Psychometric properties of the Oswestry disability index: Rasch analysis of responses in a work-disabled population.

BACKGROUND

The Oswestry disability index (ODI) is the most widely used measure of perceived disability for low back conditions. It has been adopted without adaptation in functional capacity evaluation (FCE). Rigorous testing of the ODI with modern psychometric methods, in this setting, is warranted.

OBJECTIVE

To determine the psychometric properties of the ODI in FCE: unidimensionality; differential item functioning; item coverage and to identify poorly functioning items, allowing for improvement of these items and recalibration of the scale.

METHODS

Rasch analysis, specifically Masters' partial credit model, was conducted on data.

PARTICIPANTS

133 work-disabled individuals presenting for FCE in northern British Columbia, Canada.

RESULTS

All items had one poorly functioning option. Items were rescaled from six categories to five, improving the psychometric properties of the ODI as a unidimensional (disability due to back pain) scale. Item difficulty range is sufficient for a population with mild to severe disability.

CONCLUSION

Although two of the ten ODI items functioned marginally unsatisfactorily in the unrevised state, the 5-option revised ODI appears superior. Use in clinical settings across a broad spectrum of disability levels could help establish its psychometric properties. Health professionals should be aware that the ODI may perform differently depending on client population.