Rasch analysis of the 32-item motor function measure in ambulant patients with Duchenne muscular dystrophy

Objective This study analyzed the 32-item Motor Function Measure in a cohort of ambulatory patients with Duchenne muscular dystrophy using Rasch measurement methods. Design This is a psychometric study. Setting Rehabilitation centre of a large public children's hospital in Shenzhen, China. Participants Data from 176 genetically confirmed ambulant patients with Duchenne muscular dystrophy (mean age 7.3 years, SD 2.3 years, range 3.1-13.1 years) were analyzed. Results Rasch analyses supported the Motor Function Measure domain D1 as a reliable (person reliability = 0.88, person separation index = 2.71) and valid (acceptable targeting, little misfit, minimal category disordering) measure in ambulant patients with Duchenne muscular dystrophy. Remodelling the domain D1 by collapsing item 25 from 4 to 3 response categories addressed the problematic disordered thresholds, resulting in a rebuilt domain D1 with enhanced measurement properties. However, findings for domains D2 and D3 did not fulfil most Rasch model expectations. There were disordered thresholds for most items in domains D2 and D3, with low reliability coefficients, item mistargeting and misfit, and large ceiling effects. Conclusion Rasch analyses confirmed that the Motor Function Measure domain D1 was reliable and valid and provided a unidimensional measure for motor function in ambulant Duchenne muscular dystrophy patients. Accuracy of measurement had been enhanced through remodelling, and a rebuilt domain D1 with category collapsing for item 25 was proposed. The analysis revealed multiple limitations of the domains D2 and D3 that certain essential psychometrics were poorly met and, therefore, should be used with caution in this patient group.