Even though the process of disability evaluation varies between countries, medical work capacity evaluations usually play a crucial role in deciding on a claimant’s eligibility for benefits provided by national disability insurance schemes. Because of the key role they play, such evaluations ought to be transparent and comprehensible for all persons involved [1–4]. To enhance transparency and comprehensibility, the claimant’s lived experience in relation to his or her functioning as well as with regard to influencing contextual factors should be assessed comprehensively [2, 5]. Moreover, the evaluations’ comparability in terms of both interrater reliability between medical experts and content validity is considered as an important quality criterion [6–8]. Finally, standardization is seen as one means to ensure comparability in disability assessments [9, 10].
Medical standards usually refer to features which are considered as relevant to a target group in general and less so to individuals’ unique experiences [11, 12]. As a basis for comprehensive disability evaluations, however, a suitable standard should also allow the description of relevant experiences unique to the individual, thus complementing the whole process of evaluation .
In reality, decisions on work capacity often lack transparency and comprehensibility [10, 13–15]. Also, disability assessments are often insufficiently standardized [5, 16, 17], which affects their content validity and interrater reliability negatively [8, 9, 17]. In the Swiss national disability insurance scheme, for example, there is no generally accepted tool to guide the structure and content of disability evaluations . Furthermore, decisions on work capacity for certain disorders are partly based on blanket rulings by the Swiss Federal Court . Somatoform pain disorders, for instance, do generally not lead to incapacity for work. Because they are considered to be caused by psychosocial factors, the Swiss Social Security law does not recognize them as a sufficient reason for a disability pension, except if they are accompanied by a psychiatric co-morbidity like, for example, a depressive disorder . By contrast, pain disorders caused by structural impairments (e.g. by a severe intervertebral disc disorder) normally entitle a person to receive disability benefits. However, diagnoses or impairments, are only loosely connected with functional limitations at work [19–21]. Moreover, the World Health Organization defines impairment as a loss or abnormality of a psychological, physiological, or anatomical structure or function and disability as a restriction or lack of ability to perform an activity in a manner considered to be normal for a human being . Based on these definitions, focusing only on impairments is not sufficient to give a proper statement about a claimant’s functional capacity at work.
Because pain is a subjective sensation, its impact on a claimant’s functional capacity is difficult to objectify. Claimants with somatoform pain disorders could have the same or even a lower functional capacity than persons with a disorder related to a structural impairment. Nevertheless, according to Swiss jurisprudence their work capacity is usually rated higher. With respect to this controversy between the medical and the legal view, it seems crucial to apply a disability-oriented approach and to comprehensively assess the aspects which might influence a claimant’s functioning and health in order to ensure transparent disability evaluations for persons with chronic pain.
Several attempts have been undertaken to enhance transparency and standardization in disability evaluations . The Guides to the Evaluation of Permanent Impairment of the American Medical Association (AMA) are used for disability and impairment assessment and as a standard for workers’ compensation evaluations in the United States and many English-speaking countries . Furthermore, a number of standardized procedures for work capacity assessments have been developed like, for example, the Functional Capacity Evaluation (FCE) [25–27].
FCE, however, is not appropriate for multidisciplinary assessments as it is not geared towards a comprehensive evaluation of the claimant’s functioning. It focuses on physical functional limitations and not on mental functioning , and it does not address environmental factors, an important component to ensure transparency in disability evaluations [5, 28]. The AMA Guides have been questioned regarding their applicability in disability assessments of claimants with chronic pain , because they follow a diagnosis-based and impairment-oriented rather than a disability-oriented approach .
As part of the shift in recent years from impairment-oriented to disability-oriented assessments in European social security institutions, it has been suggested that the comprehensive conceptual framework and standardized taxonomy of the International Classification of Functioning, Disability and Health (ICF)  could improve the disability determination process [16, 31–33]. Since the ICF offers a scientific basis for describing results and determinants of functioning, disability and health which also considers contextual factors , standardization and transparency in disability evaluations might be enhanced if the taxonomy would be used as a blueprint.
While the ICF framework was generally well-received, the actual application of the taxonomy has been hampered by the sheer number of categories to be assessed, i.e. 362 on the second level and up to 1,424 when applying the more detailed third and fourth levels. Consequently, ICF Core Sets (henceforth ICF-CS) have been developed in order to simplify the use of the taxonomy in clinical settings.
ICF-CS preserve the model of the ICF in a useable mode, and they come in two flavors: (1) brief ICF-CS include a minimum number of categories describing the most relevant aspects related to functioning in persons with a specific health condition or in a specific setting ; (2) comprehensive ICF-CS include all categories of the respective brief ICF-CS but also additional ones so as to facilitate multidisciplinary assessments in the clinical context .
Because they involve high costs and time resources of medical experts are limited, medical work capacity evaluations should not only be transparent but also efficient and practical . ICF-CS allow to describe a person’s lived experience in a comprehensive and systematic way , and might be applied as practical standards regarding what should be documented in disability assessments. So far there have been only few attempts to examine the applicability of ICF-CS in disability evaluations [16, 37]. To ascertain their potential it is, therefore, vital to provide further empirical evidence.
Currently ICF-CS exist for about 30 health conditions . The ICF-CS for chronic widespread pain (CWP)  and low back pain (LBP)  were published in 2004 and subsequently validated in the clinical context [41–43]. Due to the high prevalence of disability claims and large social costs based on CWP and LBP [44–47], we chose them as our index conditions. Both conditions are also often diagnosed concurrently .
Moreover, CWP has been found to be related to depression  and chronic LBP to obesity . Such co-morbidities are routinely addressed in disability assessments of claimants with chronic pain. We, therefore, also included in our analysis the ICF-CS for depression  and obesity .
The objective of the study was to establish whether or not and how the relevant content of medical work capacity evaluations can be captured by ICF-CS, using medical reports from disability claimants with the index conditions CWP and LBP as examples.
(1) We wanted to examine to what extent the relevant aspects of functioning and environmental factors in medical reports of claimants with CWP and LBP are represented by applicable ICF-CS. (2) We wanted to determine by which ICF-CS, or combinations thereof, these aspects are best represented.