With regard to the consultation load and most of the sickness certification tasks, the OHPs experienced no difference between consultations with sick-listed employees with severe MUPS and consultations with employees with less severe MUPS. Our findings are remarkable, because employees with severe MUPS present more psychiatric co-morbidity and have more functional limitations, as has been demonstrated in many other studies [5, 18, 19] Barsky et al.  found that MUPS were associated with functional limitations and more medical consumption, and psychiatric co-morbidity did not add to this effect. Furthermore, it is remarkable that the OHPs experienced no difficulties in their communication with employees with severe MUPS, unlike the treating physicians . This can be explained, at least partly, by the fact that OHPs in the Netherlands have no treatment tasks, and therefore they are not often pressurised by employees in this respect .
With regard to consultation load and sickness certification tasks, our findings suggest that OHPs manage sick-listed employees with severe MUPS in the same way as they manage sick-listed employees with less severe forms of MUPS.
According to our findings, the most important difficulties that the OHPs experienced were associated with their communication with the treating physician. This is associated with the OHPs' attribution of the physical symptoms to somatoform causes, but from our data the exact nature of these difficulties is unclear. However, difficulties associated with certification tasks can be imagined when employee and/or the treating physicians are not convinced of the somatoform nature of the physical symptoms. Return to work can be delayed if the employee is awaiting the results of diagnostic tests, or is convinced that his/her limitations are the results of a somatic disease with limitations that will last until the disease has been treated adequately.
OHPs report the need for psychiatric expertise when they attribute physical symptoms to psychiatric causes. Contrary to what could be expected, it is not the psychiatric comorbidity that is associated with this need of the OHP. This is an indication that the OHP has diagnostic uncertainties.
A major problem is that employees with severe MUPS remain unrecognized. Thus, the group with the lowest recovery rate and the most functional limitations  is missed, because more physical symptoms indicate a worse prognosis [13, 21]. There are evidence-based guidelines for the management of patients with chronic fatigue and fibromyalgia [22, 23], and one guideline in the Netherlands for medically unexplained physical symptoms . These guidelines emphasize that it is important to rule out somatic and psychiatric causes of the physical symptoms, because the prognosis depends on adequate treatment of these causes. If these causes are not present, the physician should stimulate the patient to stay active in order to prevent unnecessary inactivity, social isolation and job-loss. Guidelines for aspecific back symptoms , which can also be seen as a form of MUPS , promote a time-contingent return to work process to prevent medicalisation and lasting disabilities. Shiels and Gabbay  found that, with regard to sickness certification, the diagnosis is of more importance for the risk of long-term sickness than OHP-related factors. In combination with the under-recognition of severe MUPS, it is relevant to try to achieve more diagnostic accuracy, and this is an important starting point for solving this problem in the field of occupational health.
Strengths and limitations
To our knowledge, our study is the first survey of the impact of sick-listed employees with severe MUPS on OHPs. A strength of the study is that the 43 OHPs were recruited from 5 group practices associated with 2 large occupational health services distributed throughout the country, serving urban and rural populations in different branches and different sized organisations. Another strong point is that the information about MUPS and related aspects was gathered from OHPs and employees by means of validated questionnaires.
However, due to the cross-sectional design of the study, no conclusions can be drawn with regard to causal relationships in our findings.
Self-report questionnaires for the employees and the OHPs were used to address our main research questions. No additional medical check on the MUPS was carried out, and the only diagnoses were those made by the OHPs. As reported elsewhere, we used validated questionnaires, which have high a correlation with somatisation, depression and anxiety diagnoses in primary care and clinical practice . However, we did not ask the OHPs directly whether they diagnosed the symptoms as MUPS, in order to avoid bias in their diagnosis, and because this concept is not well known. This information is therefore missing.
There was no qualitative analysis of the OHP's answers with regard to diagnosis (e.g. when and how they consider the diagnosis of MUPS), task difficulties (e.g. what OHPs consider to be their task with regard to treatment) and their own characteristics (e.g. the differences in opinion among OHPs with regard to their tasks, and between OHPs with varying levels of burnout and engagement).
Implications for practice
Our results indicate that OHPs should be trained in how to diagnose severe MUPS, for two functions: firstly, ruling out somatic and psychiatric causes helps to rule out these disorders which can delay the return to work process. Secondly: the diagnosis of severe MUPS makes it possible to explain to employees why they have severe limitations, on the one hand, and on the other hand why they should increase their activities and continue time-contingent return to work. When improved functioning and return to work are achieved, no additional steps are required.
If uncertainty about the diagnosis and adequate treatment remains, with no improvement in the employees with severe MUPS, OHPs should have easy access to psychiatric expertise and/or psychological treatment. Research shows that liaison consultation of a psychiatrist is helpful [26, 27]. For patients with lasting MUPS, if motivated, cognitive behavioral therapy provided by a psychologist is the recommended evidence-based treatment [11, 12]. Multidisciplinary treatment is indicated for employees with long-lasting non-specific back and neck problems or other forms of MUPS .
Although in the Netherlands many projects have resulted in an improvement in communication between the OHP and the GP, there is still a need for better communication and agreement . As sick-listed employees with severe MUPS are at increased risk for lasting disabilities and health-related jobloss, their OHPs should make extra effort to prevent this as much as possible . Treating physicians have difficulties with regard to increased medical consumption and communicating with these patients. Hopefully, the results of this study will stimulate the boards of especially OHPs and GPs to make extra efforts to achieve better communication concerning patients with severe MUPS.
OHPs and GPs must focus more on the functioning of employees with severe MUPS. This is in accordance with the decision made in the United Kingdom for GPs to write 'fit for work notes' instead of sick notes [29, 30]. This was done because employees with common mental disorders are helped more by returning to (adjusted) work as soon as they can, than by remaining inactive. This is also in line with the existing evidence concerning effective methods of treatment for patients with severe MUPS [11, 12]: cognitive behavorial treatment, graded activity, and communication and casement rules. Cognitive behavorial treatment  helps patients with severe MUPS not by focusing on the (somatic) causes of their symptoms, but on helping them to handle consequences such as inactivity, isolation, etc. Graded activity focuses on a stepwise increase in activity. Communication and casement rules in consultation letters focus on empathy from the treating physician, explaining the findings, and preventing medicalisation, and this policy also results in improvement in physical functioning .
Implications for research
First, in order to formulate criteria for interventions, longitudinal research is needed to investigate the determinants of return to work in employees with severe MUPS, including their psychiatric co-morbidity and the work characteristics.
Additional research is needed to find out how the diagnostic competence of OHPs can be improved. In our opinion, the first step would be to train OHPs (in diagnosis, guidelines and communication) and provide them with diagnostic tools, such as the PHQ. In a second step the quality of the diagnosis can be improved by increasing the possibilities for OHPs to consult a psychiatrist and improving communication with GPs and other attending physicians. Finally, more research is needed to determine how these steps can be made feasible to occupational health care. A good example from primary care is the provision of collaborative care for depressed patients . Qualitative research will also be useful to identify what are the needs of the sick-listed employee with MUPS, in terms of symptoms, functioning, and return to work.
MUPS and associated limitations in functioning lead to a few, but important task difficulties for OHPs: they have diagnostic uncertainty and experience interference in their tasks from the diagnostic activities and treatment provided by treating physicians. Better recognition of severe MUPS and combining improvement of functioning and the return to work process with the diagnostic and treatment activities of the the treating physicians, are aspects that can be improved. There is a need for further training of OHPs, better communication with the GP, and easier access to the expertise of psychiatrists, psychologists and multidisciplinary treatment.