EXTENDED REPORT ICF based comparison of disease specific instruments measuring physical functional ability in ankylosing spondylitis

Objectives: To link validated and widely used instruments measuring physical functional ability in ankylosing spondylitis to the International Classification of Functioning, Disability, and Health (ICF) and to compare their contents, based on the results of the linking process. Methods: The Bath Ankylosing Spondylitis Functional Index (BASFI), the Dougados Functional Index (DFI), the Health Assessment Questionnaire modified for the spondylarthropathies (HAQ-S), and the Revised Leeds Disability Questionnaire (RLDQ) were linked to the ICF separately by two trained health professionals according to 10 linkage rules. Results: All concepts contained in the items of the selected instruments could be successfully linked to the ICF except for ‘‘illness’’ included in the HAQ-S. Altogether 55 different ICF categories were linked. Seven belonged to ‘‘body functions’’, 43 to ‘‘activities and participation’’, and five to ‘‘environmental factors’’. The component ‘‘body structure’’ was not contained in any of the four instruments. Only two ICF categories were common to all selected questionnaires, but there was a high level of concordance on the concepts represented in them. However, especially in terms of ‘‘activities and participation’’, the emphasised aspects differed. Conclusions: The ICF provides an excellent common framework for the comparison of disease specific instruments for ankylosing spondylitis. For a future revision of the ICF, a specification of major limitations in patients with ankylosing spondylitis is suggested.