Abstract: Background: The short version of the Problem Behaviours Assessment (PBA-s) is the recommended outcome measure for behavioural symptoms in Huntington’s disease. Rasch analysis was used to further investigate the measurement limitations of the PBA-s. Objectives: 1) To assess the psychometric properties of the 11 severity and frequency items within the PBA-s and 2) to determine the construct validity of using a total PBA-s score as a clinical outcome measure. Methods: PBA-s data for 517 participants from Enroll-HD were included in the Rasch analysis. Separate analyses were conducted for the severity and frequency items of the PBA-s, using RUMM2030 software. Achieving…fit to the model provides supporting evidence that all items contribute to a single underlying latent trait. This property is defined as internal construct validity. Results: The total PBA-s severity score demonstrated several important limitations, including disordered response categories for all 11 severity items, local dependency and poor targeting. However, modifying the original five-point scoring system to a four-point system resulted in ordered response categories for seven of the severity items and achieved a good overall fit to the Rasch model. For the total PBA-s frequency score, fit to the model was not achieved even after amendments to the scoring system. Conclusions: This study suggests that with reduction to a four-point scoring system, the total PBA-s severity score may be considered a valid clinical outcome measure. This study also suggests limitations in the use of a total PBA-s frequency score.
Show more
Keywords: Rasch, psychometrics, Huntington’s disease, Problem Behaviours Assessment, Enroll-HD
Abstract: Background: Huntington’s disease can present at almost any age but traditionally, those with an onset ≤20 years are described as having juvenile onset Huntington’s disease (JOHD). They are more likely to have bradykinesia and dystonia earlier in the course of the disease. The Total Motor Score of the Unified Huntington’s Disease Rating Scale (UHDRS-TMS) is often used as the principal outcome measure in clinical trials. Objective: To identify a motor scale more suitable for JOHD patients. Methods: A working group reviewed the UHDRS-TMS and modified it by adding four further assessment items. Rasch analysis was used to study the performance…of the modified scale in 95 patients with a mean age of 19.4 (SD 6.6) years. Results: The initial analysis showed a significant overall misfit to the Rasch model and a number of individual items displayed poor measurement properties: all items relating to chorea displayed significant misfit due to under-discrimination. Additionally, a number of items displayed disordered response category thresholds, and a large amount of dependency was present within the item set (96 out of 741 pairwise differences = 13%). An iterative process of scale re-structuring and evaluation was then undertaken, with a view to eliminating the largest sources of misfit and generating a set of items that would conform to Rasch model expectations. Conclusion: This post-hoc scale restructuring appears to provide a valid motor score that is psychometrically robust in a JOHD population. This scale restructuring offers a pragmatic solution to measuring motor function in a JOHD population, and it could provide the basis for the further iterative development of a more useful clinical rating scale for patients with JOHD.
Show more
Keywords: Rasch, juvenile onset Huntington’s disease, total motor score