Plasma eMTBR-tau243 and %p-tau217 for Biological Staging of Alzheimer Disease.

Importance: Biological staging of Alzheimer disease (AD) can improve diagnostic accuracy and prognostic assessment. However, existing approaches often require invasive procedures and specialized infrastructure, limiting their routine clinical use. Objective: To develop and validate a plasma-based biological staging model that closely mirrors an amyloid and tau positron emission tomography (PET)-based staging system. Design, Setting, and Participants: This observational longitudinal study included the research BioFINDER-2 cohort as the main cohort and the Knight Alzheimer Disease Research Center (ADRC) cohort as an independent validation cohort, with data acquired from November 2019 to January 2025 and from September 2007 to March 2020, respectively. Participants ranged from cognitively unimpaired (CU) to experiencing mild cognitive impairment (MCI) and dementia with AD or other neurodegenerative diseases. All individuals had both plasma biomarkers available. A Knight ADRC subsample had neuropathological data available. Data were analyzed from December 2024 to July 2025. Exposures: Plasma endogenously cleaved microtubule-binding region tau containing residue 243 (eMTBR-tau243) and phosphorylated tau at threonine 217 expressed as a percentage of its unphosphorylated form (%p-tau217). Main Outcome and Measures: The primary outcomes were PET-based biological staging according to the revised Alzheimer's Association criteria, clinical criteria, and biomarker levels assessed cross-sectionally and longitudinally. Results: The BioFINDER-2 main cohort included 872 participants, with an additional 156 participants in the Knight ADRC cohort for independent validation. In the BioFINDER-2 cohort, participants included 383 CU participants, 182 participants with MCI, 151 with AD dementia, and 156 participants with non-AD neurogenerative diseases (mean [SD] age, 72.8 [9.2] years; 438 women [50.2%]; 516 [59.2%] apolipoprotein E [APOE-E4] carriers). A plasma-based model with %p-tau217 and eMTBR-tau243 was derived, which demonstrated high concordance with established PET-based stages (C index, 0.91; 95% CI, 0.90-0.92) and was associated with clinical stage within the AD continuum (C index, 0.84; 95% CI, 0.82-0.86). In the validation cohort, concordance with PET-based stages was high (C index, 0.91; 95% CI, 0.87-0.94), as was concordance with clinical stage (C index, 0.86; 95% CI, 0.82-0.89). Concordance between fluid- and PET-based staging models was higher with this study's %p-tau217 and eMTBR-tau243 model than with a staging model using %p-tau217 alone, especially improving the intermediate or C stage (A+TMOD+) classification. In the neuropathology subsample, plasma staging was strongly aligned with autopsy-confirmed AD pathology using the Alzheimer Disease Neuropathologic Change scale (area under the curve, 0.96; 95% CI, 0.91-1.00). Conclusions and Relevance: In this longitudinal study, a plasma-based biological staging model incorporating %p-tau217 and eMTBR-tau243 showed strong concordance with PET-based staging, correlated with clinical severity and biomarker progression, and aligned with neuropathological categorization. This scalable, minimally invasive approach could facilitate broader implementation of biological staging in clinical practice and could enhance participant selection for disease-modifying treatments and clinical trials.