Background: The Scale for Outcomes in Parkinson's disease (PD) for Autonomic Symptoms (SCOPA-AUT) is a specific scale to assess autonomic dysfunction in PD patients. It was developed and validated under the classic test theory approach. This study sought to test whether the SCOPA-AUT meets item response theory standards for reliability, internal construct validity, response category ordering, and differential item functioning by gender and age group.
Method: The Rasch measurement model was applied to a sample of 385 PD patients.
Results: Model fit was obtained after the response categories were rescored and item 10-Incomplete emptying deleted because of redundancy. Person separation index, a reliability measure, was 0.82. All but two items (2-Sialorrhea and 13-Nocturia) were free of gender- and age-related bias. The strict tests of unidimensionality were met, indicating the validity of the total sumscore. Scale targeting suggested the need for items representing milder autonomic symptoms.
Conclusions: Suggestions for improving the SCOPA-AUT include a shorter scale with a simpler response scheme and a combination of sexual items for men and women. The resulting SCOPA-AUT is a reliable scale, with good internal construct validity, providing Rasch transformed results on a linear metric scale.