Measurement Invariance Gaps in Parkinson’s Disease Assessment Scale Validation: A 28-Year Systematically-Documented Analysis of Geographic and Demographic Disparities in English-Language Literature

Background: Parkinson’s disease affects over 10 million people globally, yet validation evidence for major clinical assessment scales (UPDRS, MDS-UPDRS, MoCA, MMSE) is concentrated in high-income countries. Measurement invariance-whether scales measure constructs equivalently across populations-requires geographically and demographically diverse evidence for equitable global implementation. Objective: Characterize geographic origin, demographic representation, and methodological reporting quality of validation studies for major PD assessment scales in Englishlanguage peer-reviewed literature. Methods: Systematic review (PRISMA 2020) of MEDLINE, Embase, and Cochrane CENTRAL through July 31, 2024. Included 108 studies validating UPDRS, MDSUPDRS, MoCA, or MMSE in PD populations. Assessed bias and reporting quality using COSMIN and STROBE-DOM guidelines. Geographic attribution by first-author affiliation; gender classification via three-step validated procedure (κ=0.97). Results: High-income countries: 98/108 (90.7%); low- and middle-income countries: 10/108 (9.3%). Regional distribution: Europe 44.4%, North America 36.1%, Asia 8.3%, Africa 1.9%, South America 2.8%. Critical gaps in reporting: race/ethnicity 7.4%, education adjustment 12.0%, medication state 3.7%, digital literacy 0%. Temporal trends (1996–2024) show modest improvements in race/ethnicity reporting (0% to 11.1%, p=0.019) and education adjustment (8.3% to 15.6%, p=0.048), but geographic concentration stagnant. Only 2 studies (1.9%) reported differential item functioning analysis. Female authorship: 36.6%. Conclusions: Validation literature demonstrates substantial high-income country concentration (90.7%) and critical reporting gaps, constraining measurement invariance assessment. The near-absence of differential item functioning analysis (1.9%) indicates structural unpreparedness for measurement-invariance-aware validation. Recommendations target journals, funding agencies, professional societies, and researchers to advance equitable, globally representative scale validation. Keywords: Parkinson disease, assessment scales, validation, measurement invariance, differential item functioning, UPDRS, MDS-UPDRS, MoCA, MMSE, scientometric analysis, geographic disparities, demographic representation, health equity, COSMIN, STROBEDOM, PRISMA