Phosphorylated tau in the blood, one of the most promising early markers of Alzheimer's disease, is not unique to Alzheimer's. Researchers at DZNE and the Hertie Institute for Clinical Brain Research at the University of Tubingen report in Nature Medicine that pTau levels are also raised in people with systemic amyloidosis, a group of diseases that deposits misfolded protein in the heart and kidneys rather than the brain. The analysis drew on data from 280 older adults in Germany, Italy, and the Netherlands.
The two conditions examined - transthyretin amyloidosis and immunoglobulin light-chain amyloidosis - are the most common forms of systemic amyloidosis. They share a broad mechanism with Alzheimer's in that misfolded proteins clump into amyloid deposits, but the proteins involved and the organs affected are different. Even so, the blood chemistry responded similarly, with pTau climbing in affected patients.
That cuts two ways. On the diagnostic side, pTau is comparatively easy and cheap to measure, so the researchers suggest it could help flag systemic amyloidosis earlier or help confirm or rule out suspected cases - useful in diseases that are often recognized late. On the other side, the result is a caution for anyone treating a raised pTau reading as proof of Alzheimer's.
The team is explicit on that point: pTau is not a specific marker, and it should not stand alone as a diagnostic criterion for Alzheimer's or as a gauge of its progression. They stress this matters most in people who have no cognitive symptoms yet, precisely the group where blood tests are being promoted as a way to catch disease early. Additional clinical and biological data, they argue, need to be weighed alongside the blood result.