Characteristics of the functional independence measure in patients with neurological disorders. Measurement through Rasch analysis

Abstract

The Functional Independence Measure (FIMTM) is an internationally widely used outcome measure, measuring disability in terms of dependence or need of assistance. The aim of this study was to evaluate the structural characteristics of FIMTM using the Rasch model, with regard to scoring function, cross-cultural validity in Europe and cross-diagnostic validity within rehabilitation units in Scandinavia. The requirement for comparison of sum scores between units was also emphasised.A detailed analysis of scoring functions of the seven categories of the FIMTM items of in-hospital data was undertaken prior to testing the fit to the model. Data were derived from patients with stroke and spinal cord injury (SCI) and, in the Scandinavian data, also from patients with traumatic brain injury (TBI). Categories were re-scored where necessary. Analysis of Differential Item Functioning (DIF) between countries was undertaken in pooled data from 31 European rehabilitation clinics for patients with stroke, and 9 clinics for spinal cord injured patients. Analyses of cross-diagnostic differences were undertaken in pooled data from nine rehabilitation units in Scandinavia, and in pooled data from six Swedish facilities for analyses of differences between units within the same country.Disordered thresholds were found on most items when using seven categories. Fit to the Rasch-model varied. By decreasing the number of categories in the scale the problem of disordered thresholds could be solved. The problem of misfit was not fully resolved by re-scoring. DIF was found by country for most motor items for SCI and Stroke, but only in the social-cognitive items for stroke. Cross- diagnostic DIF in the FIM motor items was also found for Spinal Cord Injury, compared with stroke and traumatic brain injury. Cross-clinical DIF was also shown to be a problem. Comparison of low sum scores between clinics should be done with caution.In conclusion a reduction of the number of categories in each item is recommended. Likewise, the removal of items that do not fit the underlying trait would improve the validity of the scale in these groups. Clinical data from FIMTM for patients with stroke or SCI cannot be pooled in its raw form or compared across countries/diagnoses/clinics. After making adjustments, appropriate comparison of sum scores between the three diagnoses was possible. Thus, when planning interventions (group or individual), evaluating rehabilitation programs, or comparing patient achievements in individual items, cross-cultural and cross-diagnostic DIF needs to be taken into account.