Responsiveness to Change of an IRT Score of Global Disability Derived from Narcoms Performance Scales

Background: At the 2013 CMSC annual meeting, we described the cross-sectional psychometric properties of a patient-reported outcome measure of global neurological disability derived from the North American Research Committee on Multiple Sclerosis (NARCOMS) Performance Scales (PS) using item response theory (IRT).

Objectives: To assess responsiveness and minimally important change (MIC) in PS IRT score by comparing changes on the new measure with changes on several external measures of disease activity and progression, including: (1) Patient Determined Disease Steps (PDDS); (2) self-evaluation of change in overall MS symptoms compared to 6 months ago; and (3) number of relapses in the last 6 months.

Methods: Each single-item PS assess perceived disability in 1 of 11 neurological domains (mobility, hand, spasticity, tremor, bladder/bowel, vision, fatigue, cognition, sensory, pain, depression). Summary IRT scores of global disability were scaled to have baseline mean 50 and standard deviation (SD) 15 among MS patients recruited by the NARCOMS registry in 1998-2011. Analyses of responsiveness were conducted using the biannual updates of NARCOMS participants enrolled in 2003-2008 who had at least an update immediately after enrollment and at 5 years of follow-up. We used linear mixed effects models for longitudinal data to assess the associations between within-person changes on the anchors and 6-month changes in PS IRT score.

Results: The study sample included 2,788 persons with MS. Median number of completed updates was 9 per subject (range, 2-to-12). Mean 6-month change in PS IRT score was 0.1 point (SD, 6.8). Change of one step on the PDDS was associated, on average, with a 2.3-point change in PS IRT score (95% CI, 2.2, 2.4; P<0.001). Patients’ assessment of symptoms now compared to 6 months ago was associated with a 0.7-point increase in PS IRT score (95% CI, 0.4, 1.1; P<0.001) when symptoms were described as worse/much worse and with a 1.0-point decrease in PS IRT score (95% CI, -1.4, -0.7; P<0.001) when symptoms were described as better/much better. PS IRT score marginally increased by 0.2 point (95% CI; -0.0, 0.5; P=0.09) when subjects experienced 1 or 2 relapses during the past 6 months, and by 0.9 point (95% CI; 0.4, 1.4; P<0.001) when they had 3 or more relapses.

Conclusions: In this preliminary analysis, associations were in the expected directions and small changes in IRT score of global disability appeared to be meaningful to the patients.