MSD S-PLEX · fg/mL Sensitivity

pT181 · pT231 · Total Tau

MSD Tau & Phospho-Tau Assay Services

Comprehensive Tau phosphorylation profiling for Alzheimer's disease and tauopathy research. Quantify total Tau, p-Tau181, and p-Tau231 individually or as a multiplex panel — all at femtogram-level sensitivity.

Different Tau phospho-epitopes — pT181, pT231, and pT217 — each reflect distinct stages of Tau pathology, from early conformational changes to late-stage aggregation. MSD S-PLEX enables ultrasensitive quantification of total Tau and site-specific phospho-Tau species in plasma and CSF, supporting disease mechanism research, cohort enrichment, and therapeutic development from as little as 25 µL of sample.

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Tau & Phospho-Tau in Alzheimer's Disease Research

Tau is a microtubule-associated protein predominantly expressed in neurons, where it stabilizes microtubules and regulates axonal transport. In Alzheimer's disease (AD) and related tauopathies, Tau becomes abnormally hyperphosphorylated at multiple serine and threonine residues, dissociates from microtubules, and aggregates into paired helical filaments (PHFs) and neurofibrillary tangles (NFTs) — hallmarks of AD neuropathology. Soluble phospho-Tau fragments released into the extracellular space and cerebrospinal fluid can be detected in the bloodstream, enabling non-invasive research into Tau pathology.

Phospho-Tau Epitopes: Distinct Windows into Tau Pathology

Different Tau phosphorylation sites provide complementary information about disease state and progression. Each epitope reflects a different aspect of Tau biology — from early phosphorylation events to late-stage aggregation — making multi-epitope Tau profiling a powerful research approach.

p-Tau181

The most extensively studied phospho-Tau biomarker in the literature. Plasma p-Tau181 elevations are associated with cerebral amyloid and Tau pathology in AD research cohorts. Established across multiple independent studies and assay platforms, p-Tau181 is a foundational blood-based biomarker for Alzheimer's pathology research.

p-Tau231

Phosphorylation at threonine 231 occurs early in the Tau hyperphosphorylation cascade and is linked to conformational changes that precede filament formation. Research indicates p-Tau231 may detect very early-stage AD pathology, including in individuals who are amyloid-positive but cognitively unimpaired.

Total Tau (t-Tau)

Total Tau measurement captures both phosphorylated and non-phosphorylated forms, providing a baseline for normalizing phospho-Tau signals. When combined with phospho-Tau measurements in a multiplex panel, t-Tau enables calculation of phospho-Tau/t-Tau ratios that may improve biological interpretation.

 Why Multi-Epitope Tau Profiling Matters
Complementary temporal windows: p-Tau231 may reflect the earliest phosphorylation changes; p-Tau181 is an established mid-stage marker; p-Tau217 tracks closely with advanced amyloid and Tau PET burden. Together they span the disease continuum.
Independent validation: Concordance across multiple phospho-epitopes strengthens the biological signal and reduces the risk of assay-specific artifacts.
Mechanistic insight: Different kinases (GSK-3β, CDK5, PKA) phosphorylate different sites — epitope-specific profiling can inform kinase activity research and target engagement studies.
Ratio analysis: p-Tau/t-Tau ratios, pT231/pT181 ratios, and other composite metrics may provide additional discriminatory power beyond individual analyte concentrations.
Therapeutic monitoring: Different investigational therapies may affect Tau phosphorylation at distinct sites — multi-epitope panels capture broader pharmacodynamic effects.

Research Applications

Cohort enrichment — identify research participants likely to have cerebral amyloid and Tau pathology using blood-based biomarkers, reducing reliance on PET imaging for screening in observational and interventional studies.
Longitudinal biomarker monitoring — track p-Tau trajectories over time to assess pharmacodynamic effects of investigational anti-amyloid and anti-Tau therapies across multiple phosphorylation sites.
Population-based research — scalable, cost-efficient blood biomarker panels for large epidemiological and aging cohort studies requiring multi-analyte Tau profiling.
Tauopathy differentiation — investigate phospho-Tau signatures across different tauopathies (AD, PSP, CBD, FTLD) to identify disease-specific phosphorylation patterns in research settings.
Translational studies — support therapeutic development with blood-based biomarkers that reflect target engagement at multiple nodes of the Tau phosphorylation network.

How MSD S-PLEX Detects Tau & Phospho-Tau at fg/mL

Why standard immunoassays are insufficient for plasma Tau quantification — and how electrochemiluminescence addresses the sensitivity gap for phospho-Tau research.

The Sensitivity Challenge for Plasma Tau

Plasma Tau and phospho-Tau species circulate at extremely low concentrations — typically in the 0.5–50 pg/mL range, depending on the epitope and research cohort characteristics. Conventional ELISA methods have lower limits of quantification (LLOQ) of 10–50 pg/mL, which is insufficient to reliably quantify these analytes at the lower end of the physiological range or to detect subtle fold-changes between research groups. Even p-Tau181, the most established blood-based phospho-Tau biomarker, requires sub-pg/mL sensitivity for robust quantification in plasma from cognitively unimpaired research participants.

The S-PLEX Solution

1. High-Capacity Carbon Electrode Surface

MSD's proprietary carbon electrode provides approximately 10× higher binding capacity than polystyrene, capturing more target molecules per well and extending the measurable range into the sub-pg/mL domain.

2. SULFO-TAG ECL Label

Non-enzymatic, chemically stable label that emits light only upon electrical stimulation at the electrode surface. Zero background from unbound detection antibody or sample autofluorescence — critical for low-abundance phospho-Tau detection.

3. S-PLEX Signal Amplification

An additional antibody amplification layer multiplies the number of SULFO-TAG labels per captured Tau molecule, pushing the lower limit of detection into the femtogram-per-milliliter range for all three analytes — total Tau, p-Tau181, and p-Tau231.

4. Electrical Excitation

ECL signal is generated only when voltage is applied to the electrode — no excitation light source means no autofluorescence, light scattering, or photo-bleaching artifacts. This is especially important for plasma samples, which can exhibit high background in fluorescence-based assays.

S-PLEX Tau & p-Tau Assay Specifications

TechnologyMSD S-PLEX (Electrochemiluminescence)

Analytesp-Tau181, p-Tau231, Total Tau

LOD<0.05 pg/mL (fg/mL range)

LLOQ~0.1 pg/mL across analytes

Dynamic Range0.1–10,000 pg/mL (5+ logs)

Sample Volume25 µL per well

Recommended MatrixEDTA Plasma

Acceptable MatricesSerum, CSF (on consultation)

Species ReactivityHuman, Non-Human Primate (NHP)

Intra-Assay CV<8%

Inter-Assay CV<12%

Plate Format96-well MULTI-ARRAY

Read Time~70 seconds per plate

Multiplex Option2-Plex: Total Tau + pT231 (LXMH02-2)

Phospho-Tau Epitope Comparison

Relative research maturity and evidence strength across Tau phospho-epitopes

p-Tau181 5/5

p-Tau231 4/5

p-Tau217 3/5

Total Tau 4/5

Bar lengths reflect volume of published literature and independent cohort validation, not analytical performance. p-Tau181 benefits from the largest published evidence base across multiple platforms. p-Tau217 is a rapidly emerging biomarker with strong recent data. For research reference only.

Tau & Phospho-Tau Assay Service Packages

Three pre-configured assay products covering p-Tau181, p-Tau231, and Total Tau — available as full-service or kit-only options.

Service Package — All Three Tau Products

Product	Analytes	Description	Species
Tau (pT231) Ultrasensitive	p-Tau231	Single-plex S-PLEX assay for phospho-Tau at threonine 231. Early phosphorylation marker associated with conformational Tau changes preceding filament formation.	Human, NHP
Tau (pT181) Ultrasensitive	p-Tau181	Single-plex S-PLEX assay for phospho-Tau at threonine 181. The most extensively published blood-based phospho-Tau biomarker in Alzheimer's disease research.	Human, NHP
Tau & p-Tau 2-Plex Panel	Total Tau + pT231	Multiplex S-PLEX panel measuring total Tau and p-Tau231 simultaneously from a single 25 µL sample. Enables p-Tau/t-Tau ratio calculation for improved biological interpretation.	Human, NHP

For Research Use Only. Not for diagnostic procedures. All products are pre-configured manufacturer-validated assays.

Sample Submission Requirements

Preferred MatrixEDTA Plasma

AcceptableSerum, CSF (consultation required for non-plasma matrices)

Typical Sample Volume50–200 µL per aliquot; as low as 25 µL per well. We'll advise based on your panel.

Collection ProtocolEDTA tube → centrifuge at 1,500×g for 10 min at 4°C → aliquot into polypropylene cryovials → freeze immediately at −80°C

ShippingDry ice, overnight courier. Include temperature logger if available.

DocumentationSample manifest form provided by our team upon project initiation

How to Get Started

Consult

Tell us about your study — samples, timeline, analytes of interest. We''ll recommend the optimal panel configuration.

Ship

Send your samples on dry ice. We provide collection kits and customs support for international shipments.

Receive Data

Receive your complete data package — quantitative report, raw data, and publication-ready methods.

Testing a hypothesis or new to this assay? Ask about a pilot feasibility run with a subset of samples before committing to a full study.

Tau & Phospho-Tau Detection: MSD S-PLEX vs Other Platforms

Platform comparison for ultra-sensitive Tau immunoassay research applications.

Feature	MSD S-PLEX (ECL)	Simoa (Quanterix)	Conventional ELISA	LC-MS/MS
Sensitivity (LOD)	fg/mL	fg/mL	~10–50 pg/mL	~1–5 pg/mL
Dynamic Range	5–6 logs	4–5 logs	2–3 logs	3–4 logs
Sample Volume	25 µL	~100 µL	100 µL	250–500 µL
Multiplex Capability	Up to 10-plex	Up to 4-plex	Single-plex only	Targeted panels
Throughput	96-well / 70 sec read	~60 samples/hr	96-well batch	Low (~20/day)
Phospho-Specificity	Good — validated antibody pairs for pT181 and pT231	Good — validated antibody pairs available	Moderate — antibody-dependent	Excellent — peptide-level isoform resolution
Matrix Tolerance	High — serum, plasma, CSF	Moderate	Low–Moderate	High — requires extensive sample prep
Best Research Fit	Cohort studies, multi-epitope Tau profiling, therapeutic monitoring	Ultra-low single markers, exotic matrices	Small pilot studies, established labs	Phospho-isoform identification, reference method development

For a detailed head-to-head analysis covering all neuro biomarkers, see our MSD vs Simoa: Ultra-Sensitive Immunoassay Platform Comparison.

What You Receive

Quantitative report — analyte concentrations (pg/mL or fg/mL), QC metrics, standard curve parameters for each assayed analyte (Excel + PDF formats)
Raw ECL data — per-well signal values, plate maps, calibration curve plots — directly usable for independent statistical analysis
QC documentation — intra-plate and inter-plate CV, LOD/LLOQ confirmation per plate, dilution linearity verification as applicable
p-Tau/t-Tau ratio — calculated phospho-Tau to total Tau ratios for 2-Plex panel orders, enabling normalized interpretation across samples
Methods summary — detailed protocol ready for manuscript Methods or supplementary materials sections
Sample tracking log — complete chain-of-custody documentation from sample receipt through final analysis
Post-delivery consultation — scheduled call with the lead scientist to discuss results, answer questions, and support data interpretation

Why Our Clients Choose Us

Multi-epitope expertise — deep understanding of Tau biology across phosphorylation sites; guidance on optimal assay selection for your research question
Dedicated scientist model — a single lead scientist owns your project end-to-end, ensuring continuity and accountability
Flexible delivery — full-service, kit-only, or custom U-PLEX combinations; single-epitope or multi-epitope panels
Transparent QC — all QC results reported honestly; flagged values with explanations, never silently removed or suppressed
Longitudinal consistency — reagent lot tracking and bridging across timepoints for multi-year cohort studies
Direct communication — talk to the scientist running your assay, not a project manager; rapid technical consultation throughout the project
Publication track record — data generated on our platform contributes to peer-reviewed biomarker research publications

Supporting Publications

Published research supporting phospho-Tau isoforms as Alzheimer's disease biomarkers.

Karikari TK, Pascoal TA, Ashton NJ, et al. (2020) The Lancet Neurology. 19(5):422–433. "Blood phosphorylated tau 181 as a biomarker for Alzheimer's disease." DOI: 10.1016/S1474-4422(20)30071-5

Ashton NJ, Pascoal TA, Karikari TK, et al. (2021) Acta Neuropathologica. 141(5):709–724. "Plasma p-tau231: a new biomarker for incipient Alzheimer's disease pathology." DOI: 10.1007/s00401-021-02275-6 · PMID: 33585983

Palmqvist S, Janelidze S, Quiroz YT, et al. (2020) JAMA. 324(8):772–781. "Discriminative Accuracy of Plasma Phospho-tau217 for Alzheimer Disease vs Other Neurodegenerative Disorders." DOI: 10.1001/jama.2020.12134 · PMID: 32722745

Janelidze S, Mattsson N, Palmqvist S, et al. (2020) Nature Medicine. 26(3):379–386. "Plasma P-tau181 in Alzheimer's disease: relationship to other biomarkers, differential diagnosis, neuropathology and longitudinal progression to Alzheimer's dementia." DOI: 10.1038/s41591-020-0755-1 · PMID: 32123385

Brickman AM, Manly JJ, Honig LS, et al. (2021) Alzheimer's & Dementia. 17(8):1353–1364. "Plasma p-tau181, p-tau217, and other blood-based Alzheimer's disease biomarkers in a multi-ethnic, community study." DOI: 10.1002/alz.12301 · PMID: 33580742

Thijssen EH, La Joie R, Wolf A, et al. (2020) Nature Medicine. 26(3):387–397. "Diagnostic value of plasma phosphorylated tau181 in Alzheimer's disease and frontotemporal lobar degeneration." DOI: 10.1038/s41591-020-0762-2

Mattsson N, Andreasson U, Zetterberg H, et al. (2017) JAMA Neurology. 74(5):557–566. "Association of Plasma Neurofilament Light With Neurodegeneration in Patients With Alzheimer Disease." DOI: 10.1001/jamaneurol.2016.6117 · PMID: 28346578

References for informational context. For Research Use Only.

Tau & Phospho-Tau Assay FAQ

Which phospho-Tau epitope should I choose for my research study?

The optimal epitope depends on your research question. p-Tau181 has the largest published evidence base and is the most established blood-based phospho-Tau biomarker — ideal for studies requiring strong literature precedent. p-Tau231 may detect very early Tau phosphorylation changes and has shown utility in preclinical AD research. p-Tau217 (available separately) shows the largest fold-change between amyloid-positive and amyloid-negative groups in published cohorts. For the most comprehensive profiling, consider measuring multiple epitopes or using our Tau & p-Tau 2-Plex Panel (LXMH02-2), which measures total Tau and p-Tau231 simultaneously. Our scientific team can help you select the optimal configuration based on your study design, cohort characteristics, and research objectives.

Are these Tau assays validated for clinical diagnostic use?

No. These Tau and phospho-Tau assays are manufacturer-validated for research use only (RUO). They are suitable for preclinical studies, cohort research, biomarker discovery, and therapeutic development applications. They are not validated for clinical diagnostic procedures, patient management decisions, or as standalone screening tools in a clinical setting. All data is reported with the explicit disclaimer that it should not be used as the sole basis for clinical decision-making.

What is the minimum plasma volume required for Tau & p-Tau assays?

For single-plex p-Tau181 or p-Tau231 assays, we require a minimum of 100 µL of EDTA plasma for singlicate measurement, or 200 µL for duplicate measurement. For the 2-Plex Panel (Total Tau + pT231, LXMH02-2), the minimum volume is 50 µL for singlicate — the multiplex format measures both analytes from the same 25 µL well. For studies with extremely limited sample availability, we can discuss minimum viable volumes with our scientific team. CSF samples typically require only 25 µL.

Why is EDTA plasma the recommended matrix for phospho-Tau measurement?

EDTA plasma has shown more consistent and reproducible phospho-Tau measurements across published research studies compared to serum. The clotting process in serum collection may release additional Tau fragments from platelets and other blood cells, potentially introducing pre-analytical variability. Furthermore, some phosphorylation sites may be susceptible to phosphatase activity during the clotting process. We strongly recommend consistent matrix use throughout a longitudinal study. If your biobank only has serum samples available, we can discuss matrix-specific considerations — our ECL platform is relatively matrix-tolerant compared to fluorescence-based methods.

Can I combine Tau & p-Tau assays with other neurodegeneration biomarkers?

Yes. Through MSD's U-PLEX custom platform, any of our Tau/p-Tau assays can be combined with up to 9 other biomarkers in a single well. Popular combinations include: Tau + NfL + GFAP (comprehensive neurodegeneration panel), pTau181 + Aβ40 + Aβ42 (AD core pathology panel), and pTau231 + pTau181 + NfL (multi-epitope Tau with axonal injury marker). For maximum sensitivity at the fg/mL level, the dedicated S-PLEX single-plex or 2-Plex formats are recommended. Contact our scientific team to determine the optimal configuration for your specific research objectives and sample volume constraints.

How should I prepare and ship samples for Tau & p-Tau analysis?

Recommended protocol: (1) Collect blood in EDTA tubes, gently invert 8–10 times. (2) Centrifuge at 1,500×g for 10 minutes at 4°C within 1 hour of collection. (3) Transfer plasma to labeled polypropylene cryovials (avoid polystyrene tubes, which can adsorb Tau). (4) Freeze immediately at −80°C. (5) Ship on dry ice via overnight courier with a completed sample manifest form. Avoid repeated freeze-thaw cycles — aliquot samples into single-use volumes at the time of initial processing. Hemolyzed samples (visible pink/red discoloration) should be noted on the manifest as hemolysis may affect Tau measurements. Contact us for a detailed sample collection and shipping guide tailored to your study.

What documentation do you provide for data integrity and reproducibility?

We provide comprehensive documentation with every project: (1) full analytical report including standard curve parameters (R², back-calculated calibrator accuracy), QC sample performance across all three levels, and analyte-by-analyte precision metrics (intra-plate and inter-plate CV); (2) sample chain-of-custody log from receipt through analysis; (3) per-well ECL signal data with plate maps in analysis-ready Excel format; (4) instrument run logs; and (5) a detailed methods section suitable for manuscript preparation. For studies that may later inform regulated work, we recommend discussing documentation requirements with our team during the study design phase so we can align our processes with your downstream needs. All services are provided under a research-use-only framework.

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