A cross-sectional study design was used.
Three hundred eight patients with MS were recruited in the context of the MS@Work study via MS outpatient clinics in the Netherlands . Inclusion criteria for the current study were (a) a diagnosis of relapsing-remitting MS (RRMS) according to the Polman-McDonald criteria 2010 , (b) 18 years and older, (c) currently having a paid job (d) completing the ‘Capability Set for Work Questionnaire’. Patients with co-morbid psychiatric or neurological disorders, substance abuse, neurological impairment that might interfere with cognitive testing or unable to speak and/or read Dutch were excluded from the study. Five patients were excluded because of an unclear diagnosis. A total of 72 patients with MS did not have a paid job. Another 68 workers with MS did not receive the Capability Set for Work Questionnaire, because the questionnaire was added at a latter point in time to the MS@Work study. This led to the inclusion of 163 workers with MS (77% females; median age 43.0, ranging from 24 to 64 years old). The group of 68 workers with MS who were excluded due to missing Capability Set for Work Questionnaire-data did not differ in terms of age, gender, educational level, disability level, disease duration and work hours.
All workers with MS completed online questionnaires on work functioning and MS symptoms, and underwent neuropsychological and neurological examinations (methods and data to be reported elsewhere). We used data from a large survey study meant to evaluate the construct validity of the Capability Set for Work Questionnaire . The validation study used a panel agency to approach a representative sample (N = 1250) of the Dutch general working population. In the current study we included 163 Dutch workers matched for gender, age, level of education and working hours. More details about the survey study can be found in Abma et al. .
The MS@Work study was approved by the Medical Ethical Committee Brabant (NL43098.008.12 1307) and the Board of Directors of the participating MS outpatient clinics. All subjects provided written informed consent. For the panel study no ethical approval was necessary according to the medical ethics committee of the University Medical Centre Groningen as it did not qualify for being tested according to the Dutch Medical Research Involving Human Subjects act of 1998 .
The panel study was performed according to the guidelines of the Association of Universities in the Netherlands . Participants provided online informed consent . Both studies were performed in agreement with the Declaration of Helsinki .
Work capabilities were examined using the Capability Set for Work Questionnaire (CSWQ) in both groups. This questionnaire represents an operationalization of the ‘capability set’ derived from the model of sustainable employability [3, 10]. The questionnaire explores whether a set of seven valued work aspects, are considered valuable by the worker (A), are enabled in the work context (B), and can be achieved (C). In relation to each of these seven valued work aspects the worker is asked (A) ‘How important is <the value> for you?’ (B) ‘Does your work offer the opportunities to achieve <the value>’ and (C) ‘To what extent do you actually achieve <the value>?’ on a scale from 0 = ‘definitely not’ to 5 = ‘very much’. An individual value is considered part of the capability set of an individual worker when it is considered important (A) (score 4–5), and the workplace offers the opportunity to achieve the value (B) (score 4–5), and the worker is able to achieve the value (C) (score 4–5). The capability set can therefore encompass up to seven values. An overall question for the capability for work was posed: ‘Taking all things together, I think I have enough opportunities to remain working’, which required a response ranging from 1 = ‘totally disagree’ to 5 = ‘totally agree’.
Education level was in both groups classified based on the Dutch classification system, according to Verhage et al. . Education level was divided into three levels; low, middle or high education. People were considered to have a low level of education up to finishing low level secondary school. Middle education corresponds with finishing secondary school at a medium level. People were considered highly educated when they finished secondary school at the highest level and/or obtained a college or university degree.
Work ability was in both groups examined using the item ‘current work ability compared with the lifetime best’ of the Work Ability Index (WAI) . Possible scores range from 0 = ‘completely unable to work’ to 10 = ‘work ability at its best’. The use of a single item of the WAI has been shown to be valid and simple indicator of work ability .
Work functioning was examined in both groups with two subscales from the Work Role Functioning Questionnaire 2.0 (Dutch Version) (WRFQ 2.0) : physical and flexibility demands. The WRFQ 2.0 measures the perceived percentage of time that physical and emotional problems impact certain work demands. Scores range from 0 to 100, with higher scores indicating better work functioning.
Self-rated health was measured in both groups with a question from the Short Form-12 : ‘In general, how would you rate your health?’. Response categories ranged from 1 (very good) to 5 (very poor), which were recoded in a way that a higher score represent a better self-rated health.
Working hours was measured in hours per week in both groups.
Absenteeism was measured using number of days absent from sickness in the past 3 months (in the general working population) or the number of days absent from work in the past year (in workers with MS). In workers with MS absenteeism was measured on an ordinal scale. 0 days, 1–3 days, 4–5 days, 6–10 days, 11–20 days, 20 days – 6 weeks, 7–13 weeks, 3–6 months, 6 months- 1 year, or not applicable. It was not possible to compare this variable between groups, due to the use of different scales. We did look at correlations between the capability set and the separate measures of absenteeism.
Presenteeism was measured only in workers with MS and represents the self-reported influence of MS symptoms on work productivity on a scale from 1 to 10 in which higher scores represent more influence of the MS symptoms on work productivity. This question is part of the Work Productivity and Activity Impairment Questionnaire .
Disability level in workers with MS was quantified using the Expanded Disability Status Scale (EDSS) . Scores range from 0 (normal neurological exam) to 10 (death due to MS) and increment with steps of 0.5. The EDSS was assessed by a neurologist during the neurological examination (as part of the MS@Work study) at the outpatient clinic where the patient with MS is being treated. Scores between 0 and 3.5 represent mild disability, 4.0–6.5 represent moderate disability and scores of 7.0 and above are seen as a severe level of disability .
Cognitive and neuropsychiatric functioning was measured in workers with MS using the Multiple Sclerosis Neuropsychological Screening Questionnaire (MSNQ) . Scores range from 0 to 60 and higher scores are indicative of greater subjective cognitive and neuropsychiatric impairment.
Information processing speed was examined using the Symbol Digit Modalities Test (SDMT)  in workers with MS. The SDMT was administered by a trained research nurse or (neuro)psychologist during a cognitive examination (as part of the MS@Work study) at the outpatient clinic where the patient with MS is being treated. The SDMT is often used as an indicator of cognitive functioning in MS . Possible total scores range from 0 to 110. Higher scores indicate better performance. Z-scores were retrieved from the SDMT manual, Z-scores below − 1,5 were considered indicative for an impairment in information processing speed .
Anxiety and depression were examined in workers with MS using the Hospital Anxiety and Depression Scale (HADS) . Possible scores per domain, i.e. anxiety or depression, range from 0 to 21 and scores at or above 8 are considered indicative of major depression or generalized anxiety disorder as validated in MS .
Fatigue impact was measured in workers with MS using the Modified Fatigue Impact Scale (MFIS) . This scale assesses the impact of fatigue on daily functioning in physical, cognitive, and psychosocial dimensions. Possible total scores range from 0 to 84. Total scores at or above 38 are considered indicative of MS-related fatigue .
SPSS for Windows (release 23.0) was used for data analysis. Workers from the general population were matched with workers with MS using fuzzy case-control matching for gender (tolerance = 0), age (tolerance = 8), working hours (tolerance = 5) and level of education (tolerance = 1). Differences in demographics, self-rated health, work ability, work functioning and capability aspects between workers with MS and the general working population were analysed using parametric or non-parametric tests. Additionally, using Wilcoxon Signed Rank Tests, we examined discrepancies between the importance of each value (A), whether the value was enabled (B) and whether the person was able to achieve the value (C), but only for important values (score 4–5). Spearman’s rho correlation analyses were performed to examine whether the capability set and overall capability item were related to measures of work functioning and health. Correlation coefficients between 0.15–0.29 were interpreted as weak, 0.3–0.59 were interpreted as moderate and 0.6–1 were interpreted as strong . To correct for multiple testing a Bonferroni correction was used for the interpretation of statistical significance. Due to the exploratory nature of the analyses for discrepancies between A, B and C, a p value of 0.01 was considered trend significant.