Blood-Based Biomarkers Reveal Molecular Overlap Between Alzheimer's and Parkinson's Disease: A Comprehensive Review

Abstract

Alzheimer’s disease (AD) and Parkinson’s disease (PD) together affect over 65 million people worldwide, yet no disease-modifying treatment exists for either condition. For decades, they were studied as separate entities, involving different proteins, different brain regions, and different clinical teams. Blood- based biomarker research is changing that picture, revealing that the two conditions share far more molecular machinery than their clinical presentations would indicate. This review focuses on four clinically validated blood markers: p-tau217, α-synuclein seed amplification assay (SAA), glial fibrillary acidic protein (GFAP), and neurofilament light chain (NfL). The central finding is that no single biomarker reliably separates these diseases across all clinical contexts. Instead, multiplex panels that pair disease-specific markers (p-tau217 for AD; α-synuclein SAA for PD) with shared neurodegeneration markers (NfL, GFAP) offer both the strongest diagnostic performance and the clearest window into pathological overlap. Realizing this potential will require solving problems that are not purely scientific: assay standardization across platforms, better representation of global populations in validation studies, and regulatory frameworks that do not yet exist in most countries.