Validation of a numerical rating scale to assess functional impairment in hip and knee osteoarthritis: comparison with the WOMAC function scale

Objectives (1) To investigate the psychometric properties of a patient-reported numerical rating scale (NRS) for evaluating functional disability in osteoarthritis (OA), in comparison with the WOMAC function scale and with a physician-reported function NRS; (2) to estimate the patient acceptable symptomatic state (PASS) and the minimal clinically important improvement (MCII) values for treatment with non-steroidal anti-inflammatory drugs (NSAIDs). Methods Data were extracted from a prospective multicentre study involving 1186 patients with knee or hip OA. The psychometric properties assessed were feasibility : percentage of responses, floor and ceiling effects; construct validity by examining the correlations with classically used OA outcomes measures; responsiveness by comparing the results of before and 1 month after treatment with NSAIDs using standardised response mean (SRM) and effect size (ES). The MCII and PASS values of each function scale were calculated by an anchoring method. Results No floor or ceiling effect was observed. High correlations were observed as expected between the patient NRS and WOMAC function, pain visual analogue scale and patient global assessment. The responsiveness was moderate to large, with SRM and ES ranging from 0.6 (hip OA) to 0.9 (knee OA) and higher than that of the WOMAC function scale. The PASS was close to 3 for the NRS scales. The MCII appears to be the change that makes the OA functional disability decrease from baseline to the PASS. Conclusion The patient-reported NRS demonstrated good psychometric properties, similar to the WOMAC function scale and can be regarded as a promising tool in therapeutic evaluation and decision-making in OA.