Establishing score equivalence of the Functional Independence Measure motor scale and the Barthel Index, utilising the International Classification of Functioning, Disability and Health and Rasch measurement theory

Introduction: Two widely used outcome measures to assess functioning in neurological rehabilitation are the Functional Independence Measure (FIM™) and the Barthel Index. The current study aims to establish the equivalence of the total score of the FIM™ motor scale and the Barthel Index through the application of the International Classification of Functioning, Disability and Health, and Rasch measurement theory. Methods: Secondary analysis of a large sample of patients with stroke, spinal cord injury, and multiple sclerosis, undergoing rehabilitation was conducted. All patients were assessed at the same time on both the FIM™ and the Barthel Index. The International Classification of Functioning, Disability and Health Linking Rules were used to establish conceptual coherency between the 2 scales, and the Rasch measurement model to establish an exchange of the total scores. Results: Using the FIM™ motor scale, items from both scales linked to the International Classification of Functioning, Disability and Health d4 Mobility or d5 Self-care chapters. Their co-calibration satisfied the assumptions of the Rasch model for each of 3 diagnostic groups. A ceiling effect was observed for the Barthel Index when contrasted against the FIM™ motor scale. Conclusion: Having a Rasch interval metric to transform scores between the FIM™ motor scale and Barthel Index is valuable for monitoring functioning, meta-analysis, quality audits and hospital benchmarking.