The Alzheimers Association Global Biomarker Standardization Consortium (GBSC) plasma phospho-tau Round Robin study

BACKGROUNDPhosphorylated tau (p-tau) is a specific blood biomarker for Alzheimers disease (AD) pathology. Multiple p-tau biomarkers on several analytical platforms are poised for clinical use. The Alzheimers Association Global Biomarker Standardisation Consortium plasma phospho-tau Round Robin study engaged assay developers in a blinded case-control study on plasma p-tau, aiming to learn which assays provide the largest fold-changes in AD compared to non-AD, have the strongest relationship between plasma and cerebrospinal fluid (CSF), and show the most consistent relationships between methods (commutability) in measuring both patient samples and candidate reference materials (CRM). METHODSThirty-three different p-tau biomarker assays, built on eight different analytical platforms, were used to quantify paired plasma and CSF samples from 40 participants. AD biomarker status was categorised as "AD pathology" (n=25) and "non-AD pathology" (n=15) by CSF A{beta}42/A{beta}40 (US-FDA; CE-IVDR) and p-tau181 (CE-IVDR) methods. The commutability of four CRM, at three concentrations, was assessed across assays. FINDINGSPlasma p-tau217 consistently demonstrated higher fold-changes between AD and non-AD pathology groups, compared to other p-tau epitopes. Fujirebio LUMIPULSE G, UGOT IPMS, and Lilly MSD p-tau217 assays provided the highest median fold-changes. In CSF, p-tau217 assays also performed best, and exhibited substantially larger fold-changes than their plasma counterparts, despite similar diagnostic performance. P-tau217 showed the strongest correlations between plasma assays (rho=0.81 to 0.97). Plasma p-tau levels were weakly-to-moderately correlated with CSF p-tau, and correlations were non-significant within the AD group alone. The evaluated CRM were not commutable across assays. INTERPRETATIONPlasma p-tau217 measures had larger fold-changes and discriminative accuracies for detecting AD pathology, and better agreement across platforms than other plasma p-tau variants. Plasma and CSF markers of p-tau, measured by immunoassays, are not substantially correlated, questioning the interchangeability of their continuous relationship. Further work is warranted to understand the pathophysiology underlying this dissociation, and to develop suitable reference materials facilitating cross-assay standardisation. FUNDINGAlzheimers Association (#ADSF-24-1284328-C) RESEARCH IN CONTEXTO_ST_ABSEvidence before this studyC_ST_ABSPhosphorylated tau (p-tau) in cerebrospinal fluid is an established biomarker of Alzheimers disease (AD), and several studies of plasma phosphorylated tau (p-tau) biomarkers now show evidence for its diagnostic accuracy in AD. We reviewed the literature on plasma p-tau in AD by searching without language restrictions in PubMed for articles from 1st January 2014 to 1st March 2024 using the terms ((("Alzheimer Disease"[Mesh]) OR ((Alzheimer[Title/Abstract]) OR (Alzheimers [Title/Abstract))) AND ((("Blood"[Mesh]) OR ("Plasma"[Mesh])) OR (plasma[Title/Abstract]))) AND ((phospho tau[Title/Abstract]) OR (phosphorylated tau[Title/Abstract]) OR (p-tau[Title/Abstract]) OR (P-tau[Title/Abstract]) OR (pTau[Title/Abstract])), filtering for studies in humans. Several studies examined plasma p-tau181, p-tau217 and p-tau231 in well-characterised research cohorts including cognitively impaired and unimpaired individuals, demonstrating high diagnostic accuracy when utilising CSF or amyloid PET imaging "gold standard" biomarkers, and in relation to post-mortem amyloid and tau pathology. Most studies utilised single-method assays to measure biomarkers of interest, including immunoprecipitation-mass spectrometry and a variety of immunochemical methods including single molecule array, enzyme-linked immunosorbent assays, electrochemiluminescence, and immunomagnetic reduction. Some later studies have performed cross-method and cross-phospho-form comparisons (p-tau181, p-tau217 and p-tau231), including six single molecular array assays in the Amsterdam Dementia Cohort, nine assays in the BIODEGMAR memory clinic-based cohort from Spain, ten assays in individuals with mild cognitive impairment in the Sk[a]ne University memory clinic cohort from Sweden, and have generally reported the various individual methods for p-tau217 to have the highest diagnostic accuracy for the presence of cerebral {beta}-amyloid deposition. No study to date has compared assays for p-tau217 on semi- and fully automated platforms, described comparisons with p-tau212 and p-tau205 in plasma, or assessed commutability of candidate reference materials between assays in comparison to patient samples. Added value of this studyOur study included the largest number of methods for assaying plasma p-tau to date, with engagement from 12 assay developers, allowing for blinded cross-assay comparisons of 33 plasma assays (for p-tau181, p-tau205, p-tau212, p-tau217, and p-tau231). Among these were included several semi-automated and fully automated methods that have potential for widespread clinical application. This is also the first study, to our knowledge, in which the commutability of candidate reference materials was assessed for plasma p-tau, as a first step in efforts to standardise between assays. The study design capitalised on a large volume of paired CSF and plasma drawn from well-characterised attenders to a specialist cognitive disorders service, allowing for 26 intra-method comparisons between CSF and plasma measurements within the same individuals. We found that CSF p-tau217 assays consistently outperformed other CSF p-tau assays in terms of fold-change between individuals with and without Alzheimers pathology, and the top ten assays in terms of fold change in plasma were also p-tau217 assays. This high discriminative value of plasma p-tau217 contrasted with overall only moderate correlations between CSF and plasma within individuals, which did not persist within the AD group alone. We also did not observe commutability of the four types of candidate reference materials we investigated, for any of the assay pairs. Implications of all the available evidenceOur study adds to the growing evidence for plasma p-tau217 as a candidate biomarker for translation into clinical practice by virtue of its superior discriminative ability between AD vs non-AD in symptomatic individuals, when compared head-to-head with other phosphorylated tau forms. The observed high and clinically interpretable fold-changes are pivotal for this biomarkers possible future success. Further studies should examine whether blood biomarker-supported diagnosis will extend access to disease-specific (and potentially disease-modifying) treatments and impact patient-relevant outcomes such as quality of life, particularly in diverse and resource-limited settings. Standardization between different assays will also be important for real-world applications, and this will require further concerted efforts in developing reference materials.