Plasma phosphorylated tau 217 detects amyloid-β in neuronal synuclein disease

Multiple proteinopathies commonly coexist in neurodegenerative diseases, making it essential to evaluate plasma biomarker performance in these complex diseases. While plasma biomarkers accurately detect amyloid-β pathology in Alzheimer's disease (AD), their performance is unknown in neuronal synuclein disease (NSD). We aimed to determine the accuracy of plasma pTau217, pTau181, Aβ42/40, GFAP, and NfL to detect amyloid-β in NSD, then establish and validate cut points for the most promising marker. We included 253 participants (180 discovery; 73 validation). In the discovery cohort, NSD status was defined by CSF α-synuclein seed amplification assay and amyloid-β status by CSF Aβ42/40. Participants included individuals with clinical Lewy body disease (LBD), AD, and cognitively unimpaired. Validation cohorts consisted of clinically diagnosed LBD participants. In the discovery cohort, plasma pTau217, pTau181, Aβ42/40, and GFAP significantly differed by amyloid-β status regardless of NSD status, while NfL was highest in NSD+/Aβ+ participants. Among all biomarkers, plasma pTau217 showed the best diagnostic performance (AUC = 0.92, 95% CI = 0.81-0.98). Applying plasma pTau217 cut points to pre-screen clinically diagnosed LBD participants reduced the need for confirmatory amyloid-β PET or CSF in 41-56%. These findings support plasma pTau217 as a minimally-invasive tool for identifying pathological amyloid-β in neuronal synucleinopathies with mixed Alzheimer's disease pathology.