Objective: Preference-based measures, such as the Short Form-6D (SF-6D), allow quality-adjusted life-years, used in cost-utility evaluations, to be calculated. We investigated the construct and criterion validity of the SF-6D in patients with systemic lupus erythematosus (SLE).
Methods: Female patients with SLE were recruited from outpatient clinics at 2 timepoints, 5 years apart. Cross-sectional correlation of the SF-6D with domains of the disease-specific LupusQol health-related quality of life (HRQOL) measure, the Systemic Lupus International Collaborating Clinics Damage Index (SDI; for damage) and Systemic Lupus Erythematosus Disease Activity Index (SLEDAI; for activity) measures, and patient characteristics was tested. The ability to discriminate between groups defined by smoking status, presence/absence of carotid plaque, depression, and fatigue was tested using the t-test.
Results: In total 181 patients were recruited at baseline. The SF-6D correlated moderately to strongly with all domains of the LupusQoL (0.6-0.8) apart from intimate relationships (0.42) and body image (0.34). Correlations of the SF-6D with the demographic and disease-specific measures at baseline were small for the SDI score (-0.23) and age (-0.19) and in the expected direction. The SF-6D did not correlate with disease activity (SLEDAI -0.08). The SF-6D could distinguish those who smoked, had carotid plaque, had depression, and reported fatigue from those who did not, with the largest effect size being for depression (0.75).
Conclusion: The SF-6D displays construct and criterion validity for use in patients with SLE, but the low correlation with aspects of intimate relationships and body image represents a concern and reinforces the need to collect disease-specific measures of HRQOL alongside generic preference-based instruments.