Diagnostic performance of plasma pTau217 in genetically admixed South American populations.

Plasma pTau217 is a leading biomarker for Alzheimer's disease, but evidence from genetically admixed populations in low- and middle-income countries remains limited. We evaluated the diagnostic performance of pTau217 and pTau217/Aβ42 measured using Simoa technology in memory-clinic cohorts from Brazil (Cog-Aging-Study, n = 353) and Argentina (GeNED.ar, n = 134). Here we show that both biomarkers accurately identified cerebrospinal fluid (CSF)-defined amyloid pathology in Cog-Aging-Study-Brazil and showed high concordance with clinical diagnosis across both cohorts. Classification performance was improved using two-cut-off approaches that account for diagnostic uncertainty. We further show that APOE-ε4, lower body mass index, and reduced kidney function were associated with higher pTau217 in Cog-Aging-Study-Brazil, whereas education also influenced biomarker levels in GeNED.ar-Argentina. African ancestry modified APOE-ε4 effect on CSF but not plasma biomarkers. These findings support the use of plasma pTau217-based biomarkers in genetically admixed South American populations and in settings with limited access to CSF testing and brain imaging.