Kurtzke scales revisited: the application of psychometric methods to clinical intuition

Abstract

When developing his disability scales for multiple sclerosis, Kurtzke demonstrated perception and insight. However, 45 years later, the evaluation of his clinically derived scales remains limited, particularly for more disabled patients. Indeed, many of Kurtzke's assumptions underpinning the development of the Expanded Disability Status Scale (EDSS) and Functional Systems (FS) are untested. This study aims to build on previous work and provide a more detailed examination using psychometric methods of the EDSS and FS. There are three study objectives: (i) to examine comprehensively the psychometric properties of the EDSS in more disabled people with multiple sclerosis undergoing in-patient rehabilitation; (ii) to examine the reliability of the FS and test Kurtzke's assumptions that they measure different aspects of the neurological examination and measure different constructs from that measured by the EDSS; and (iii) to examine whether the FS can be summed to generate a summary score. The EDSS was examined for its acceptability (score distributions), reliability (inter- and intra-rater reproducibility, standard error of measurement), validity (convergent and discriminant validity, measurement precision, discrimination between individuals) and responsiveness (effect size). The FS were examined for their reliability (inter- and intra-rater reproducibility), intercorrelations, correlations with the EDSS and the extent to which they satisfy Likert's criteria as a summed rating scale. In this more disabled sample of people with multiple sclerosis, the EDSS is an acceptable measure but demonstrates limited variability. Inter-rater reproducibility (intraclass correlation coefficient; ICC = 0.78) is adequate for group comparison studies, but intra-rater reproducibility is variable (ICC = 0.62-0.94). Convergent and discriminant validity for the EDSS is supported, but its measurement precision relative to the Functional Independence Measure is limited (56%). Also, the EDSS has a limited ability to distinguish between individuals in terms of their disability and its responsiveness is poor (effect size = 0.10). Results indicate that the FS measure constructs distinct from each other (intercorrelations = -0.23 to +0. 52) and from the EDSS (correlations = -0.10 to +0.59). Intra-rater, but not inter-rater reproducibility is adequate for group comparison studies. The FS do not satisfy criteria as an eight-, seven- or six-item summed rating scale. Despite being based on sound clinical intuition, the lack of psychometric input into the development of the EDSS and FS has limited their usefulness as evaluative outcome measures in multiple sclerosis.