New methodologies allow the scores for the Health Assessment Questionnaire-Disability Index (HAQ-DI) to be translated into preferences/utility scores. We evaluated the construct validity of the HAQ-DI-derived Short Form-6D (SF-6D) score and assessed its responsiveness to change over 6- and 12-month followup periods in patients with early aggressive rheumatoid arthritis (RA).
Patients (n=277) participating in an RA observational study completed self-reported measures of symptoms and the HAQ-DI at baseline and at 6 and 12 months. Total Sharp scores, C-reactive protein, and erythrocyte sedimentation rate were assessed along with clinical data. Construct validity was assessed by examining the association between SF-6D score and patient-reported and clinical measures using Spearman correlation coefficients. The responsiveness of SF-6D to change was assessed using patient and physician assessments of the disease as clinical anchors. The magnitude of responsiveness was calculated using SF-6D effect size (ES).
Mean SF-6D scores were 0.690, 0.720, and 0.723 at baseline and 6 and 12-month followup, respectively. Baseline patient-reported measures had moderate to high correlations with baseline SF-6D (r=0.43 to 0.52); whereas clinical measures had negligible to low correlations with SF-6D (r=0.001 to 0.32). ES was moderate for the groups that were deemed to have improved (ES 0.63-0.75) but negligible to small for those that did not (ES 0.13-0.46).
Our data support the validity and responsiveness of the HAQ-DI derived SF-6D score in an early RA cohort. These results support the use of the HAQ-DI derived SF-6D in RA cohorts and clinical trials lacking preference-based measures.