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)1.
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 validity1. 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/m2 (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 standards1. 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.
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