Towards an ICF-based clinical measure of functioning in people with ankylosing spondylitis: A methodological exploration

Objective. To explore whether it is possible to construct clinical measures of functioning for patients with ankylosing spondylitis (AS) by integrating information obtained across categories of the International Classification of Functioning, Disability and Health (ICF). Methods. Sixty-eight ICF categories that were identified as relevant by patients with AS and that covered body functions, structures, and activity and participation were analysed based on the Rasch model for ordered response options. The following properties were studied: unidimensionality, reliability, fit of the ICF categories to the Rasch model, the appropriateness of the order of the response options of the ICF qualifier, and the targeting between the ICF categories and the person's abilities. Results. After accounting for disordered thresholds and misfitting ICF categories, a clinical measure of functioning for AS was proposed that contained 64 ICF categories. On the basis of a transformation table, the raw scores obtained by adding the answers to the 64 ICF categories can be transformed to the Rasch logit scale and to a meaningful interval scale ranging from zero to 100. Conclusion. For the first time, it has been shown that clinical measures of functioning, in principle, can be constructed based on the comprehensive ICF framework covering body functions and structures and activities and participation domains. The results of this investigation are preliminary and must be validated, but they are promising and can contribute to the acceptance and usefulness of the ICF in clinical practice.