p-tau181 (Phosphorylated Tau 181)
A form of tau protein in blood plasma. A result at or below the lab's cutoff makes amyloid changes less likely; a result above it calls for more testing.
AI-verified against cited sources. Awaiting expert reviewAlso called p-tau181, pTau181, P-tau181LOINC 103675-5
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What it is
Tau is a protein inside nerve cells. In Alzheimer's disease, tau picks up extra phosphate groups, and some of this phosphorylated tau reaches the blood. p-tau181 is tau with a phosphate group at position 181. This test measures it in plasma using a lab-specific method. Kidney disease can raise it too, and many people without amyloid have results above the cutoff, so a high result is not specific to Alzheimer's disease.
Why it matters
p-tau181 is one of several blood tests used when someone has memory or thinking changes. The FDA cleared one version, the Roche Elecsys test, in 2025 as an aid in the first assessment of adults 55 and older with cognitive complaints. In its main study, about 98 of every 100 negative results matched a negative amyloid PET scan. But only about 22 of every 100 positive results matched a positive scan. So a negative result is more informative than a positive one, and a positive result is a reason for follow-up, not a diagnosis. p-tau217 tests usually separate people better.
If it’s low
A result at or below the cutoff (a negative result) makes the amyloid changes of Alzheimer's disease less likely as the reason for memory or thinking changes. In the FDA study, about 98 of every 100 negative results matched a negative amyloid PET scan. Other causes of the symptoms should still be looked into.
Common causes
- No amyloid build-up in the brain
- Another cause of memory or thinking changes, such as depression, certain medicines or another kind of dementia
What can help
- Ask your clinician which other causes of your symptoms should be checked
- Tell your clinician if your symptoms change or get worse
If it’s high
A result above your lab's cutoff means more p-tau181 than the cutoff allows. This can happen in people with the amyloid changes of Alzheimer's disease, but also in many people without them. In the FDA study, about 3 in 4 positive results did not match a positive amyloid PET scan. It is a reason for follow-up, not a diagnosis.
Common causes
- A result close to the lab's cutoff, where small measurement differences can change the category
- Reduced kidney function
- Early brain changes in an older adult with no symptoms: in the FDA reference study of 174 cognitively normal adults aged 55 to 80, 39% were above the cutoff
- Amyloid build-up in the brain linked to Alzheimer's disease
What can help
- Ask your clinician whether a p-tau217 blood test, an amyloid PET scan or a spinal fluid test would help sort out a result above the cutoff
- Ask whether your kidney function has been checked, since kidney problems can raise p-tau181
- Tell your clinician about all of your symptoms and the medicines you take
What can affect your result
- Method: each lab and assay has its own scale and cutoff
- Kidney function: reduced function is linked to higher levels
- Age: levels rise slightly with age, mostly after about 70, in cognitively normal adults
- How common amyloid is in the group tested, which changes how often a positive result is right (13% of the Roche study group had a positive PET scan)
- Sample type: the cleared Roche test uses K2-EDTA plasma
- In the FDA studies of the Roche assay, hemoglobin, bilirubin, lipids, biotin, rheumatoid factor, albumin, immunoglobulins and 43 common and special-use drugs did not interfere significantly.
- Reduced kidney function is linked to higher plasma p-tau181 in studies, so kidney disease can raise a result on its own.
- The cleared Roche test uses K2-EDTA plasma. Other lab methods have different scales and handling rules, so results are not interchangeable.
- Levels rise slightly with age, mostly after about 70, in cognitively normal adults.
When to talk to a clinician
A blood p-tau181 result is not an emergency on its own. Talk with a clinician if you or your family notice memory or thinking changes that are new, getting worse or getting in the way of daily life. Call 911 or your local emergency number for sudden confusion, trouble speaking or face drooping.
Questions you could ask
- Is this test appropriate for me, given my age and symptoms?
- Which lab and method was used, and what cutoff does it apply?
- Would a p-tau217 test, an amyloid PET scan or a spinal fluid test help sort out this result?
- Could my kidney function or another condition explain it?
- What other causes of my symptoms should we look into?
- If my symptoms change, should we repeat the test or try a different one?
Questions people ask
Why is a negative p-tau181 result more useful than a positive one?
In the FDA study, a negative result matched a negative amyloid PET scan about 98 times out of 100, so it helps make amyloid changes less likely. A positive result matched a positive scan only about 22 times out of 100, because many people without amyloid also test positive. A positive result is a reason for more testing.
Is p-tau181 or p-tau217 the better test?
p-tau217 tests have generally done a better job of telling people with amyloid changes from people without them. p-tau181 is still used, and the Roche p-tau181 test is cleared as a first-step aid for adults with cognitive complaints. Your clinician can explain which test fits your situation.
Show the science and sources
For clinicians and researchers
p-tau181 is tau phosphorylated at threonine 181. The Roche Elecsys Phospho-Tau (181P) Plasma assay (ECLIA on cobas e analyzers, K2-EDTA plasma) was cleared by the FDA on 2025-10-08 (K252163) as an aid in the initial assessment of adults aged 55 or older with signs, symptoms or complaints of cognitive decline, with a single cutoff: above 0.722 pg/mL is positive. Its measuring range is 0.300–10 pg/mL. In 312 people with cognitive complaints (average age 69; 13.1% amyloid-PET positive), positive percent agreement was 92.7%, negative percent agreement 51.3%, PPV 22.4% and NPV 97.9%, and 45.5% of people had a negative (rule-out) result. In 174 cognitively normal adults aged 55–80, the median was 0.658 pg/mL (2.5th–97.5th percentile 0.323–1.91) and 39% were above the cutoff. The labeling says the test is not recommended for people already referred to a specialist and that performance is not established for predicting dementia or monitoring therapy. Earlier research cohorts found plasma p-tau181 separated Alzheimer's dementia from other neurodegenerative diseases with AUCs of 0.94–0.98 (Janelidze 2020). In head-to-head studies of people with mild cognitive impairment, p-tau181 immunoassays generally separated amyloid-positive from amyloid-negative people less well than the best p-tau217 methods (Janelidze 2023), and chronic kidney disease is linked to higher p-tau181 and p-tau217 (Janelidze 2023, JAMA Neurology).
LOINC 103675-5 (primary). Specimen: plasma.
Sources
- Elecsys Phospho-Tau (181P) Plasma, 510(k) K252163 decision summary, U.S. Food and Drug Administration (2025)
- Head-to-head comparison of 10 plasma phospho-tau assays in prodromal Alzheimer's disease, Brain (2023)
- Mitigating the Associations of Kidney Dysfunction With Blood Biomarkers of Alzheimer Disease by Using Phosphorylated Tau to Total Tau Ratios, JAMA Neurology (2023)
- Plasma P-tau181 in Alzheimer's disease: relationship to other biomarkers, differential diagnosis, neuropathology and longitudinal progression to Alzheimer's dementia, Nature Medicine (2020)
- The Alzheimer's Association appropriate use recommendations for blood biomarkers in Alzheimer's disease, Alzheimer's & Dementia (2022)
- Alzheimer's Association Clinical Practice Guideline on the use of blood-based biomarkers in the diagnostic workup of suspected Alzheimer's disease within specialized care settings, Alzheimer's & Dementia (2025)
- LOINC 103675-5: Phosphorylated tau 181 [Mass/volume] in Plasma by Immunoassay, LOINC via NLM Clinical Tables
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.