Impact of co-morbidities on measuring indirect utility by the Medical Outcomes Study Short Form 6D in lower-limb osteoarthritis.

INTRODUCTION: Co-morbidities can influence generic measurement of health indirect utility. We investigated their impact to assess indirect utility with the Medical Outcomes Study Short Form 6D (SF-6D) in patients with osteoarthritis (OA).
METHODS: In patients with hip and knee OA from the Knee and Hip Osteo-Arthritis Long-term Assessment (KHOALA) study, co-morbidities were assessed by the Functional Co-morbidity Index. Multivariate linear regressions were used to determine predictors of utility score.
RESULTS: For the 878 patients included, the mean (standard deviation (SD)) utility score for 808 patients was 0.66 (11; range 0.32-1.00) and mean number of co-morbidities 2.05 (1.58). Number of co-morbidities (beta = -0.30; p = 0.002), psychiatric disease (beta = -0.043; p < 0.0001) and degenerative disc disease (beta = -0.014; p = 0.018) were predictors of low utility score. The WOMAC functional score had a higher significant effect (beta = -0.003; p < 0.0001) and explained a higher percentage of the model variance. DISCUSSION: Compared to greater negative effect of functional severity of OA, co-morbidities have a negative but relatively marginal impact on indirect utility score. This suggests that, clinically, considering the functional severity of OA remains a first priority.