The costs and burdens of long-term sick leave are substantial both for the individual, the employer and society [3,7]. Medical diagnoses related to mild or moderate mental health problems and musculoskeletal conditions, are the most common diagnoses for long term sick leave and disability pension in Norway [8-11], and in other European countries [12]. These diagnoses are often characterized more by symptoms and distress, than by consistent demonstrable tissue abnormalities [13,14], and are often based on the patients’ own reports of pain and complaints [15,16], and may be classified as subjective health complaints [13,15,16]. For some it may be experienced as an increased burden when there are no objective findings to explain their symptoms [17]. Research on work-related rehabilitation emphasize the importance to be seen, heard and taken seriously by the professionals, for individuals on long-term sick leave [18]. The increase in long-term sick leave and disability pension is not due to ill health alone, but could be related to changes in working life and health expectations [8]. Length of the sick leave and the possible burden of being out of work should also be emphasized in the outcome measures, made visible in form of transitions and shifts [1,12,19]. Further when the purpose of a study is to analyse the effects of different treatments or rehabilitation efforts, partial recoveries should also be considered, along with full-time return to work [1,19]. There is still limited knowledge of the long-term occupational status for patients after participation in work-related rehabilitation [3,20]. This may in part, be due to the different challenges described above. Outcome measures in follow-up studies on RTW tend to distinguish only between those working full-time and those not working at all, at a particular point in time [19].

During the 4-year follow-up the probability for working increased and the probability for being on sick leave decreased (see Figure ​Figure11 and Table ​Table2).2). There was also an increased probability to be on disability pension, both partial, time-limited, and permanent.

When summing up cumulative days within each of the different benefits and working during the 4-year follow-up, the participants worked an average of 2.2years (95% CI, 2.08-2.32). The average time on partial sick leave was 4.8months (95% CI, 4.20-5.40), and 3.6months (95% CI, 3.24-3.96) on full-time sick leave. The average time on medical rehabilitation allowance was 6months (95% CI, 5.28-6.72), and 9.6months (95% CI, 8.40-10.80) for vocational rehabilitation allowance. For disability pensions, the average time on partial disability was 4.8months (95% CI, 3.84-5.76) and 6months on full-time disability pension (95% CI, 4.80-6.72).

The probability of working at different points in time during the follow-up was not related to the benefit the participant was receiving when they departed from the rehabilitation clinic (see Table ​Table3).3). For those working (i.e. no registered benefit) at departure, the probability for working after 1year was 52%, and it was 54% at 4-year follow-up. Those that were on partial sick leave at departure were more likely to work during follow-up, compared to those that were on full-time sick leave, but this was only during the first years, and the confidence intervals were wide. Those receiving partial disability pensions at departure were more likely to work, compared to those on full-time disability pension. However the numbers of patients on these benefits at departure was very low, thus these results should be interpreted with caution. For those receiving medical rehabilitation allowance or vocational rehabilitation allowance at departure, it was an increased probability for working over time.

There was an average of 3.7 transitions between different benefits and working, during the 4-year follow-up. The median number of transitions was 3.0. The minimum number of transitions for a single patient was zero and the maximum number was 18 transitions. 10 individuals had no registered transitions after leaving the rehabilitation clinic, thus they kept the same benefit as they had during their stay at the rehabilitation clinic. 4 of these were registered with vocational rehabilitation allowance and two were receiving disability pension. The last four were not registered on any benefit; thus they were defined as if they were working from the day they left the rehabilitation clinic.

For full-time sick leave 61% of the transitions were to work and 31% of the transitions were to medical or vocational rehabilitation allowances (see numbers in Table ​Table4).4). From partial sick leave a total of 86% of the transitions were to work. The main pathway to permanent disability pension was through rehabilitation allowances; a total of 54% transitions, while for partial disability pension the main pathway was from work; 45% of the transitions, and from partial sick leave; 35% of the transitions.

Our findings are in line with other studies showing that RTW can be a long and resource consuming process [19,25]. It also emphasises the need for analyses of multiple, longitudinal and repeated events for a better understanding of the RTW process [3,5,26]. RTW is not an absorbing state and cannot be measured at a single point of time [25,26]. Multi-state models, as used in our study, can be a useful method to analyze longitudinal RTW data. However there is no clear agreement about how long of a follow-up period is needed to get the best measurement of the effect on work and benefits, after sick leave and work-related interventions [27,28]. Our data indicate that several years are needed to get an adequate picture of the RTW outcome. Differences in design, populations and follow-up period in RTW studies make it difficult to compare results between studies [1,27,28].

Our data also demonstrates the importance of including information on partial sick leave benefits and part-time work, when evaluating RTW after rehabilitation programmes [29]. This is in accordance with recommendations in research on sick leave [1,5,19]. For individuals on long-term sick leave, with limited ability to function, working part-time may be a sufficient goal after rehabilitation, especially when the alternative would be full-time disability pension. Since work ability is not a fixed either/or category, but changes over time, and with varying circumstances, working part-time may also play a role in transitions to full-time employment [30]. It may therefore be important to have a flexible working life, giving opportunities for part-time work for workers with temporary low work ability [30].

We found an increased probability for working after receiving either medical or vocational rehabilitation allowances, at the time of departure from a rehabilitation clinic. This finding is in accordance with the idea behind the legal regulations for sick leave benefits in Norway. Medical rehabilitation allowances should only be granted to individuals with treatable medical conditions, whereas vocational rehabilitation allowance is restricted to individuals with an expected likelihood for RTW after e.g. work training or professional re-education. After the first year on sick leave benefits, medical and vocational rehabilitation allowances constitute a secure source of income for long-term sick workers in Norway. These allowances are equivalent to the sickness benefits system in Sweden before 2010, where sick leave benefits could be expanded beyond the first year in certain medical cases [5].

However receiving such benefits has been claimed to be a step toward disability pension, indicating that RTW is difficult to achieve after more than one year out of work. In Sweden the sick leave track record was found to be the most important predictor of the probability of being granted a disability pension [31,32]. The probability of being granted disability pension following sick leave often begins with short term sick leave periods followed by episodes increasing in length, and with shorter and shorter intervals between them [31]. Few studies have explored outcomes after such allowances in Norway, but Landstad and co-authors [33] found that only 27% returned to work after receiving medical rehabilitation allowance. RTW was, among other factors, associated with prior work life affiliation, contact and satisfaction with the work place during rehabilitation, and experiencing a high degree of influence over their own rehabilitation process [33]. Even though this was not an intervention study, the findings may have relevance in the interpretation of the optimistic RTW outcomes in our study. The rehabilitation program emphasized client participation and contact with the workplace, and the final part of the intervention was to agree on a RTW plan. Workplace engagement, in terms of coordination and cooperation between client, employer and different stakeholders involved in the RTW process is stated to be crucial for success in re-integrating individuals on sick leave in working life [3,20,34-36]. Offering adequate rehabilitation efforts may alter a negative trend out of working life when it is given to the right type of patients.

Furthermore we found an increase in medical and vocational rehabilitation allowance and a simultaneously decrease in sick leave benefit during the first months after the rehabilitation program. This may be interpreted as an effect of the social security legislation on time limits for sick leave benefit to maximum one year continuously. As the participants on average already had been out of work for 9months before the rehabilitation stay, some were about to cross the time limit at departure from the clinic. These legislations, and the decrease in social security disbursement to 66% of the salary after one year, may also be an important incentive to RTW.

The current study adds to previous literature by a detailed analysis exploring the RTW process and pathways back to and out of work after work-related rehabilitation. To our knowledge, a multistate model on sick leave data has not earlier included this amount of possible outcomes. Lie et al. (2008) restricted their analysis to three different states, in their follow-up of low back pain patients in Norway [1]. Analysis on register data in the Nordic countries has been restricted to sick leave benefits and disability pension [1] and to unemployment benefits [32].

The use of social security benefits and RTW is a frequent issue in public health research, irrespective of nationality [31,37]. Comparison between studies may be difficult due to the large variation between countries in the regulation of sick leave compensations and the granting of disability pension [31]. Within the Nordic countries the social security system, representing The Nordic welfare model, is more similar, and makes comparison between the Nordic countries more suitable [5,27,37,38]. These countries are facing the same challenges towards undesirable high rates of sick leave and disability pension [5]. The countries are also somewhat similar in the access to official registers, making it possible to follow individuals on sick leave through the social security system [31]. Our findings may therefore be generalized to the Nordic countries.