While several instruments and measures are available to assess function and mobility, there was no exhaustive list of impairments, limitations and restrictions that are the consequence of ankylosing spondylitis (AS). The International Classification of Functioning, Disability and Health (ICF) facilitates agreement on a comprehensive description of aspects of functioning that are relevant and typical for a specific disease by using ICF categories. The Comprehensive ICF Core Set for AS is the selection of 80 ICF categories that are typical and relevant for AS. Physical functioning and mobility have an essential but partial role in the broad view of functioning and health in AS. Consistent with the bio-psycho-social model, the ICF Core Set for AS also recognises the role of contextual factors, either environmental or personal, when understanding functioning. This new reference for functioning is now available for clinical practice and research. It can help to increase insight into the complexity of functioning and can serve as the starting point for the development of new instruments that address either global functioning or aspects of functioning.
For many centuries the major challenges of medical research were the recognition of disease entities , later described in classification criteria, and the understanding of disease in terms of aetiology and pathophysiology . Initially, observation was the major tool physicians used to describe diseases but substantial progress was made when tests were developed to measure physiological and pathophysiological processes. An obvious example is microbiological analysis, which enabled isolation of pathogenic micro-organisms, laboratory testing that made it possible to detect disturbance and failure of different organ systems or imaging techniques that helped to assess organs not visible from the outside. However, it was only in the second half of the twentieth century that the consequences of disease on the functioning and health of the patient received research interest. This was boosted not only by the need to evaluate the effectiveness of new treatments, but also by the increasing empowerment of the individual and the attention to the quality of life of ill persons. Impact on functioning and health became a major issue, especially in chronic rheumatological diseases, where the impact on functioning is more important than that on life expectancy. In rheumatoid arthritis (RA), the first functional classification was introduced in 1948 by Steinbrocker and the health assessment questionnaire (HAQ) was proposed by Fries in 1982 . In the last two decades of the 20th century, several other patient reported outcome measures for functioning were developed and these were either disease specific or generic , or addressed specific aspects of functioning such as hand function in RA . However, the different instruments were not based on a common theoretical framework of functioning and health. Along with the improvement in worldwide mortality rates, the World Health Organisation (WHO) recognised the need for a model and information system to describe the impact of disease on functioning. In 2001, the International Classification of Functioning, Disability and Health (ICF) was endorsed by the World Health Assembly as the universal framework and classification .
The ICF belongs to the larger family of international classifications endorsed by the WHO among which the International Classification of Diagnoses (ICD) is the best known. The ICF framework adheres to the bio-psycho-social model of disease and recognises that functioning and health results from a complex interplay of the health components, body functions and body structures and activities and participation, which can be influenced by contextual factors that consist of environmental and personal factors ( Fig. 1 ). In addition to the framework, the ICF also offers a universal and hierarchical classification of functioning by means of so-called categories that can be seen as the units that are necessary to classify functioning .
It should be noted that the ICF aims to address the extent to which a person’s ‘ability’ is affected, which is a different construct than the ‘importance’ of that impact to the individual and different from preferences in situations of choice (decisions). Although interesting, the exact relation between these constructs is still a subject of debate and research ( Fig. 2 ).
The strength of the ICF framework and classification is that it can be applicable for different stakeholders, including the different types of health professionals, researchers, decision makers and last, but not least, the patients. One of the advantages of the ICF is that it offers a universal language to describe the impact of any condition. It needs to be emphasised that the ICF classification primarily describes ‘what to measure’ and not ‘how to measure’. This article describes the development of the ICF Core Sets for AS, discusses possible applications and reviews instruments to assess physical functioning and spinal mobility in the framework of the ICF.
ICF framework, ICF classification and ICF core sets
Functioning is a broad construct, going far beyond physical function and mobility. Following the ICF framework, functioning is understood as the umbrella concept covering body functions and structures, activities and participation . Functioning is not a fixed state but the result of a dynamic interaction between the health condition, the environment and the person. Full functioning is achieved if there are no health-related problems. This would be the case if, for example, early and effective treatment in a person with AS successfully prevents structural impairment, minimises pain and stiffness and enables the person to perform a full range of activities and participations. Vice versa, disability, defined as the negative connation of functioning, would be present if there was structural impairment, pain, stiffness and limitations in activities and participation despite medical and rehabilitative management, including the management of environmental and personal factors. The relationships between impairments, activity limitations, and participation restrictions can be bidirectional and can be influenced by contextual factors that consist of environmental factors such as availability of healthcare or social support systems and personal factors such as age, gender, health beliefs or coping.
The ICF classification comprises 1545 categories divided over the four ICF components (1) body functions (psychological and physiological functions), (2) structures (anatomical parts), (3) activities (execution of tasks) and participation (involvement in a life situation) and (4) environmental and personal factors. The definition of each component is listed ahead and can be seen in Box 1 . :
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