Plasma Aβ42/p-Tau217 ratio and p-Tau217 independently predict CSF-defined Alzheimer’s disease pathology in a Brazilian admixed cohort

Introduction Blood-based biomarkers offer a promising, minimally invasive approach to Alzheimer’s disease (AD) diagnosis, yet validation in admixed populations remains limited. We investigated whether plasma biomarkers predict CSF-defined AD pathology in a Brazilian cohort. Methods Seventy-eight older adults including individuals with mild cognitive impairment (MCI), subjective cognitive decline (SCD), and cognitively unimpaired controls underwent cognitive testing, neuroimaging, and plasma biomarker assessment. CSF data were available for symptomatic participants (MCI and SCD; n = 61), and regression and ROC analyses were performed in the subset with both CSF and APOE genotyping data ( n = 53). Plasma Aβ42, p-Tau181, p-Tau217, t-Tau, and derived ratios were quantified. Multivariable logistic regression and ROC analyses evaluated prediction of abnormal CSF p-Tau181/Aβ42 and t-Tau/Aβ42, adjusting for age, sex, and APOE ε4 status. Results Approximately 25% of individuals with MCI exhibited abnormal CSF p-Tau181/Aβ42 and t-Tau/Aβ42 ratios. Moderate correlations were observed between plasma and CSF biomarkers (r 0.4), particularly for Aβ42/p-Tau217 and p-Tau217. In adjusted models, plasma p-Tau217 and the Aβ42/p-Tau217 ratio independently predicted abnormal CSF pathology. Each one standard deviation increase in p-Tau217 was associated with 3.53–4.83-fold higher odds of abnormal CSF ( p ≤ 0.003). In contrast, higher Aβ42/p-Tau217 ratios were associated with substantially lower odds of pathology, with each one standard deviation increase corresponding to a 91%–93% reduction in risk ( p ≤ 0.002). The ratio showed stronger associations than p-Tau217 alone. ROC analyses demonstrated good discrimination. For CSF p-Tau181/Aβ42, Aβ42/p-Tau217 achieved an AUC of 0.88 (83% sensitivity, 85% specificity), compared with 0.83 for p-Tau217. For CSF t-Tau/Aβ42, both biomarkers yielded AUCs of 0.89. Discussion Plasma Aβ42/p-Tau217 and p-Tau217 effectively identify CSF-defined AD pathology in an admixed cohort. While higher p-Tau217 levels were associated with increased odds of pathology, higher Aβ42/p-Tau217 ratios were associated with lower pathological burden and demonstrated stronger effect sizes, supporting the added value of combining amyloid and tau biomarkers. These findings provide initial evidence for local validation of blood-based AD biomarkers in Brazil.