Psychometric properties of MG-ADL items and MG-ADL score: An assessment of distributional characteristics, validity and factor structure in two large datasets

Background: The Myasthenia Gravis–Activities of Daily Living scale (MG-ADL) is an 8-item outcome measure to assess symptoms and functional limitations in myasthenia gravis (MG) patients. The MG-ADL score is an equally weighted level sum score that is used as primary outcome measures in clinical trials, in clinical practice, and as an end-point in health economic evaluation. This data analysis aims to obtain detailed knowledge of measurement properties of MG-ADL items and the MG-ADL score. Methods: Cross-sectional data from a real-world prospective study (MRW) were combined with longitudinal data from the ADAPT trial. Outcome measures included were MG-ADL, Quantitative Myasthenia Gravis score (QMG), MG 15-item Quality of Life (MG-QOL15r) and EQ-5D-5L. Patients were categorized by their Myasthenia Gravis Foundation of America (MGFA) clinical classification. The following measurement properties were assessed: distributional characteristics, inter-item correlation, convergent, known groups and construct validity and internal factor structure. Results: Correlations of items within MG-ADL dimensions were moderate, while MG-ADL correlations between comparable MG-QOL15r and QMG items were mixed. Known groups validity for the MG-ADL score was demonstrated for MGFA class. Mean MG-ADL item level scores by MGFA class demonstrated construct validity. PCA, including all four outcome measures, resulted in a nine factor solution. Discussion: Psychometric properties of individual MG-ADL items were moderate to good. This study showed that the MG-ADL adequately captures the multidimensional heterogeneous nature of MG. This is, however, accompanied by mixed psychometric performance of the MG-ADL score, which may complicate health economic modelling. Registration: MyRealWorld-MG was registered on November 25, 2019, with registration number NCT04176211. The ADAPT randomized clinical trial is registered at ClinicalTrials.gov (NCT03669588).