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

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.