Background and purpose: The quality of a disability scale should dictate when it is used. The purposes of this study were to examine the validity of a global rating of change as a reflection of meaningful change in patient status and to compare the measurement properties of a modified Oswestry Low Back Pain Disability Questionnaire (OSW) and the Quebec Back Pain Disability Scale (QUE).
Subjects: Sixty-seven patients with acute, work-related low back pain referred for physical therapy participated in the study.
Methods: The 2 scales were administered initially and after 4 weeks of physical therapy. The Physical Impairment Index, a measure of physical impairment due to low back pain, was measured initially and after 2 and 4 weeks. A global rating of change survey instrument was completed by each subject after 4 weeks.
Results: An interaction existed between patients defined as improved or stable based on the global rating using a 2-way analysis of variance for repeated measures on the impairment index. The modified OSW showed higher levels of test-retest reliability and responsiveness compared with the QUE. The minimum clinically important difference, defined as the amount of change that best distinguishes between patients who have improved and those remaining stable, was approximately 6 points for the modified OSW and approximately 15 points for the QUE.
Conclusion and discussion: The construct validity of the global rating of change was supported by the stability of the Physical Impairment Index across the study period in patients defined as stable by the global rating and by the decrease in physical impairment across the study period in patients defined as improved by the global rating. The modified OSW demonstrated superior measurement properties compared with the QUE.