all versions; only items 4 and 12 of the "sleep adequacy" dimension for the Australian and Hungarian versions, item 11 of the "somnolence" dimension for the German, Hungarian and South African versions, and item 3 of the "disturbance" dimension for the Hungarian version showed poorer results when compared to the other versions. This might be related to translation difficulties in these language versions. Most of the items satisfied the requirements for item discrimination across dimensions, that is items were more strongly correlated with their hypothesized dimensions than with the other dimensions of the instrument.
Internal consistency reliability of the sleep problems index, "sleep disturbance" and "sleep somnolence" showed good to excellent results whether the populations of the 6 countries were pooled or analyzed separately. Only the results for the "sleep adequacy" dimension were the least satisfactory, with Cronbach's alpha reaching the recommended threshold value of 0.70 only for the German and Polish language versions. This is likely to be explained by "sleep adequacy" is a 2-item scale, as dimensions with only 2 or 3 items are susceptible to have lower Cronbach's alpha than dimensions with a greater number of items. Because of the study design that lasted 12 weeks, test-retest reliability of the MOS-Sleep could not be evaluated in the trial, as this property requires a short enough time between the two points of data collection so that there is minimal or no change on the attribute being measured. So far, there is no published test-retest reliability assessment for the MOS-Sleep.
Correlations between the MOS-Sleep and SF-36 dimensions provided evidence for construct validity. The sleep problems index was moderately correlated (0.40 ≤ r < 0.70) with most of the SF-36 dimension scores, highlighting the relationship between sleep and other aspects of patients' health-related quality of life. This relationship is in agreement with previous work in general and chronic disease populations, which demonstrated that sleep disorders adversely affected general health as well as functional status, work performance, mood and everyday functioning [1, 37–40]. It also suggested that sleep has an impact not only on physical but also on mental and social functioning, confirming previous reports in DPN patients using the SF-12 tool or the Nottingham Health Profile [1, 16]. As hypothesized, the highest overall correlations were observed between the "sleep disturbance" and the "sleep adequacy" scores and the SF-36 scores, whereas the "snoring" dimension was the most weakly correlated with the SF-36 scores. The sleep problems index shared the strongest correlation (-0.53) with the SF-36 "vitality" dimension.
The MOS-Sleep dimensions were able to discriminate between patients with various levels of sleep interference and pain severity, with greater impairment observed for more severe patients thus providing further support for the validity of the instrument. However the categories used to classify patient severity could be discussed. Indeed because of the study design (patients had to have pain at baseline), the majority of our trial population was classified in the "severe" group (> 7) as defined by Zelman et al. . Therefore, in order to better reflect the distribution of the pain severity of our population, grouping was performed based on slightly different cut-points which are specific to our population.
The responsiveness of the MOS-Sleep observed here was consistent with previous studies performed in patients with neuropathic pain [24, 26]. The sleep problems index score proved to be highly responsive to clinical changes, whether sleep interference, pain, patient or clinician global impressions were used to define improvement, stability or worsening over the 12-weeks period of study. Overall, the magnitude of change as measured by ES for the sleep problems index was the largest for patients who improved over the 12 weeks, and decreased as patients' health status became stable or worsened based on sleep interference, pain, patient or clinician global impression of change. The changes observed were statistically different from zero for all the improved subgroups. Surprisingly, these changes in scores were also significantly different from zero for patients classified as stable according to the change in mean sleep interference and mean pain scores. These unexpected findings could be explained by the fact that as patients are being treated, most of the patients classified as stable are actually minimally improved. It also suggests that pain and the other factors, taken individually, can not be the only determinants of the effect observed on patients' sleep.
Results and observations that could be drawn from the pooled language versions should be taken with caution. Even if some results showed the reliability and validity of the translated versions, they do not allow the cross-cultural validity of the MOS-Sleep to be fully addressed. Cross-cultural validity is a very complex notion including several levels of equivalence that should be checked and fulfilled in order to conclude that the language versions of an instrument are cross-culturally equivalent . In order to address the cross-cultural equivalence of the different versions of the MOS-Sleep, further analyses would be necessary, requiring a larger number of patients than what we had in the present study.
The MOS-Sleep had good psychometric properties in this study's DPN population, which vary from one language version to an other. While Australian, Hungarian and South African versions of the MOS-Sleep showed limited psychometric properties when taken individually in the specific conditions of the clinical trial, German, Polish and English language versions display good internal consistency reliability and structure.
We would like to thank all patients and clinical investigators who participated in this clinical trial. We also thank Christine de la Loge for her involvement in the analysis and interpretation of the results.
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