Comparison of 4 functional indexes in psoriatic arthritis with axial or peripheral disease subgroups using Rasch analyses.

OBJECTIVE: Rasch item response theory analysis is essential in evaluating measurement tools in specific disease cohorts. We compared the performance of 4 functional indexes in patients with psoriatic arthritis (PsA) in axial or peripheral disease subgroups.
METHODS: A cross-sectional study was performed in a single center. Functional outcomes assessed by the Health Assessment Questionnaire (HAQ), Bath Ankylosing Spondylitis Functional Index (BASFI), Dougados Functional Index (FI), and the physical functioning scale of the Medical Outcome Study Short-form 36 (SF-36-PF) were analyzed by the Rasch model for item fit, item separation, measurement span, and distribution properties. Patient subgroups with axial or peripheral disease were analyzed for differential item functioning (DIF).
RESULTS: One hundred eight patients with PsA were assessed. The 4 functional indexes were highly correlated with each other and moderately correlated with patients' perception of health and pain scores. Floor effects were less marked in SF-36-PF. The 4 indexes satisfied the unidimensionality assumption of the Rasch model. HAQ and SF-36-PF had better information-weighted fit statistics (INFIT) and outlier-sensitive (OUTFIT) statistics. HAQ had the poorest item separation. SF-36-PF had the highest item separation (6.99), reliability (0.85), and the longest span of item threshold (9.03 logits). Only 1 and 2 items in BASFI and Dougados-FI had DIF in patients with sacroiliitis.
CONCLUSION: HAQ, BASFI, Dougados-FI, and SF-36-PF provide unidimensional measures of functional disability in PsA. SF-36-PF was the best in terms of less floor effect, highest item separation, longest span of item threshold, and better distributional properties. BASFI and Dougados-FI behaved similarly in patients with and without sacroiliitis and conferred no superiority in patients with axial disease.