Refining Alzheimer's disease biological diagnosis with plasma biomarkers: Resolving p‐tau217 “gray zone” with p‐tau181 integration

Abstract BACKGROUND Blood‐based biomarkers offer a less invasive and more scalable alternative to cerebrospinal fluid (CSF) analysis and amyloid‐positron emission tomography (PET) for the biological diagnosis of Alzheimer's disease (AD). Among blood‐based biomarkers (BBMs), plasma phosphorylated tau217 (p‐tau217) has shown the highest accuracy, although intermediate (“gray zone”) values remain challenging to interpret. METHODS In this study, 401 individuals across the Alzheimer's Disease (AD) continuum (Subjective Cognitive Decline, Mild Cognitive Impairment, and AD dementia) underwent clinical and biomarker assessment. Plasma p‐tau217, p‐tau181, neurofilament light chain (NfL), and glial fibrillary acidic protein (GFAP) were measured. Core1 status was defined through CSF or amyloid‐PET. RESULTS Plasma p‐tau217 demonstrated the strongest discrimination of Core1 positivity (area under the curve AUC = 0.95) and showed the steepest increase with disease progression. A two‐cutoff strategy improved diagnostic accuracy (94%), though 18% of patients fell into the gray zone. Within this subgroup, p‐tau181 was the only predictor of Core1 status and correctly reclassified 77.4% of indeterminate cases. DISCUSSION These findings support a sequential plasma biomarkers approach for reliable AD detection.