Design and validation of a patient-reported Functional Mobility Composite Score (FMCS) in people with Parkinson’s disease

BACKGROUND: While patient-reported instruments are less costly and invasive than the administration of physical performance tests, there is no gold standard for a patient-reported outcome measure (PROM) of functional mobility in people with Parkinson’s disease (PwP).
METHODS: We designed the PROM functional mobility composite score (FMCS) and investigated its construct validity. Convergent validity was assessed using the objective Timed Up and Go (TUG) and discriminant validity was assessed by comparing the FMCS with patient-reported (MDS-UPDRS II) and clinician-assessed (MDS-UPDRS III) motor symptoms as well as between disease stages (H&Y) and PIGD phenotypes. RESULTS Spearman correlation coefficients (rs) ranging from -0.41 to -0.79 (p < 0.001) indicated convergent validity. Hence, multiple regressions followed by a t-test to test the standardized regression coefficients for differences were conducted and these suggested sufficient ability of the FMCS to discriminate (p < 0.001) between patient-reported and clinician-assessed motor symptoms. FMCS was more strongly associated with patient-reported MDS-UPDRS II (rs=- 0.79) than clinician-reported MDS-UPDRS III (rs=-0.32) and can discriminate between disease stages as between PIGD phenotypes (p < 0.001). CONCLUSION The FMCS is a valid composite score to assess functional mobility through patient reports in people with PD. Further research, including the validation of the FMCS in independent PD cohorts is required.