Assessment of the factorial validity and reliability of the ALSFRS-R: a revision of its measurement model

The amyotrophic lateral sclerosis functional rating scale-revised (ALSFRS-R) is a widely used primary outcome measure in amyotrophic lateral sclerosis (ALS) clinical practice and clinical trials. ALSFRS-R items cannot, however, validly be summed to obtain a total score, but constitute domain scores reflecting a profile of disease severity. Currently, there are different measurement models for estimating domain scores. The objective of the present study is, therefore, to derive the measurement model that best fits the data for a valid and uniform estimation of ALSFRS-R domain scores. Data from 1556 patients with ALS were obtained from a population-based register in The Netherlands. A random split of the sample provided a calibration and validation set. Measurement models of the ALSFRS-R were investigated using both exploratory factor analyses and confirmatory factor analyses. The measurement model with a four-factor structure (i.e., bulbar, fine motor, gross motor, and respiratory function), with correlated factors and cross-loading items on dressing and hygiene and turning in bed and adjusting bed clothes on both motor function scales, provided the best fit to the data in both sets. Correlation between factors ranged from weak to modest, confirming that the ALSFRS-R constitutes a profile of four clinically relevant domain scores rather than a total score that expresses disease severity. The internal consistency of the four domain scores was satisfactory. Our revision of the measurement model may allow for a more adequate estimation of disease severity and disease progression in epidemiological studies and clinical trials.