Structural validity for the Health Assessment Questionnaire-Disability Index (HAQ-DI) has recently been provided for patients with rheumatoid arthritis (RA). The goal of the current study was to examine the structural validity of the HAQ-DI in patients with systemic sclerosis (SSc, scleroderma) and to compare its performance with that in patients with RA.
The HAQ-DI structural validity was first assessed in a sample of 100 scleroderma patients using confirmatory factor analysis. Second, the similarity of factor structures between SSc patients (n = 291) and RA patients (n = 278) was tested using a multigroup structural validity model to assure that comparison of scores between these two diagnostic groups is appropriate.
Results yielded a single-factor HAQ-DI score which favored the current scoring system of the HAQ-DI (model fit was CFI = 0.99 and RMSEA = 0.04). Moreover, even the most stringent model of multigroup structural validity affirmed the similarity between SSc and RA patients on the HAQ-DI (model fit was CFI = 0.99 and RMSEA = 0.04) nor was it different from a model without any demands on group similarity: CFI difference = 0.007; chi(2) = 4.29, df = 26, p=0.99.
The current results indicate that a single-factor HAQ-DI is appropriate for future clinical trials in scleroderma and, in addition, HAQ-DI scores among patients with SSc and early RA can be compared legitimately with one another.