Background The impact of ankylosing spondylitis (AS) on a patient’s life can be considerable. The International Classification of Functioning, Disability and Health (ICF) is a model to systematically classify and describe functioning, disability, and health in human beings. However no ICF-based patient-reported outcome measure has been developed.
Objectives To develop a measure to assess health in patients with AS, the ASAS (Assessments of SpondyloArthritis international Society (ASAS)) Health Index, based on the Comprehensive ICF Core Set for AS.
Methods Development has beenperformed in five phases: Ia Preparatory: Development of an item pool representing the categories of the Comprehensive ICF Core Set for AS, Ib Patient meeting: Patient preference and weighting of the items per ICF category, II 1st postal patient survey: Item reduction (within and across ICF categories), III Expert consultation: Agreement on item reduction, IV 2nd postal survey: Validation of the draft version and further item reduction, V consensus meeting: Agreement on a final version.
Results Phase I: The item pool contained 251 items in 44 categories by linking existing items from over 60 questionnaires to the comprehensive ICF core set for AS. Phase II was performed based on data collected along an international cross sectional study among 1915 AS patients (mean age 51.2±3.6, 53% male, BASDAI 5.5±2.4) in 4 continents. For 82 items of the functioning part a unidimensional scale, fit to the Rasch model and absence of Differential Item Function (DIF) could be confirmed. Phase III: An expert committee selected 50 functioning items using predefined selection criteria. Phase IV: An international cross sectional study with 628 AS patients (mean age 48.5±14.2, 51.6% male, BASDAI 5.6±2.3) was conducted in 4 continents. Misfit was identified in 4 items and DIF in 15 items. More than 50% of the items showed a residual correlation between each other above a value of 0.2 in the initial round. Phase 5: Based on results of the analyses in step 4, the consensus members agreed on 17 final items. In the 17 items fit to the model, no residual correlation and absence of constant DIF could be confirmed with a Person-Separation Index of 0.82. The item location has been shown to be 0.00 ± 1.84 with a fit residual of 0.06 ± 1.24 and the person location has been shown to be 0.01 ± 1.80 with a fit residual of -0.30 ± 0.67. The chi-square probability was 0.73.
Conclusions The ASAS HI measure contains 17 dichotomous items addressing categories of pain, emotional functions, sleep, sexual functions, mobility, self-care, community life and employment. In covering much of the ICF Core Set for AS, these items represent a whole range of abilities of patients with AS. The questionnaire will be translated and further tested for sensitivity to change. ASAS HI can be used in clinical trials and clinical practiceas a new composite index that captures relevant information on the health status of the patients.
Disclosure of Interest None Declared
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