ASSESSMENT AND COMPARISON OF RESPONSIVENESS OF FOUR PATIENT REPORTED OUTCOME MEASURES TO ASSESS PHYSICAL FUNCTION IN KNEE OA: WOMAC-PF SUBSCALE RESPONDS BEST

Background: Although physical function is one of the core outcome domains in knee OA, the ability of a measurement instrument to detect changes over time in the construct (physical function) being measured, i.e. the responsiveness, has never been tested as currently recommended by the Consensus-based Standards for the selection of health status Measurement Instruments (COSMIN). Objectives: The aim of the current study was to compare the responsiveness of four disease-specific patient related outcome measures (PROMs) of physical function for patients with knee osteoarthritis receiving conservative treatment. Methods: Consecutive patients fulfilling the clinical ACR criteria for knee OA visiting our specialized knee OA outpatient clinic were invited to complete questionnaires at baseline and 3 month follow-up. Physical function was measured with four PROMs: the Lequesne Algofunctional index (LAI), Lower Extremity Functional Scale (LEFS), Knee Injury and Osteoarthritis Outcome Function Short Form (KOOS-PS) and Western Ontario and McMaster University Osteoarthritis Index Physical Function subscale (WOMAC-PF). Responsiveness was investigated by testing a priori defined hypotheses formulated by an expert group in analogy to construct validity. These a priori defined hypotheses addressed expected correlations between changes in physical function with changes in other (un)related measures (pain, fatigue, self-efficacy, coping, anxiety, depression and mental health) or expected differences in correlation in changes between groups. The expert group reached consensus on 15 a priori defined hypotheses and responsiveness was considered positive if >75% of the hypotheses were confirmed. Results: Of the 161 included patients, 61% was female with a mean age of 59.0 years (SD 9.3) and BMI of 29.7 kg/m (SD 5.0). The majority (n=129, 80.6%) remained stable and 14 patients (8.8%) indicated (very) much worsening in their physical function assessed by the transition question after 3 months. The 17 patients (10.6%) who indicated (very) much improvement, showed significant mean improvement in physical function after 3 months in all four disease-specific PROMs compared with baseline (p<0.05). We could confirm 12 out of 15 of the a priori defined hypotheses (80%) using the physical function subscale of the WOMAC. For the LAI, LEFS and KOOS-PS, respectively 10 (67%), 11 (73%) and 11 (73%) hypotheses were confirmed respectively. Conclusions: This is the first study that comprehensively assessed and compared the responsiveness of physical function measured with LAI, LEFS, KOOS-PS and WOMAC-PF according to the latest standards. Our results suggest that the WOMAC-PF is potentially better able to detect changes over time in physical function than the LAI, LEFS and KOOS-PS in a population of patients with knee OA receiving conservative treatment. We therefore recommend that clinicians and researchers should prefer the WOMAC-PF subscale in future clinical trials to evaluate the effectiveness of (conservative) treatment. References: 1. Mokkink LB, Terwee CB, Knol DL, Stratford PW, Alonso J, Patrick DL et al. The COSMIN checklist for evaluating the methodological quality of studies on measurement properties: a clarification of its content. BMC Med Res Methodol 2010; 10: 22 Disclosure of Interest: None declared DOI: 10.1136/annrheumdis-2015-eular.4922Citation: Annals of the Rheumatic Diseases, volume 74, supplement 2, year 2015, page 1185Session: Osteoarthritis (Abstracts Accepted for Publication )