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

Authors

  • M. J. Forjaz,

    1. National School of Public Health; and Consortium for Biomedical Research in Neurodegenerative Diseases (Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas – CIBERNED), Madrid
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  • A. Ayala,

    1. National School of Public Health, Madrid
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  • C. Rodriguez-Blazquez,

    1. Area of Applied Epidemiology, National Center for Epidemiology; and CIBERNED, Carlos III Institute of Health, Madrid, Spain
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  • B. Frades-Payo,

    1. Area of Applied Epidemiology, National Center for Epidemiology; and CIBERNED, Carlos III Institute of Health, Madrid, Spain
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  • P. Martinez-Martin,

    1. Area of Applied Epidemiology, National Center for Epidemiology; and CIBERNED, Carlos III Institute of Health, Madrid, Spain
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  • on behalf of the Longitudinal Parkinson’s Disease Patient Study (Estudio longitudinal de pacientes con enfermedad de Parkinson – ELEP) Group

    1. National School of Public Health; and Consortium for Biomedical Research in Neurodegenerative Diseases (Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas – CIBERNED), Madrid
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M. J. Forjaz, Escuela Nacional de Sanidad, Instituto de Salud Carlos III, Avda Monforte de Lemos 5, 28029 Madrid, Spain (tel.: +34 91 822 2062; fax: +34 91 387 7869/2;
e-mail: jforjaz@isciii.es).

Abstract

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.

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