Measuring and monitoring health outcomes is of utmost importance in rheumatology and in medicine in general. Yet quantifying health outcomes, such as disease activity, physical function and health related quality of life, is a complex and challenging process.

Composite scores can be particularly useful in measuring health outcomes because they integrate several different aspects of the same outcome into one single numerical value, resulting in a more precise estimate of the outcome than the individual components of the composite score. They also have the potential to increase the statistical power of clinical trials and observational studies. Furthermore, they improve the consistency of patient care across different clinical settings and help patients and doctors better understand the disease and its impact. This does not dispute the fact that there may be times when single construct measures are more appropriate for judging a specific outcome, because the intervention is directed primarily at one construct and not necessarily to produce a more global health change.

The OMERACT (Outcome Measures in Rheumatology) has served a critical role in the development and validation of outcome measures in rheumatology (http://www.omeract.org/). The OMERACT filter (Boers et al. J Rheumatol. 1998 Feb;25(2):198-9) is a set of criteria that can serve as guide to develop valid and meaningful indices: 1) Truth: Is the measure truthful, does it measure what it intends to measure? Is the result unbiased and relevant? This criterion captures the issues of face, content, construct and criterion validity. 2) Discrimination: Does the measure discriminate between situations that are of interest? The situations can be states at one time (for classification or prognosis) or states at different times (to measure change). This criterion captures the issues of reliability and sensitivity to change/responsiveness. Feasibility: Can the measure be applied easily, given constraints of time, money, and interpretability? This criterion addresses the pragmatic reality of the use of the measure, one that may be decisive in determining a measure's success.

Regarding the ability to detect change there are two levels at which it can be evaluated: 1) at the level of statistical significance and 2) at the level of clinical meaningfulness. The first level relates to the concept of “minimal detectable change” (the smallest change detectable between two time points) while the second level relates to the concept of “clinically important change” (the smallest change, which individuals experience, that can be considered important or clinically meaningful). Knowing if the amount of change in an index is clinically meaningful is essential to provide adequate patient care.

The Ankylosing Spondylitis Disease Activity Score (ASDAS) is a new ASAS (Assessment of SpondyloArthritis international Society) and OMERACT-endorsed composite disease activity index developed for axial Spondyloarthritis (Machado et al. Ann Rheum Dis. 2011 Jan;70(1):47-53). Using the ASDAS as a model, this presentation will discuss the approach to the development of valid and meaningful indices to assess health outcomes in rheumatology.

Disclosure of Interest None declared

DOI 10.1136/annrheumdis-2014-eular.6289