What is Tau217?
What is tau217?
Tau is a normal protein inside neurons. One of its jobs is to help stabilize the cell's internal "railroad tracks"—the microtubules that transport materials through the neuron.
The 217 in p-tau217 refers to a specific location on the tau protein: threonine 217. When tau becomes phosphorylated at that location, it is called phosphorylated tau 217, or p-tau217.
In Alzheimer's disease, the appearance and increasing amount of p-tau217 in blood appears to be closely associated with the biological process connecting amyloid pathology to tau pathology. Importantly, p-tau217 can rise before widespread tau tangles can be detected by tau PET.
So what is the blood test actually measuring?
Think of it this way:
Amyloid begins accumulating → neurons respond → tau becomes abnormally phosphorylated → small tau fragments containing p-tau217 enter the bloodstream → the laboratory measures those fragments.
So the blood test is looking for a chemical signal produced by the Alzheimer's process, rather than scooping up tau tangles themselves.
This is why p-tau217 is particularly interesting. Early in Alzheimer's, its increase appears to be strongly associated with amyloid pathology. Later, as tau pathology spreads into the neocortex, p-tau217 becomes increasingly associated with the amount of tau pathology as well.
What does the number mean?
This is the important part: there is no single universal p-tau217 number that applies to every laboratory.
Different companies use different assays, antibodies, equipment, units and cutoffs. Consequently, you should interpret the result using the reference ranges printed on your particular laboratory report, rather than comparing your number with a cutoff from another test. Studies have demonstrated meaningful differences between p-tau217 assays.
For example, Mayo Clinic Laboratories' current plasma p-tau217 assay reports:
Result
Mayo interpretation
≤ 0.185 pg/mL
Negative
0.186–0.324 pg/mL
Intermediate
≥ 0.325 pg/mL
Positive
For that particular assay, a negative result is consistent with a low likelihood of Alzheimer-type neuropathological changes, while a positive result is consistent with amyloid pathology. An intermediate result requires additional testing.
Those numbers should NOT be applied to another laboratory's p-tau217 test.
Does a high p-tau217 mean "I have a lot of tau tangles"?
Not necessarily.
This distinction is especially important for your book.
A high p-tau217 tells us that the Alzheimer's biological process is likely active, but p-tau217 is not a simple "tangle meter."
Research suggests that:
Early increase: p-tau217 can rise in response to amyloid pathology, even before substantial widespread tau accumulation.
Later increase: as tau pathology spreads, p-tau217 becomes increasingly related to tau accumulation.
Therefore, p-tau217 reflects both amyloid-related tau changes and, later, more extensive tau pathology.
This is one reason the Alzheimer's Association's newer diagnostic framework distinguishes T1 biomarkers, such as plasma p-tau217, from biomarkers that more directly indicate aggregated tau pathology. Tau PET and certain later-appearing tau fragments can provide different information about actual tau aggregation.
A useful analogy
Imagine Alzheimer's disease as a fire in a house.
Amyloid plaques are like fuel accumulating in the house.
Tau pathology is like the fire spreading through different rooms.
p-tau217 is like smoke escaping through the chimney.
A little smoke tells you something is happening inside, but measuring the smoke doesn't tell you exactly how many rooms are burning.
Early in the process, you may detect smoke before the fire has spread very far. Later, increasing smoke can correspond to a much larger fire.
That is roughly how p-tau217 should be thought of: an early and highly informative signal of the Alzheimer's disease process, not a direct count of tau tangles.
Why p-tau217 is so valuable
One of the remarkable findings is that p-tau217 can become abnormal around the same general time that amyloid PET becomes abnormal, before tau PET becomes clearly abnormal.
That means a person can potentially have:
Amyloid pathology → elevated p-tau217 → little or no detectable tau PET abnormality → later widespread tau pathology
rather than:
Amyloid → immediately measurable tau tangles → p-tau217
That distinction is very important when explaining why amyloid, p-tau217, tau tangles, symptoms, and cognitive test results don't all appear at the same time.
And as of the current 2025 Alzheimer's Association clinical guideline, high-performing blood biomarkers can be used in specialized memory-care settings as part of the diagnostic workup; however, the guideline emphasizes that performance varies by assay and that blood biomarkers should be interpreted in the context of the person's clinical evaluation