The ASAS health index (ASAS HI) - a new tool to assess the health status of patients with spondyloarthritis

Within the variable course of ankylosing spondylitis (AS), peripheral arthritis, enthesitis, and involvement of other organs can add to the burden of the disease. The primary complaints of patients with spondyloarthritis (SpA) are pain, stiffness, fatigue, and limitation in activities and social participation. Instruments currently available for the assessment of patients with SpA focus predominantly on specific aspects of health such as pain, disease activity, and physical function and measure specific concepts like physical function and health-related quality of life (HR-QoL). However, the overall picture of impairments, limitations and restrictions in activities or social participation of patients with AS is not adequately assessed in SpA-specific questionnaires. Most of the existing questionnaires are not conceptualised with regard to their underlying construct. The International classification of functioning, disability and health (ICF) Core Set for AS may serve as an appropriate model and underlying construct to develop a health index, since the whole range of functioning and disability of patients with AS is captured. Based on these assumptions, ASAS developed for patients with SpA an instrument assessing health as operationalised by the ICF. The questionnaire was developed by preparing an item pool, linkage of the items to the comprehensive ICF core set for AS and test of the item pool in two cross-sectional studies. The analysis of the questionnaire and the response scale were done with Rasch analysis. Emphasis was on optimal targeting, the capacity of items to differentiate between different levels of health, and optimal coverage of items to the spectrum of ICF categories, so that the final questionnaire could represent as much of the entire range of difficulty levels as possible. The ASAS HI is a linear composite measure and includes 17 items which cover most of the ICF core set. Preliminary validity has been confirmed in a field test in 4 English-speaking countries. The ASAS HI should soon be used in clinical trials and in clinical practice to test its real life performance and to confirm that this new composite index captures relevant information on functioning and health of patients with AS.