Highlights
- Plasma p-tau217 has emerged as a high-performance stand-alone biomarker for identifying amyloid-beta (Aβ) pathology in cognitively unimpaired (CU) individuals, achieving an accuracy of approximately 81%.
- A sequential 2-step screening workflow (plasma p-tau217 followed by confirmatory PET/CSF) significantly increases the positive predictive value (PPV) to 91%, drastically reducing the need for invasive or expensive imaging.
- Combining plasma %p-tau217 with the Aβ42/40 ratio allows for the prediction of future brain Aβ accumulation even in individuals currently below the traditional threshold for amyloid positivity.
- While tau PET remains superior for tracking real-time disease progression and localized neurodegeneration, plasma p-tau217 is a more sensitive prognostic indicator for early cognitive changes in preclinical stages.
Background
The therapeutic landscape for Alzheimer disease (AD) has undergone a paradigm shift with the advent of amyloid-targeting monoclonal antibodies. Evidence increasingly suggests that disease-modifying treatments (DMTs) are most effective when initiated during the preclinical stage—the period where individuals are cognitively unimpaired (CU) but harbor significant amyloid-beta (Aβ) and tau pathology. However, identifying these individuals at scale remains a monumental challenge. Traditionally, Aβ status has been determined via cerebrospinal fluid (CSF) analysis or amyloid positron emission tomography (PET), both of which are limited by high costs, invasiveness, and limited accessibility.
Phosphorylated tau 217 (p-tau217) has recently been identified as a particularly sensitive blood-based biomarker (BBM) for AD pathology. Unlike other tau isoforms, p-tau217 levels rise very early in response to Aβ deposition, often before overt tau tangles are visible on PET. This review synthesizes findings from three pivotal studies published in JAMA Neurology between 2024 and 2025 to evaluate the clinical utility, prognostic value, and predictive power of p-tau217 in the preclinical AD continuum.
Key Content
Diagnostic Accuracy and Optimized Workflows (Salvadó et al., 2025)
In a massive cross-sectional study involving 2916 CU participants from 12 international cohorts, Salvadó and colleagues evaluated the real-world performance of plasma p-tau217. The study is particularly notable for its comparison between mass spectrometry (MS) and immunoassay platforms, as well as its assessment of sequential screening strategies.
As a stand-alone test, plasma p-tau217 (measured via immunoassay) achieved a PPV of 79% and an overall accuracy of 81% for classifying Aβ status. When the researchers modeled a 2-step workflow—where individuals with intermediate or positive plasma results received a confirmatory PET or CSF test—the PPV soared to 91%. This strategy is highly efficient: it reduced the requirement for PET scans to only 124 per 100 Aβ-positive individuals identified, compared to the 536 scans needed in an unscreened population. While MS platforms showed higher accuracy than immunoassays (88% vs 82%), both modalities proved robust enough to revolutionize clinical trial recruitment and specialized clinical assessments.
Longitudinal Dynamics and Cognitive Prediction (Insel et al., 2025)
While Salvadó focused on cross-sectional identification, Insel et al. utilized data from the A4 (Anti-Amyloid Treatment in Asymptomatic Alzheimer’s Disease) and LEARN studies to investigate how p-tau217 changes over time in relation to tau PET and cognition. Analyzing 1707 participants over 36 months, the study revealed critical temporal dynamics.
In Aβ-positive individuals, p-tau217 levels increased significantly at the start of the study but eventually showed a deceleration, suggesting that plasma p-tau217 may reach a plateau or change at a slower rate as the disease progresses into overt neurodegeneration. Conversely, tau PET accumulation (particularly in the inferior temporal gyrus) continued linearly and was more strongly associated with concurrent cognitive decline as measured by the Preclinical Alzheimer Cognitive Composite (PACC). However, baseline p-tau217 remained a powerful predictor of future cognitive change (correlation ρ = -0.47), emphasizing its role as a prognostic screening tool to identify those at the highest risk for imminent decline.
Predicting Early Brain Aβ Accumulation (Janelidze et al., 2024)
A critical question in primary prevention is whether we can identify individuals who are “pre-amyloid positive”—those with subthreshold Aβ levels (<40 Centiloids) who are destined to accumulate significant pathology. Janelidze et al. addressed this by combining plasma p-tau217 with the Aβ42/40 ratio in the Swedish BioFINDER-2 and replication cohorts.
The study found that a combination of plasma %p-tau217 (the ratio of phosphorylated to non-phosphorylated tau) and Aβ42/40 yielded an AUC of 0.949 for detecting abnormal CSF Aβ status. Most importantly, in participants who were PET-negative at baseline, these combined biomarkers significantly predicted the rate of future Aβ accumulation. This suggests that the p-tau217/Aβ42/40 combination can identify the very earliest biological changes of AD, potentially years before current PET thresholds are met.
Mechanistic Insights and Gut-Brain Axis Considerations
The biological rationale for p-tau217’s performance lies in its close coupling with Aβ soluble species. Emerging research also points to systemic factors that may exacerbate these pathways. For instance, recent studies in Gut Microbes have identified that gut-derived metabolites like Trimethylamine N-oxide (TMAO) can exacerbate tau phosphorylation via HIF1α signaling pathways. While primarily preclinical, these findings suggest that the rise in p-tau217 measured in the plasma may be influenced by systemic inflammatory and metabolic states, providing a broader context for why blood-based biomarkers are so reflective of overall disease progression.
Expert Commentary
The evidence presented here marks the end of the “biomarker research phase” and the beginning of the “clinical implementation phase” for p-tau217. The 2-step workflow proposed by Salvadó et al. addresses the primary concern of clinicians: the risk of false positives when using a blood test as a stand-alone diagnostic for a terminal condition. By using p-tau217 to filter out likely negative cases, the medical system can focus its limited PET and CSF resources on the high-probability cases, making population-level screening for AD trials economically viable for the first time.
However, controversies remain. The plateauing of p-tau217 observed by Insel et al. suggests that while it is an excellent entry marker for trials, it may not be the ideal pharmacodynamic marker to track the effects of drugs that target tau tangles specifically; for that, tau PET remains the gold standard. Additionally, the field must standardize cut-off values across different immunoassay platforms (e.g., ALZpath, Janssen, Quanterix) to ensure that a “positive” result in one lab means the same as in another. There is also a pressing need for more data in diverse populations, as most current cohorts are predominantly white and highly educated.
Conclusion
Plasma p-tau217, particularly when combined with Aβ42/40, is a transformative tool for the identification of preclinical Alzheimer disease. It offers high diagnostic accuracy for Aβ status, predicts future amyloid accumulation in subthreshold individuals, and forecasts cognitive decline. As we move toward a future of primary prevention, p-tau217 will likely become the first-line screening tool in clinical practice, guiding the selection of patients for confirmatory testing and eventual access to disease-modifying therapies.
References
- Salvadó G, Janelidze S, et al. Plasma Phosphorylated Tau 217 to Identify Preclinical Alzheimer Disease. JAMA Neurol. 2025;82(11):1122-1134. PMID: 40952756.
- Insel PS, Mattsson-Carlgren N, et al. Concurrent Changes in Plasma Phosphorylated Tau 217, Tau PET, and Cognition in Preclinical Alzheimer Disease. JAMA Neurol. 2025;82(10):985-993. PMID: 40853684.
- Janelidze S, Barthélemy NR, et al. Plasma Phosphorylated Tau 217 and Aβ42/40 to Predict Early Brain Aβ Accumulation in People Without Cognitive Impairment. JAMA Neurol. 2024;81(9):947-957. PMID: 39068669.
- Zheng et al. Gut microbe-derived metabolite trimethylamine-N-oxide exacerbates Alzheimer’s disease progression via targeting HIF1α signaling. Gut Microbes. 2026;18(1):2605768. PMID: 41459734.
