Objective: To compare in patients with fibromyalgia (FM) utilities derived by rating scale and standard gamble methods; to gain insight into construct validity by relating utility values to other outcome measures; to assess the sensitivity to change of utilities.
Methods: A total of 73 patients with FM were randomized into one of 3 groups: low impact fitness training, biofeedback, or controls. At baseline and after 6 mo the Maastricht Utility Measurement Questionnaire was applied. By means of both the rating scale and standard gamble method patients were asked to value their own health status. Construct validity of patient utility measurements was evaluated by Spearman correlation and multiple regression of baseline values with pain, stiffness, patient's global assessment, Sickness Impact Profile (SIP), modified Health Assessment Questionnaire and Arthritis Impact Measurement Scale (AIMS). Sensitivity to change was assessed against changes in these outcomes.
Results: Rating scale utilities correlated significantly (p < 0.05) with patient's global assessment (rs = 0.53), pain (rs = -0.47), SIP (rs = -0.43), and with 9 of 11 dimensions of the AIMS (rs ranging from 0.23 to 0.62). Standard gamble utilities correlated significantly with mobility, pain, and arthritis impact of the AIMS scale (rs from 0.22 to 0.36) and with pain by visual analog scale (rs = -0.24) and patient's global assessment (rs = 0.32). Multiple regression analysis showed that patient's global assessment explained 41% (rating scale) and 10% (standard gamble) of total variance in baseline utilities. Also, 16% of the variance in change in rating scale utility values was explained by changes in patient's global assessment. In contrast, variance of changes in standard gamble utility values was not explained significantly by changes in other disease outcomes.
Conclusion: Rating scale utilities correlated more strongly with disease outcome measures than standard gamble utilities. Also, construct validity for the rating scale was better than for the standard gamble. In FM, utility measurement is sensitive to the method chosen to elicit patient priorities.