Circulating Vesicular Biomarkers in Alzheimer's Disease: From Mechanistic Insights to Clinical Applications

Alzheimer's disease (AD) is moving toward earlier, biology-driven diagnosis, which increases the need for blood-based markers that are reliable, scalable, and interpretable across populations. This review integrates the AT(N) framework with a maturity model for circulating biomarkers. We first describe core and largely validated plasma measures, including LC-MS or automated immunoassay Aβ42/Aβ40 ratios, p tau217 and p tau231, glial fibrillary acidic protein (GFAP), and neurofilament light, and we relate them to recent multi-stakeholder recommendations on analytical performance and regulatory status. We then summarize replicated but context-dependent markers, such as soluble TREM receptors, CHI3L1, and MCP 1, which improve risk stratification when interpreted together with amyloid and tau. A separate section examines emerging readouts that capture central nervous system (CNS) processes indirectly, focusing on neuron-enriched extracellular vesicles (EVs) and EV-carried microRNA panels. These signatures are biologically plausible and often precede symptoms, although current datasets are small, Alzheimer's disease neuroimaging initiative (ADNI)-based, and require standardized pre-analytical handling and external validation before clinical triage can be recommended. We also discuss platform selection, comparing automated electrochemiluminescence (ECL) and single-molecule assays with LC-MS, and outline how composite plasma panels that include APOE genotype can support screen-confirm-monitor workflows in memory clinics. Finally, we propose a tiered implementation path in which genomic risk profiling and blood tests identify candidates for cerebrospinal fluid (CSF) or positron emission tomography (PET) studies. This shows how circulating and multi-omics biomarkers can be layered onto established plasma Amyloid beta (Aβ) and p tau assays to widen the measurable blood space in Alzheimer's disease.