Elevated serum levels of phosphorylated tau are not specific for Alzheimer's disease. These levels can serve as a diagnostic tool for the most common types of systemic amyloidosis. Phosphorylated tau in blood has the potential to differentiate polyneuropathy caused by amyloidosis from other types of polyneuropathy. The study focuses on immunoglobulin light chain amyloidosis and transthyretin amyloidosis. The article was published in Nature Medicine on March 11, 2026. These findings extend the use of the blood biomarker phosphorylated tau beyond neurodegenerative brain diseases.