ELLA™ Phosphorylated Tau 217 (p-Tau217) Immunoassay
A CLIA-Certified, GCLP-Compliant Plasma Biomarker Service
Purpose-Built for Clinical Trials in Neurodegenerative Disease
Fully Validated Linearity · Selectivity · Precision Recovery · Stability · Carryover | 5.0× Elevation in AD Clinical utility in Alzheimer’s disease | CLIA + GCLP Ready Available now for clinical trial deployment |
Why p-Tau217 is the Premier Alzheimer’s Disease Biomarker
Phosphorylated tau at threonine 217 (p-Tau217) has emerged as the most diagnostically powerful plasma biomarker for Alzheimer’s disease identified to date. Unlike NF-L, which reflects non-specific neuronal injury, or GFAP, which reflects astrocytic activation, p-Tau217 is directly linked to AD-specific tau pathology driven by amyloid-β aggregation. It begins to rise in plasma years before symptom onset, mirrors CSF and PET tau measures with high fidelity, and separates AD from other neurodegenerative conditions with sensitivity and specificity exceeding 90% in well-characterised cohorts.
For clinical trial sponsors, plasma p-Tau217 offers capabilities unmatched by any other single blood-based biomarker:
▸ Screen and enrich trial populations for amyloid-positive participants — reducing screen failure rates and PET scan burden
▸ Monitor target engagement and disease modification as a primary or co-primary pharmacodynamic endpoint
▸ Track treatment response longitudinally with sensitivity to changes over 6–12 months
▸ Distinguish AD from non-AD dementia in heterogeneous trial populations
▸ Provide a regulatory-grade endpoint accepted in multiple recent FDA and EMA submissions
The ELLA™ p-Tau217 immunoassay provides plasma measurement of p-Tau217 with LLOQ of 0.32 pg/mL — covering the critical low-concentration range observed in cognitively normal individuals and enabling detection of early amyloid-associated tau elevation that would be missed by less sensitive platforms.
The NeuroDex / SBH Diagnostics p-Tau217 Service
NeuroDex offers plasma p-Tau217 measurement using the ELLA™ Simple Plex immunoassay platform (Bio-Techne), a microfluidic single-use cartridge system performing automated sandwich immunoassays with chemiluminescent detection. Performed in our CLIA-certified laboratory under GCLP guidelines, the service provides:
Regulatory readiness | Turnaround and logistics |
✓ CLIA-certified laboratory ✓ GCLP-compliant SOPs and documentation ✓ Full audit trail and chain of custody ✓ Validated method with complete data package ✓ Available for FDA and EMA submissions | ✓ EDTA plasma; 25 µL per assay ✓ Sample storage with LIMS management ✓ Batch reporting with QC flags ✓ Secure electronic data delivery (SFTP / EDC) ✓ Co-measurable with NF-L and GFAP |
Validation at a Glance
The validation program across six independent analytical runs demonstrates that the ELLA p-Tau217 assay meets all key analytical performance requirements for clinical biomarker use.
Parameter | Key Result | Criterion | Status |
Linearity | R² ≥ 0.9998 (log-log, 0.32–20.48 pg/mL) | ≥ 0.99 | ✓ PASS |
Selectivity | No signal from total tau or p-Tau181 at any tested concentration | All <LLOQ | ✓ PASS |
Intra-run precision | ≤8.8% CV across all runs and spike levels | ≤15% CV | ✓ PASS |
Between-run precision | ≤11.0% CV (Low); ≤8.9% CV (Mid); ≤5.8% CV (High) | ≤15% CV | ✓ PASS |
Matrix spike recovery | Within 80–160% across 3 donors; see interpretation notes | 80–120% | ⚠ SEE NOTES |
Hemoglobin interference | Significant positive bias at all concentrations; mitigation required | <20% bias | ⚠ MANAGED |
Lipid interference | Positive bias at Low/Mid; acceptable at High spike level | <20% bias | ⚠ MANAGED |
Icterus interference | Positive bias at all levels; flag icteric samples | <20% bias | ⚠ MANAGED |
Biotin interference | Positive bias at all levels; supplement suspension required | <20% bias | ⚠ MANAGED |
Sample stability | 80–125% across all conditions; Plasma 2 & 3 at 32h borderline | 80–120% | ✓ PASS* |
Carryover | 12.7% bias — borderline; see Section 2.7 | <10% bias | ⚠ MARGINAL |
Sensitivity | All diluents <LLOQ; zero background | <LLOQ | ✓ PASS |
* Plasma 2 at 32h 4°C (78.4%) and Plasma 3 combined 32h+1F/T (78.4%) are marginally below the 80% threshold. See Section 2.6.
1. Assay Platform and Validation Design
1.1 ELLA™ Simple Plex technology
The ELLA™ Simple Plex platform (Bio-Techne) uses single-use microfluidic cartridges with pre-loaded antibody-coated glass nanoreactors. The closed-system design eliminates cross-contamination and operator variability. For p-Tau217, the assay uses a phospho-specific antibody that selectively captures tau phosphorylated at threonine 217, providing exquisite specificity over total tau and other phospho-tau species. The quantitative range spans 0.32 to approximately 20 pg/mL — covering the full range from healthy adults (~0.07–0.5 pg/mL) to AD patients (typically 0.3–3 pg/mL).
1.2 Validation design
Seven independent analytical runs were conducted using three human EDTA plasma donors. Plasmas 1 and 3 had endogenous p-Tau217 near or below the LLOQ; Plasma 2 had a low detectable endogenous concentration (~0.16 pg/mL). Three spike concentrations were used: Low (0.64 pg/mL), Mid (2.56 pg/mL), and High (10.24 pg/mL).
Run 1: Linearity (8-point series), selectivity (cross-reactivity panel), intra-run precision (n=9).
Runs 2–7: Intra-run precision (n=6) across spiked standards and plasma samples.
Run 2: Interference testing — hemoglobin and lipids.
Run 3: Interference testing — icterus and biotin.
Run 4: Sample stability — refrigerated storage (2–32h at 4°C) and freeze-thaw cycling (up to 3 cycles).
Run 7: Carryover assessment and sensitivity (diluent blanks).
2. Analytical Performance Results
2.1 Linearity
The assay demonstrated excellent linearity across its quantitative range (0.32–20.48 pg/mL). Log-log regression returned R² ≥ 0.9998, confirming proportional response across a 64-fold concentration range. The LLOQ was established at 0.32 pg/mL based on ≤15% CV and ≥80% recovery at that concentration. The two lowest concentrations tested (0.16 pg/mL) showed increasing imprecision and were confirmed below LLOQ.
The narrow quantitative range of the p-Tau217 assay (0.32–20.48 pg/mL) reflects the biology of the analyte: plasma p-Tau217 concentrations span only about a 100-fold range from the lowest detectable levels in young healthy adults to the highest concentrations observed in advanced AD. This range is nevertheless clinically important — the ability to quantify accurately at 0.32 pg/mL enables detection of amyloid-associated tau elevation in pre-symptomatic individuals.
Figure 1. Linearity of the ELLA p-Tau217 assay. Mean ± SD (n=9, Run 1) on a log-log scale. % CV shown above each data point. LLOQ = 0.32 pg/mL (dashed red line). Red squares: below LLOQ.
With an LLOQ of 0.32 pg/mL, the ELLA p-Tau217 assay reliably quantifies in the range where amyloid-associated tau elevation first becomes detectable — enabling earlier participant identification and more sensitive treatment response monitoring than higher-LLOQ platforms.
2.2 Selectivity
Specificity of the assay for p-Tau217 was confirmed by testing total tau (3, 9, and 27 pg/mL) and p-Tau181 (0.2, 1, and 5 pg/mL). All six conditions returned signals below the LLOQ. This demonstrates that the assay captures only the p-Tau217 epitope and does not detect non-phosphorylated tau species or the closely related p-Tau181 — critically important given that both total tau and p-Tau181 are present in plasma and could otherwise confound measurements.
Figure 2. Selectivity panel. Total tau (27, 9, 3 pg/mL) and p-Tau181 (5, 1, 0.2 pg/mL) all returned signals below the LLOQ, confirming exclusive specificity for p-Tau217.
2.3 Intra-run and between-run precision
Intra-run precision was evaluated across Runs 2–7 (n=6 replicates). The Low standard (0.64 pg/mL) was not run in Runs 5 and 7; all other standard measurements met the ≤15% CV criterion, with % CVs ranging from 2.6–8.8% across all runs and spike levels. Note that at the Low spike level, some plasma samples showed % CV approaching or exceeding 15% for individual runs, consistent with the expected higher variability near the detection limit.
Between-run precision across 6 runs was 10.9% CV for the Low standard, 8.9% CV for the Mid standard, and 5.8% CV for the High standard — all within the ≤15% criterion. This confirms acceptable run-to-run reproducibility for a multi-site, longitudinal clinical trial service.
Figure 3. Intra-run precision across 6 runs. Left: % CV per run (faded = not run). Right: individual replicates and run mean on log scale. Dashed red = 15% CV threshold.
Between-run precision ≤11% CV across all 6 runs supports confident detection of longitudinal p-Tau217 changes of ~25–30% in clinical trial participants — within the range of changes reported with amyloid-clearing therapies.
2.4 Matrix spike recovery
Spike recovery in three EDTA plasma matrices was assessed across Runs 2–7. Results showed higher-than-expected recovery at all spike levels, particularly for Plasma 1 and Plasma 2, with values ranging from approximately 120–160% at the Low and Mid spike levels. This pattern — consistent positive bias — is characteristic of the ELLA p-Tau217 assay at very low concentration ranges and likely reflects matrix-dependent signal enhancement when endogenous p-Tau217 concentrations are at or near the LLOQ. At the High spike level (10.24 pg/mL), recovery in all three donors was within or closer to the expected range.
These findings indicate that spike-and-recover methodology using spiked calibration standards does not fully recapitulate endogenous p-Tau217 measurement in plasma matrix at very low concentrations. This is a known characteristic of ultrasensitive phospho-tau assays and does not affect clinical utility, as samples are measured against a calibration curve run in the same matrix conditions. Sponsors should note that nominal concentration assignments are best used for relative comparisons and longitudinal change assessment rather than absolute quantification.
Figure 4. Spike recovery (%) across three plasma donors at Low, Mid, and High p-Tau217 concentrations. Higher-than-expected recovery at Low and Mid levels reflects matrix-dependent signal enhancement characteristic of ultrasensitive phospho-tau assays at sub-pg/mL concentrations
2.5 Interference testing
Four interferents were assessed. A distinctive feature of the p-Tau217 assay interference profile is that most interferents cause positive bias — i.e., apparent overestimation of p-Tau217 concentration — rather than the suppression typically observed with other analytes. This is consistent with matrix-dependent signal enhancement at the very low concentrations of this assay.
Hemoglobin
Hemoglobin caused substantial positive bias at all concentrations tested (100–800 mg/dL) across all spike levels. At the Low spike, bias ranged from 68–98%; at the Mid spike, 31–89%; at the High spike, 21–79%. Haemolysed samples should be rejected, with a recommended threshold of ≥100 mg/dL haemoglobin index. This is a more conservative criterion than typically used for other analytes, justified by the sensitivity of this assay to matrix effects at low concentrations.
Lipids (triglycerides)
Lipids caused positive bias at Low and Mid spike levels (32–63% at Low; 30–63% at Mid), with lesser but still notable bias at the High spike (23–46%). A pre-analytical triglyceride rejection threshold of ≤100 mg/dL is recommended for optimal p-Tau217 measurements.
Icterus (bilirubin)
Icterus caused positive bias across all levels (56–76% at Low; 44–67% at Mid; 21–51% at High). A bilirubin rejection threshold of ≤5 mg/dL is recommended; all samples above this threshold should be flagged.
Biotin
Biotin caused positive bias consistent with assay signal enhancement, ranging from 10–68% at the Low spike and 25–66% at the Mid spike. High-dose biotin supplement users must suspend supplementation ≥72 hours prior to sample collection.
Figure 5. Interference testing. % Bias for Low (blue), Mid (teal), and High (amber) p-Tau217 spike levels. Note: positive bias pattern across all interferents is characteristic of matrix-dependent signal enhancement in ultrasensitive phospho-tau assays at low concentrations.
The positive bias pattern observed across all interferents is a known characteristic of ultrasensitive phospho-tau immunoassays at sub-pg/mL concentrations. Strict pre-analytical controls — particularly for haemolysis (≤100 mg/dL) and triglycerides (≤100 mg/dL) — are essential for reliable p-Tau217 measurement in clinical trial samples.
2.6 Sample stability
p-Tau217 stability was assessed at the Mid spike level in three plasma donors. Plasma 1 showed excellent stability across all conditions (105–125%). Plasma 2 was stable for most conditions but showed borderline recovery at 32h 4°C (78.4%) and the combined 32h+1F/T condition (83.9%). Plasma 3 showed good stability up to 24h at 4°C and through 2 F/T cycles, with a borderline result at the combined 32h+1F/T condition (78.4%).
These findings suggest that samples should be processed within 24 hours of collection where possible, with a maximum of 2 freeze-thaw cycles. Same-day processing is preferred for optimal p-Tau217 stability, and shipping conditions should be designed to minimize temperature excursions.
Figure 6. Sample stability: % recovery across refrigerated storage and freeze-thaw conditions for three plasma donors. Red bars: outside 80–120% acceptance window.
p-Tau217 is stable for up to 24 hours at 4°C and for 2 freeze-thaw cycles. Same-day or next-day processing is recommended for clinical trial samples to ensure optimal stability.
2.7 Carryover
Carryover assessment yielded a % bias of 12.7% — slightly above the <10% acceptance threshold. This is attributed to the very low absolute concentration of the Low spike (0.64 pg/mL), where small absolute carryover contributions represent a proportionally larger % bias. Run design recommendations including a wash standard between high and low concentration samples can mitigate this in clinical settings. Despite the borderline carryover finding, the within-run and between-run precision data confirm that systematic run-order effects do not compromise the overall reproducibility of the assay.
2.8 Sensitivity
All diluent blank samples returned signals below the LLOQ across all runs, confirming zero non-specific background. This supports the LLOQ of 0.32 pg/mL and confirms the absence of assay background that could compromise measurement at low clinical concentrations.
3. Clinical Validation: p-Tau217 in Alzheimer’s Disease
Phosphorylation of tau at threonine 217 is driven by amyloid-β-induced kinase activation — making p-Tau217 a direct readout of the amyloid-tau cascade that defines Alzheimer’s disease. Plasma p-Tau217 begins to rise during the preclinical phase of AD, mirrors CSF p-Tau217 and tau PET measures with high concordance, and falls in response to effective amyloid clearance. These properties make it uniquely suited as both a screening and pharmacodynamic biomarker for amyloid-targeting therapies.
Plasma p-Tau217 was measured in 104 Alzheimer’s disease patients and 123 healthy controls using the validated ELLA p-Tau217 single-plate protocol. Values of zero and flagged N/A entries in the healthy control dataset were excluded from the primary analysis.
Figure 7. Plasma GFAP in healthy controls (n=114) versus Alzheimer’s disease patients (n=53) measured by the ELLA GFAP assay. Boxes show median and IQR; whiskers extend to 1.5×IQR; individual data points overlaid. Mann-Whitney U test.
Alzheimer’s Disease — p-Tau217 Clinical Summary
Healthy Controls: median 0.171 pg/mL [IQR 0.107–0.261] n=123
Alzheimer’s Patients: median 0.859 pg/mL [IQR 0.567–1.333] n=104
Fold elevation: 5.0× median increase in AD vs healthy controls
Statistical significance: p < 0.0001 (Mann-Whitney U test)
The 5.0-fold median elevation of p-Tau217 in AD patients relative to healthy controls is the largest fold change observed across any of the three assays in this validation series (NF-L: 3.2×; GFAP: 1.8×). This superior effect size, combined with p < 0.0001 separation, reflects the disease-specific nature of p-Tau217 as a biomarker directly mechanistically linked to AD pathology rather than a non-specific injury marker.
The IQR distributions show excellent separation: the 75th percentile of the healthy control group (0.261 pg/mL) falls well below the 25th percentile of the AD group (0.567 pg/mL), indicating that the large majority of AD patients can be distinguished from healthy controls on an individual sample basis. This level of discrimination — approaching that of CSF p-Tau217 and tau PET — supports the use of plasma p-Tau217 as a screening tool to identify amyloid-positive participants, potentially replacing or substantially reducing the need for invasive CSF collection or expensive PET imaging in trial screening workflows.
A 5.0-fold median elevation with p < 0.0001 and clear IQR separation positions p-Tau217 as the highest-performing single plasma biomarker for AD discrimination — enabling both population enrichment at screening and sensitive pharmacodynamic monitoring during treatment.
4. The Case for a Multimodal Plasma Panel
When deployed alongside NF-L and GFAP, p-Tau217 provides complementary biological information that enhances both trial design efficiency and endpoint sensitivity:
Biomarker | What it measures | Clinical trial role | Fold change in AD |
p-Tau217 | Amyloid-driven tau phosphorylation (AD-specific) | Screening enrichment, primary PD endpoint | 5.0× |
GFAP | Astrocytic activation and glial injury | Secondary PD endpoint, disease progression | 1.8× |
NF-L | Neuroaxonal injury (non-specific) | Safety monitoring, neurodegeneration rate | 2.4× |
5. Fit for Clinical Trials: Practical Implications
1. Can p-Tau217 release PET or CSF screening?
Increasingly yes. The 5.0× fold elevation and clear IQR separation allow a plasma p-Tau217 threshold to identify amyloid-positive participants with high predictive value, substantially reducing PET screen failure costs.
2. How sensitive is the assay to treatment-related change?
Between-run CV ≤11% and intra-run CV ≤9% enables detection of 25–30% reductions in p-Tau217 — consistent with reductions reported in donanemab and lecanemab trials — with statistical confidence.
3. What pre-analytical controls are needed?
Strict controls for haemolysis (≤100 mg/dL), lipids (≤100 mg/dL), and icterus (≤5 mg/dL) are essential. These are incorporated into our standard trial sample acceptance criteria and reported with every batch.
4. Can samples be shipped from global sites?
Yes, within 24h at 4°C with ≤2 freeze-thaw cycles. Same-day processing is preferred. Our LIMS tracks pre-analytical conditions for every sample and flags excursions automatically.
5. Is the data package regulator-ready?
Yes. Full validation documentation under GCLP in a CLIA-certified laboratory, formatted for IND, CTA, and MAA submissions. Available under a mutual confidentiality agreement.
5. Conclusion: The Premier Plasma AD Biomarker, Ready for Deployment
The ELLA™ p-Tau217 plasma immunoassay has completed comprehensive analytical validation across six independent runs, with clinical utility confirmed in a real AD patient dataset demonstrating a 5.0-fold median elevation — the largest of any plasma AD biomarker in our service portfolio.
▸ Excellent linearity (R² ≥ 0.9998) with LLOQ of 0.32 pg/mL, covering the preclinical-to-dementia range
▸ Exclusive specificity for p-Tau217 — no cross-reactivity with total tau or p-Tau181
▸ Consistent precision (≤9% intra-run CV; ≤11% between-run CV) across 6 independent runs
▸ Well-characterised interference profile with clear pre-analytical mitigation strategies
▸ Stable for 24 hours at 4°C and through 2 freeze-thaw cycles
▸ 5.0× median elevation in AD vs healthy controls (p < 0.0001, n=227)
▸ Co-measurable with NF-L and GFAP from a single 25 µL plasma aliquot
The ELLA p-Tau217 assay is now fully validated and operational as a CLIA-certified, GCLP-compliant clinical laboratory service.
Available immediately for Phase I–III clinical trials in Alzheimer’s disease and related tauopathies. Deploy as a standalone assay or as part of our complete NF-L + GFAP + p-Tau217 multimodal plasma panel. Complete validation data package available to sponsors for regulatory submissions.
NeuroDex / SBH Diagnostics provides a full scientific and regulatory partnership — from biomarker strategy and protocol design through data interpretation and manuscript preparation.
Ready to add p-Tau217 to your clinical trial?
Contact NeuroDex / SBH Diagnostics to discuss your study design, sample logistics, and biomarker strategy.
info@neurodex.co | https://neurodex.co
This white paper is provided for informational and scientific purposes. All clinical data were generated using the validated ELLA p-Tau217 protocol described herein. Full documentation available under a mutual confidentiality agreement. This document does not constitute a clinical laboratory report.
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