OBJECTIVES: The Quick Motor Function Test (QMFT) is a 16-item functional scale constructed for Pompe disease for which content and construct validity have been previously established. Structural validity, to evaluate the degree to which a unidimensional conceptual framework is supported, has not yet been investigated. Hence, this study aimed to evaluate QMFT’s structural validity and individual items’ performance.

METHODS: Blinded pooled data from baseline and Week 49 of the COMET study (NCT02782741) of patients with late-onset Pompe disease (LOPD) were analyzed. QMFT item response distributional characteristics were evaluated. Item-to-item and item-total correlations to assess items’ interrelatedness were calculated via Spearman coefficients. Structural validity was assessed by comparing one-factor, second-order, and bifactor confirmatory factor analysis (CFA) models. Goodness-of-fit measures, together with factor loading pattern and magnitude, were investigated to confirm appropriateness of the unidimensional factor structure. Internal consistency reliability was assessed via Cronbach’s alpha and McDonald’s omega coefficients.

RESULTS: In total, 97 patients were evaluated on all QMFT items at baseline and 91 at Week 49. Ceiling effects were pronounced for 9/16 items across timepoints, meaning that ≥20% of responses fell into the best possible health state response category. Item-to-item correlations ranged from 0.04–0.80, while the majority of item-total correlations were sufficiently strong (≥0.50). The most appropriate model was the one-factor model (unidimensional), with factor loadings ranging between 0.39–0.91 at baseline and 0.47–0.94 at Week 49. The unidimensional QMFT showed excellent internal consistency reliability both at baseline (Cronbach’s alpha=0.906; McDonald’s omega=0.946) and Week 49 (Cronbach’s alpha=0.917; McDonald’s omega=0.966).

CONCLUSIONS: Our study has empirically demonstrated that the QMFT is a unidimensional measure of motor function characterized by high measurement precision. The psychometric evidence established in this work offers support for the application of the QMFT total score in assessments of motor function in patients with LOPD.