Item response theory validation of the Oxford Knee Score and Activity and Participation Questionnaire: a step towards a common metric.

OBJECTIVE: The Oxford Knee Score (OKS) and OKS Activity and Participation Questionnaire (OKS-APQ) are patient reported outcome measures used to assess people undergoing knee replacement surgery. They have not explicitly been tested for unidimensionality (whether they measure one underlying trait such as 'knee health'). This study applied item response theory (IRT) to improve the validity of the instruments to optimise for ongoing use. STUDY DESIGN AND SETTING: Participants undergoing primary total knee replacement (TKR) provided pre-operative and post-operative responses for OKS and OKS-APQ. Confirmatory factor analyses (CFA) were performed on the OKS and OKS-APQ separately, and then on both when pooled into one. An IRT model was fitted to the data. RESULTS: 2972 individual response patterns were analysed. CFA demonstrated that when combining OKS and OKS-APQ as one instrument, they measure one latent health trait. A user-friendly, free-to-use, web app has been developed to allow clinicians to upload raw data and instantly receive IRT scores. CONCLUSIONS: The OKS and OKS-APQ can be combined to use effectively as a single instrument (producing a single score). For the separate OKS and OKS-APQ the original items and response options can continue to be posed to patients, and this study has confirmed the suitability of IRT-weighted scoring. Applying IRT to existing responses, converts traditional sum scores into continuous measurements with greater granularity including individual measurement error.