Patient utilities in chronic musculoskeletal pain: how useful is the standard gamble method?

The main goal of current pain management approaches is to increase the patients' quality of life by improving pain coping skills and by reducing the levels of disability in daily life, often despite persistent pain. Direct measurement of quality of life is of crucial importance in economic evaluation research, in which not only is the estimation of financial costs and benefits included, but so is the evaluation of costs and benefits in terms of changes in health states. The purpose of this study is to compare the psychometric qualities of two instruments for assessing patients' utilities, the rating scale (RS) and the standard gamble (SG). Such instruments are designed for their application in economic evaluation research, but have seldomly been used in chronic pain trials. Both methods provide a single measure between 0 and 1. The relationship between these utility measures and descriptive and domain-specific quality of life measures was examined in 133 fibromyalgia patients and 148 patients with chronic non-specific low back pain. Mean utility score at baseline was 0.43 with the RS and 0.78 for the SG. The correlation between both methods was found to be poor (r = 0.21). Both measures appeared to be fairly stable in a 2-week test-retest period (intra class correlation coefficient (ICC) = 0.74 and 0.77). Scores on the description of patient's own health on six domains, global assessment of change and domain specific measures correlated moderately with the RS scores and low with the SG. Multiple regression analyses demonstrated that 32% of the variance in RS values and only 13% of the variance in SG utilities could be explained by domain-specific measures. These results suggest an acceptable construct validity for the RS but insufficient construct validity for the SG. Valuations of ones own health appear only partially to be related to the assessment of the pain-specific measures and measures of distress. It can be concluded that the RS and domain-specific measures assess partly different, but nevertheless complementary aspects of health-related quality of life. It is therefore recommended to include in economic evaluation studies both domain-specific measures and valuation measures. Finally, in chronic musculoskeletal pain patients, RS scores were found to be more responsive in detecting significant changes in preferences than SG scores. For use in patients with chronic musculoskeletal pain, the RS is preferred to the SG for establishing accurate decisions about the impact of new interventions on their health outcomes.