The Health Assessment Questionnaire (HAQ) disability index (DI) is the most common self-reported measure of physical disability in rheumatoid arthritis (RA). Recently, the HAQ-II was developed in the US as a short, valid, and reliable alternative using Rasch analysis. Our objective was to compare the scaling properties of the HAQ DI and HAQ-II in Dutch patients with RA.
We used data from 472 patients with confirmed RA. Internal construct validity of the HAQ versions was assessed using Rasch analysis. Additionally, external construct validity was assessed by examining correlates with other outcome measures.
The HAQ DI had a large floor effect, with 9.5% of the patients indicating no disability compared with 4.3% for the HAQ-II. Both versions were unidimensional and adequately fit the Rasch model, containing only 1 nonfitting item. Additionally, 2 HAQ-II items demonstrated overfit and a high residual correlation, suggesting overlap or redundancy in item content. The HAQ-II demonstrated better item separation, indicating that it covered a wider range of physical function. Item difficulty estimates were reasonably well spread for the HAQ-II, whereas the HAQ DI items tended to cluster around similar difficulty levels. Both scales contained several items with differential item functioning by sex, age, or disease duration. Both scales demonstrated the expected pattern of correlations with other outcome measures.
The results indicate that both the HAQ DI and HAQ-II are psychometrically robust measures of physical function. The Rasch-developed HAQ-II, however, has several favorable scaling properties, including a better scale length and a reduced floor effect.