To develop a formula to translate Western Ontario and McMaster University Osteoarthritis Index (WOMAC) scores collected in clinical trials of patients with osteoarthritis (OA) into Health Utilities Index Mark 3 (HUI3) utility scores for application in economic evaluation.
Data from a previously published open-label randomized controlled trial of appropriate care with hylan G-F 20 versus appropriate care without hylan G-F 20 in 255 outpatients with knee OA. We estimated linear regression models of HUI3 scores using various functions of WOMAC, demographics, and clinical variables. Out-of-sample predictive performance of the models was assessed using the mean absolute error and several other criteria.
The preferred formula included WOMAC pain, stiffness, function subscales, demographic variables; it accounted for almost 40% of the variation in the HUI3 utility scores. At the group level, absolute differences between predicted and actual overall HUI3 utility scores were < 0.001 and not statistically significantly different from zero.
A formula was derived from the WOMAC index to estimate overall utility scores based on the HUI3 for studies of patients with OA for whom utility has not been recorded. Researchers can estimate overall utility scores, compute quality-adjusted life-years, and perform cost-utility analyses within a defined range of certainty.