Alzheimer's Blood Marker pTau Also Spikes in Heart and Kidney Diseases

Published in Nature Medicine. Research led by Mathias Jucker, DZNE and Hertie Institute for Clinical Brain Research, University of Tubingen.

For years, phosphorylated tau protein -- pTau -- has been the darling of Alzheimer's diagnostics. Measured in blood, it promised something close to a screening revolution: a simple test that could flag neurodegeneration before symptoms appear. But a new study published in Nature Medicine complicates that tidy narrative in a way that may ultimately prove more useful than the original story.

Researchers at DZNE (the German Center for Neurodegenerative Diseases) and the Hertie Institute for Clinical Brain Research at the University of Tubingen have found that pTau levels are also elevated in two conditions that have nothing to do with the brain -- at least not directly. The diseases in question, transthyretin amyloidosis and immunoglobulin light-chain amyloidosis, primarily attack the heart and kidneys. And yet the blood tells a similar biochemical story.

Amyloid deposits, different organs, same signal

The connection is amyloid -- misfolded proteins that clump together in tissues where they do not belong. In Alzheimer's disease, amyloid-beta plaques and tau tangles accumulate in the brain. In systemic amyloidosis, the two most common forms involve different proteins -- transthyretin or immunoglobulin light chains -- depositing primarily in the heart and kidneys.

Despite the different proteins and different organs involved, the Tubingen researchers found that pTau was elevated in patients with systemic amyloidosis. The study examined 280 older individuals from Germany, Italy, and the Netherlands, comparing blood pTau levels across diagnostic groups.

The implication is that pTau may not be a specific marker for brain amyloid, as has been widely assumed. Instead, it appears to reflect a more general cellular stress response to amyloid deposits -- regardless of where in the body those deposits form.

A stress response that crosses organ boundaries

Mathias Jucker, professor and scientist at DZNE and the Hertie Institute, suspects that cells release pTau as a reaction to the presence of amyloid -- a kind of molecular distress signal. This response may occur in many organs, not just the brain.

There is precedent for this idea in an unexpected place: hibernation biology. In hibernating animals, temporary increases in pTau have been described as a protective mechanism -- part of the cellular toolkit for surviving extreme metabolic slowdowns. The parallel suggests that pTau release may have evolved as a general-purpose stress response that happens to be most studied in the context of Alzheimer's.

If confirmed, this reframing would shift pTau from being a brain-specific biomarker to being a more versatile indicator of amyloid-related cellular stress across the body. That is a loss for Alzheimer's specificity but potentially a gain for systemic amyloidosis diagnosis.

New possibilities for diagnosing systemic amyloidosis

Systemic amyloidosis is notoriously difficult to diagnose early. The symptoms -- fatigue, swelling, shortness of breath, numbness in the extremities -- overlap with dozens of other conditions. By the time amyloid deposits are large enough to cause organ dysfunction, significant damage has often already occurred.

A blood marker that can be measured relatively easily, as Jucker described pTau, could help clinicians detect systemic amyloidosis earlier or at least narrow the diagnostic workup. It would not replace cardiac imaging or tissue biopsy, but it could serve as a flag -- a signal that amyloid-related disease should be investigated.

The findings also have specific implications for polyneuropathy (PNP), a condition causing tingling and numbness in the hands and feet. PNP has many possible causes, and systemic amyloidosis is one of them. Jucker suggested that pTau could help distinguish amyloidosis-related PNP from forms with other underlying causes, potentially speeding diagnosis in a condition where delays are common.

A warning for Alzheimer's testing

The flip side of this discovery is a caution for Alzheimer's diagnostics. If pTau rises in response to amyloid deposits anywhere in the body, then an elevated pTau blood test cannot, on its own, confirm that a patient has Alzheimer's disease. This is particularly problematic in the growing push toward blood-based Alzheimer's screening in people without cognitive symptoms, where the risk of false positives from non-brain amyloid disease becomes relevant.

Jucker was direct on this point: blood levels of pTau are not a specific marker and further data should be considered when diagnosing Alzheimer's or assessing its progression. He emphasized that pTau should not serve as a standalone diagnostic criterion, particularly in the absence of cognitive deficits when Alzheimer's disease is at an early stage.

This does not invalidate pTau as an Alzheimer's biomarker -- it remains one of the most promising tools in the field. But it does mean that clinicians interpreting elevated pTau levels need to consider the patient's full clinical picture, including the possibility of systemic amyloid disease, before concluding that Alzheimer's is the cause.

Boundaries of the evidence

The study included 280 participants, a meaningful but relatively small sample drawn from three European countries. Larger, more diverse cohorts will be needed to establish whether the association holds across different populations, age ranges, and disease severities.

The research also cannot determine whether pTau levels in systemic amyloidosis patients change over time with treatment, which would be necessary to evaluate pTau as a monitoring tool rather than just a diagnostic one. Longitudinal data are not yet available.

Additionally, the mechanism -- that cells release pTau as a general stress response to amyloid -- remains a hypothesis. The study demonstrates the association but does not prove the causal pathway. Alternative explanations, including the possibility that systemic amyloidosis somehow affects brain metabolism indirectly, have not been ruled out.

Still, the finding opens a genuinely useful two-way door: better context for interpreting Alzheimer's blood tests, and a potential new tool for catching diseases that currently slip through diagnostic nets until they have already caused serious harm.