Multiple sclerosis (MS) is a complex, debilitating, and chronic disease of the central nervous system that has affected more than two million people worldwide. It is the most common neurological disease leading to the disability of young adults [1]. Although the etiology of this multi-factorial and costly disease has yet to be identified, it is widely believed that genetics, vitamin D deficiency, the autoimmune mechanism, and several environmental factors play major roles in its development. Of significant concern are the varied prevalence rates of MS worldwide. For example, 12 cases per 100,000 are reported in Mexico compared to 55 per 100,000 in Spain. MS prevalence is also high in Iran at approximately 50 per 100,000 and women are affected at approximately 3 times the rate of men [2].

The nature of this disabling disease can affect the activities and social participation of MS patients. A Germany study found that anorgasmia in women and erectile dysfunction in men are the most common problems in sexual relationships among MS patients. It may also result in unemployment, with at least one study finding that persons with MS lost their jobs after being diagnosed with the disease based solely on their employers’ bias toward the disease. This in itself often leads to reduced quality of life of patients [1,3]. Measuring activity limitations— especially participation restrictions—instead of the level of impairment seems to be more important for finding ways to increase the participation of patients in order to integrate those with disabilities into the community [4].

Although numerous instruments have been developed to measure disabilities, the World Health Organization Disability Assessment Schedule 2.0 (WHODAS 2.0) was designed to assess the general health and disability levels based on the International Classification of Functioning, Disability, and Health (ICF) concepts as outlined in its activity and participation chapter [5,6]. The instrument has been used to assess activity limitation, disability, functioning, and intervention effectiveness for diseases and disorders in clinical and research centers [5,7]. Moreover, the majority of studies have shown that WHODAS 2.0 has acceptable psychometric properties. More than 28 countries have translated and adapted it to their specific cultures [8]. The questionnaire’s simplicity, short completion time, and a structure based on the bio-psychosocial model of ICF set WHODAS 2.0 apart from other instruments [9,10]. The aim of this paper was to assess the psychometric properties (test-retest reliability, internal consistencies of reliability, floor and ceiling effects in every domain, and construct validity) of the Persian version of WHODAS 2.0 for persons with MS.

The demographic variables of the participants (for both whole and test–retest samples) are displayed in Table 1.

This study was designed and conducted to examine the psychometric properties of the Persian version of the 36-item WHODAS 2.0 using a sample of persons with MS in Iran. The results excellent psychometric properties. They also show that Cronbach’s α coefficients for each subscale were between 0.70 (understanding and communication) and 0.88 (life activities), and the value was also high (α=0.94) for the WHODAS 2.0 total score. This is greater than a Portuguese version of WHODAS 2.0 with persons with musculoskeletal pain (α=0.84) [19] and similar to the Chisolm study with persons with hearing loss (α=0.94) [20]. Cronbach’s α coefficient for the Persian version was very similar to the Taiwanese study (α=0.91) [5].

In Table 2, the results for test–retest show that the ICC of all domains were greater than 0.80, and the cognition (D1) and participation in society (D2) domains had the minimum and maximum ICC at 0.82 and 0.99, respectively. Guilera et al. [21] (who studied schizophrenia) argued that the minimum and maximum amount of ICC for D1 and D2 was 0.86 and 0.89, respectively. These findings are similar to those obtained using the Persian version. In the present study, the total ICC for WHODAS 2.0 was 0.98, a value greater than the Guilera study. Similarly, the Portuguese study of Silva et al. [19] also reported that the total ICC score for WHODAS 2.0 was 0.95. This is also consistent with the Persian version. Further, the 0.80 ICC of the Taiwanese study is not consistent with the present study that revealed a lower coefficient [5].

The results of the content validity analysis show that the Persian version of WHODAS 2.0 had floor effects over 30% for the following domains: “self-care,” “getting along,” and “life activities (work/school).” In a study carried out by Garin et al., [9] lower floor and ceiling effects were found for WHODAS 2.0 compared to the results of the present study. Chiu et al. [5] (Taiwan study) reported that all WHODAS 2.0 domains had floor effects greater than 35%, and the “self-care,” “getting along,” and “life activities” dimensions were over 40%; however, the total floor effect of WHODAS 2.0 was only 7.2%. The results obtained in the Taiwan study were similar to the Persian version of WHODAS 2.0 that showed a 4.6% floor effect.

The correlation between WHODAS 2.0 and MSQoL-54 confirmed the construct validity between the two instruments. The results of the correlation are shown in Table 3. Compared to other studies, this result is similar to that obtained for the English version of WHODAS 2.0 originally developed by WHO that shows an acceptable correlation between other instruments, such as the SF-36, SF-12, and WHOQoL (r=0.43–0.58) [7]. The correlation between the Persian version of WHODAS 2.0 and MSQoL-54 (r=-0.67) was more than the previous studies (Taiwan and Guilera) [5,21] but less than the correlation observed in the study of Thomas et al. [22] that was conducted with persons with lymphatic filariasis (r>0.7). Magistrale et al. [23] reported moderate correlations between mental health in a study frame comprising MSQoL-54 and WHODAS 2.0 and strong correlation between physical health based on the same combined framework with MS patients. The results of the correlation analysis between WHODAS 2.0 and MSQoL-54 in this study indicate that the Persian WHODAS 2.0 shows good external construct validity.

The EFA results showed that approximately 75% of the total variance was influenced by seven factors, more than both the English [9] and Japanese versions [24] of WHODAS 2.0. The identification of seven factors for the Persian version is similar to previous research, including the Taiwan [5], English [7], and European [25] studies; however, it differs from the Italian [26] version that explained only six factors. The CFA results are displayed in Figure 1, wherein the Persian WHODAS 2.0 is shown to have an acceptable construct validity. Consistent with this finding, Chiu et al. [5] reported that the traditional Chinese version of WHODAS 2.0 items loaded greater than 0.4 in CFA.

In general, the results of the current study show that the Persian WHODAS 2.0 has acceptable internal consistency, excellent ICC, a strong construct, and good content validity. Most of these results align with those reported in the original WHODAS 2.0, and there is little difference between the factor structures of the Persian and original versions. In this study, our factor analyses explored and divided the instrument’s questions into seven factors, thus yielding one more factor or domain than the original. It should be noted that during the study when the original WHODAS 2.0 was being developed, the life activities (school/work) domain was not considered for the factor structure analysis. This in itself could be due to the decreased number of WHODAS 2.0 factors.

The results of this study indicate that the Persian WHODAS 2.0 is a valid and reliable instrument to evaluate the disability for persons with MS. Its limitations are as follows. First, we did not include a healthy control sample for proper comparison with the patient sample to potentially derive more accurate results. Also, in order to assess test–retest reliability, only 60 patients cooperated by completing the WHODAS 2.0 the second time. It is therefore recommended that a larger, more comprehensive and heterogeneous sample be included in future studies. One final point is that the “sensitivity to changes” factor in the WHODAS 2.0 was not evaluated in this study.