Metric properties of the ASIA motor score: subscales improve correlation with functional activities

OBJECTIVE: To apply item response theory (IRT) methods to neurologic and functional scales to determine the value of using American Spinal Injury Association (ASIA) motor subscores and ability estimates, rather than total ASIA motor scores, to predict motor FIM instrument scores. DESIGN: Secondary analysis of prospectively collected data. SETTING: Model Spinal Cord Injury Systems centers. PARTICIPANTS: People with traumatic spinal cord injury (SCI) (N=4338) discharged from inpatient rehabilitation between January 1, 1994, and March 31, 2003. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Total discharge motor FIM scores, FIM subscale scores, and IRT-derived ability estimates of motor FIM scores. RESULTS: Use of separate ASIA upper-extremity and lower-extremity motor scores improved prediction of motor FIM scores over that of total ASIA motor score (R(2) for motor FIM score, .71 vs .59). Use of IRT-based ability estimates derived by applying a 2-parameter graded response model to the raw scores, however, did not improve prediction of motor FIM scores above that of the ASIA motor subscale scores. CONCLUSIONS: Consistent with the metric properties of the ASIA motor score, and with recent models of disablement, impairment in SCI is more accurately characterized by using separate ASIA upper- and lower-extremity motor scores than by using a single motor score. Use of subscores for impairment should improve prediction of functional abilities and enhance more complex models of disability.