Factor structure of Guy's Neurological Disability Scale in a sample of Dutch patients with multiple sclerosis

Background: Guy's Neurological Disability Scale (GNDS) is designed to assess disability (i.e. activity limitations) in patients with multiple sclerosis. It contains 12 functional domains, each indicating a level of disability. Four domain scores and a total score can be calculated. Objective: The aim of this study was to evaluate the structural validity of the GNDS in a Dutch population who were definitely diagnosed as having multiple sclerosis. Methods: Data of 974 patients were available. The structural validity of the GNDS was evaluated by confirmatory item factor analysis (CIFA). Two first-order models and two bifactor models were investigated. Results: The best fitted model was a bifactor model with a general factor underlying all items, and 10 items loading on 3 group factors. Cronbach's alpha on the general factor (0.78) and on the group factor spinal-plus (0.74) were satisfying. Cronbach's alpha on the group factors mental (0.56) and bulbar (0.48) were low. Reliability based on CIFA was 0.85. Conclusion: Results showed a clear factor structure of the GNDS. It justifies the use of the total score of the GNDS. In addition, three sub-scale scores could be used.