USE OF A DISEASE-SPECIFIC INSTRUMENT IN ECONOMIC EVALUATIONS: MAPPING THE WESTERN ONTARIO AND MCMASTER UNIVERSITIES OSTEOARTHRITIS INDEX (WOMAC) TO THE EQ-5D

Purpose: To map the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) onto the EQ-5D in patients with knee osteoarthritis (OA).

Method: A consecutive sample of patients (n=258) completed the WOMAC and the EQ-5D. Regression models with the ordinary least squares (OLS) or the censored least absolute deviations (CLAD) as the estimator were used to establish the mapping function. Different presentations of the WOMAC scores were the explanatory variables. Goodness-of-fit criteria included mean absolute error (MAE) and root mean squared error (RMSE). An iterative random sampling procedure was used to account for variability in these goodness-of-fit diagnostics. Prediction precision was evaluated at both individual and group levels. At individual level, the prediction error was computed using the difference between observed and predicted EQ-5D scores for each of the 258 patients. At the group level, the prediction error was estimated by applying a non-parametric bootstrapping with replacement method. Specifically, various group sizes of patients (n=50, 100, 200, and 400) were randomly sampled. A patient was randomly chosen from the original dataset and his/her predicted EQ-5D score and prediction error were recorded. This process was repeated until the sample size of each group was reached. For each group, mean predicted EQ-5D scores and mean prediction error were calculated which formed one bootstrapping replicate. By repeating the above-mentioned process 5000 times, we generated a distribution for group mean predicted EQ-5D scores and corresponding group mean prediction errors for each of the groups. The 2.5^th and 97.5^th percentiles of the distribution were therefore used to estimate 95% CI for the prediction error.  

Result: The model using OLS estimator and WOMAC domain scores as the explanatory variables was chosen as the preferred model: EQ-5D = 0.83414 - 0.00166×WOMAC pain score – 0.00092×WOMAC stiffness score – 0.00330 × WOMAC function score The prediction error at individual level exceeded the maximal tolerance value (i.e. the minimally important difference of the EQ-5D) in about 16% of the patients. At the group level, the width of the 95% CI of prediction errors varied from 0.0176 at a sample size of 400 to 0.0359 at a sample size of 100.

Conclusion: EQ-5D scores can be predicted using WOMAC domain scores with an acceptable precision at both individual and group levels in patients with knee OA.