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.
Introduction
The concept of utility in health was developed by economists to explain individual choices and preferences for health states . The health-state utility score ranges from 0, for ‘death’, to 1, ‘perfect’ health. Utility methodology in assessing health-related quality of life (HRQoL) allows for comparing different treatment strategies for one or more diseases by a single unit of measurement and may be used in medico-economic evaluations for allocating resources .
There are two approaches for estimating utility in health: direct or indirect. Direct measurement usually involves face-to-face interview methods such as the standard gamble (SG) and the time trade-off methods (TTO) . The utility by SG is calculated from the risks (of death usually) that the patient would take to improve the health state . The TTO method, developed by Torrance et al. , is a method that determines how many healthy days (months or years) one is willing to trade against a better health state. Indirect measurement, also called multi-attribute utility , involves estimation by generic HRQoL instruments with weighting for the general population for each health state. The complexity of the direct method and its implementation have led to the increasing use of indirect methods because they can be easily and quickly applied. The most popular instruments are the Health Utility Index (HUI) , the EuroQol (EQ-5D) and the Medical Outcomes Survey Short Form 6D (SF-6D) . Evaluation of the health state differs in two ways. First, the method used to determine the weight assigned in the general population to health state varies between instruments: health state values for the EQ-5D are derived with the TTO method while values for the SF-6D and HUI are obtained from the SG. Second, the number of dimensions and levels (variation in descriptive system) are lowest in the EQ-5D, compared to the SF-6D , and highest in the HUI. The SF-6D, obtained from the SF-36, is likely most popular.
Co-morbidities refer to chronic co-occurring disorders in the same individual. The concept was described by Feinstein et al. as “any distinct additional entity that has existed or may occur during the clinical course of patient who has the index disease under study”. Co-morbidities are inversely and negatively correlated with HRQoL . As indirect utility measurement involves HRQoL, it should consider co-morbidities, which can affect the measurement and subsequent utility assessment. The treatment of a related disorder can be influenced by a chronic condition and therefore disturb the management of disease .
Osteoarthritis (OA) is a chronic and frequent degenerative joint disease influencing QoL and incurs important economic burden . As it develops with ageing, patients with OA have a risk of developing co-morbidities . QoL and utility are associated linearly in OA as in other illnesses – with decreased QoL, utility decreases and vice versa – and as for QoL, the range of utility values varies by co-morbidity . Therefore, QoL with OA can be influenced by co-morbidities, which significantly affects utility measurement by a multi-attribute instrument. Whether the relation of utility and co-morbidity is of the same magnitude as with QoL and co-morbidity is of interest.
Multi-attribute utility involves quality-adjusted life years resulting from utility weighting of life expectancy and is generally used in cost-utility analysis to compare the differential costs of strategies related to differential utility among those strategies. In a literature review conducted using the terms ‘comorbidity’ and ‘multimorbidity’ with ‘utility’, we found only one study that analysed the impact of co-morbidities on utility scores for patients with OA . Studies have focussed on the QoL of patients with OA without taking into account the impact of multimorbidities or co-morbidities in utility measurement . The possible influence of co-morbidities on the condition of individuals and the cost of illness may affect utility measurement and cost-utility analysis.
We aimed to determine in a cross-sectional study the effect of co-morbidities in utility assessment by the Medical Outcomes Study SF-6D of OA patients.
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