Assessing autonomic symptoms of Parkinson's disease with the SCOPA-AUT: a new perspective from Rasch analysis.
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.


METHODS
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.
Forjaz
M J
MJ
National School of Public Health and Consortium for Biomedical Research in Neurodegenerative Diseases, Madrid, Spain. jforjaz@isciii.es
Ayala
A
A
Rodriguez-Blazquez
C
C
Frades-Payo
B
B
Martinez-Martin
P
P
D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001342:Autonomic Nervous System Diseases / Q000175:diagnosis*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D010300:Parkinson Disease / Q000175:diagnosis*; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D012737:Sex Factors