Inadequate structural validity of the Oxford Hip Score: Evidence from Rasch and Confirmatory Factor Analysis in a prospective, Danish total hip arthroplasty cohort.

BACKGROUND AND OBJECTIVE: The Oxford Hip Score (OHS) is widely used as a patient reported outcome measure in clinical studies and national quality databases. The content of the questionnaire suggests a potential two-subscale structure, a structure sparsely evaluated. Thus, the aim was to evaluate the validity of both the original unidimensional structure (i.e., one total score) of OHS and a two-dimensional structure reflecting separate "Pain" and "Function" subscales. METHODS: Preoperative and postoperative OHS item scores were available from a prospective cohort of patients undergoing total hip arthroplasty. Eight homogeneous subsamples, each comprising 100 randomly chosen patients with hip osteoarthritis were separately assessed using Confirmatory Factor Analysis (CFA) exact fit and close fit indices, and Rasch analysis item fit statistics. Pooled analyses were added for robustness. Both unidimensional models and two-dimensional models were considered for each subsample. Item thresholds, targeting plots, internal consistency, and floor and ceiling effects were also evaluated. RESULTS: Exact CFA fit was rejected in seven out of eight subsamples for both the unidimensional, two-dimensional structure and pooled data (χ2; p-values <0.007). Close fit indices showed substantial variability across subsamples. Two of eight subsamples reached acceptable thresholds for the unidimensional structure, and three of eight subsamples for the two-dimensional structure (RMSEA [0.031-0.120]; CFI [0.876-0.997]; TLI [0.842-0.996]; SRMR [0.059-0.106]). Rasch analysis consistently identified item misfit for all four preoperative subsamples, and minimal item misfit for the one-year data. Pooling data increased misfit to the models. Substantial ceiling effects were evident at one-year follow-up, inflicting disordered item threshold and poor targeting of the scale for one-year data. CONCLUSION: Consistent fit to the statistical models was not demonstrated for the Danish OHS based on robust psychometric analyses of OHS data from a Danish prospective cohort of patients undergoing primary THA. These findings raise concerns regarding the validity of reporting the OHS as a single total score and separately as two subscales of pain and function.