Phosphorylated tau at threonine 181 (p-tau181) is a highly specific blood-based biomarker for Alzheimer's disease pathology. It was one of the first phosphorylated tau species to be developed as a plasma assay and remains widely used in clinical research and increasingly in clinical practice. Plasma p-tau181 correlates with both amyloid-beta and tau pathology as measured by PET imaging, making it a valuable biomarker for detecting Alzheimer-type neurodegeneration.[1][2]
The tau protein (encoded by the MAPT gene) is a microtubule-associated protein that stabilizes neuronal axons. In Alzheimer's disease, tau becomes abnormally hyperphosphorylated, leading to neurofibrillary tangle formation. Threonine 181 is one of the earliest and most studied phosphorylation sites on tau.[1:1]
The phosphorylation at position 181 occurs via several kinases, including:
These kinases are activated in Alzheimer's disease by various pathological triggers including amyloid-beta oligomers, neuroinflammation, and mitochondrial dysfunction.
p-tau181 is released into the cerebrospinal fluid (CSF) and subsequently into blood through:
The phosphorylated form (p-tau181) appears to be more specific to AD pathology than total tau, as it reflects the disease-specific post-translational modifications occurring in AD brains.[1:2][2:1]
Plasma p-tau181 has demonstrated excellent performance for distinguishing Alzheimer's disease from:
Key performance characteristics:
p-tau181 is particularly valuable for predicting progression from MCI to AD dementia:
Plasma p-tau181 correlates with:
This makes p-tau181 useful as a screening tool to identify individuals likely to have AD pathology who might benefit from further diagnostic workup or clinical trial enrollment.[2:2][3]
Fujirebio Lumipulse G
Roche Elecsys
Quanterix Simoa
ALENTO (Next Generation)
UCB-MS (Ultrasensitive Immun-MS)
| Biomarker | Specificity | Sensitivity (AD) | CSF/Plasma | Key Advantage |
|---|---|---|---|---|
| p-tau181 | High | 85-90% | Both | First validated, widely available |
| p-tau217 | Very High | 90-95% | Both | Best for differential diagnosis |
| p-tau231 | High | 80-85% | CSF only | Earliest detection in preclinical AD |
| Total tau | Moderate | 70-80% | Both | Non-specific neuronal damage |
Recent research (2026) has revealed that p-tau181 can also be elevated in systemic amyloidoses:[4]
This finding has important implications for differential diagnosis in patients with both AD and systemic amyloidosis, as well as for clinical trial interpretation.
In corticobasal syndrome (CBS), plasma p-tau181 helps distinguish:[5][6]
The p-tau181/NfL ratio has emerged as a useful discriminator between AD and 4R tauopathies.
In PSP, p-tau181 shows:
p-tau181 should be interpreted in clinical context:
p-tau181 serves as the T (tau) biomarker in the ATN (Amyloid/Tau/Neurodegeneration) research framework:
A+ (Amyloid positive): Aβ42/40 ratio, amyloid PET
T+ (Tau positive): p-tau181, p-tau217, tau PET
N+ (Neurodegeneration): NfL, FDG-PET, MRI atrophy
Tau Pathology Pathway
Amyloid-beta Biomarkers
Blood Biomarkers for Neurodegeneration
CSF Biomarkers Comparison
p-tau217
Neurofilament Light Chain (NfL)
Palmqvist S, Janelidze S, Quiroz YT, et al. Discriminatory accuracy of plasma phospho-tau217 for Alzheimer disease vs other neurodegenerative disorders. JAMA. 2020. ↩︎ ↩︎ ↩︎
Janelidze S, Teunissen CE, Zetterberg H, et al. Head-to-head comparison of 8 plasma amyloid-beta 42/40 and phospho-tau assays in Alzheimer disease. JAMA Neurology. 2021. ↩︎ ↩︎ ↩︎
Karikari TK, Pascoal TA, Ashton NJ, et al. [Blood phosphorylated tau 181 as a biomarker for Alzheimer's disease: a systematic review and meta-analysis](https://doi.org/10.1016/S1474-4422(22). Lancet Neurology. 2022. ↩︎
Elevated blood p-tau181, p-tau217 not unique to Alzheimer's disease. AlzForum Research News. 2026. ↩︎
Altmann M, et al. A Biomarker-Based Classification of Corticobasal Syndrome. Mov Disord. 2025. ↩︎
Stamelou M, et al. Neuropathological hints from CSF and serum biomarkers in corticobasal syndrome (CBS): a systematic review. Neurol Res Pract. 2023. ↩︎