The symptom inventory disability-specific short forms for multiple sclerosis: reliability and factor structure

Abstract

Objective: To further the development of the 99-item Symptom Inventory (SI) for multiple sclerosis (MS) using modern test theory methods to create 3 disability-specific short forms for MS patient subgroups identified using Performance Scale (PS) items.

Design: A web-based cross-sectional study.

Setting: National MS Registry.

Participants: People with MS (N=1532) who participate in the North American Research Committee on Multiple Sclerosis Registry.

Interventions: None.

Main outcome measures: The SI; the disease-specific PS and the Patient-Determined Disease Steps; and the generic Short-Form 12.

Results: When the original SI subscales did not demonstrate unidimensionality, exploratory factor analysis was conducted yielding 14 factors that could be classified using the structure of the PS. Confirmatory factor analysis confirmed the unidimensionality of the hand function, vision, fatigue, cognitive, bowel/bladder, spasticity, and pain scales. The mobility scale was split into mobility and use of assistive devices; the sensory scale was split into sensory and vasomotor. Item response theory analyses revealed good model fit.

Conclusions: This study provides empirical support for a 10-scale symptom measure for use in MS clinical research, with short forms in 5 scales tailored to have good specificity for people with mild, moderate, and severe disability and single forms for the remaining 5 scales. The PS items can serve as a screener for these disability-specific short forms, which provide choice and flexibility that are similar to a computerized adaptive test but without the reliance on real-time computer infrastructure.