Differential Item Functioning on the Cochin Hand Function Scale Among People With Systemic Sclerosis by Language, Sex, and Disease Subtype: A Scleroderma Patient-Centered Intervention Network (SPIN) Cohort Study

Objective: To evaluate the degree that the Cochin Hand Function Scale (CHFS) generates scores that are comparable across language, sex, and disease subtype. Methods: We included participants enrolled in the Scleroderma Patient-centered Intervention Network (SPIN) Cohort who completed the CHFS at their baseline assessment between April 2014 and September 2020. Confirmatory factor analysis (CFA) was used to test unidimensionality, and multiple indicator multiple cause (MIMIC) models were used for differential item functioning (DIF) analysis based on language, sex, and disease subtype. Both intraclass correlation coefficient (ICC) and Pearson's correlation were calculated using factor scores obtained from unadjusted and DIF-adjusted MIMIC models to evaluate agreement and correlation between scores. Results: A total of 2,155 participants were included. CFA with covarying error terms supported a good fit of the model (χ²[127] = 1,754.671; P < 0.001; Tucker-Lewis index = 0.985; comparative fit index = 0.987; root mean square error of approximation = 0.077). Nine items displayed statistically significant DIF for language of administration, 10 items for sex, and 10 items for disease subtype. However, the overall impact of DIF was negligible when comparing factor scores that did and did not account for DIF (ICC = 0.999; r = 0.999). Conclusion: The CHFS has score comparability in systemic sclerosis regardless of participants’ language, sex, and disease subtype.