Diagnosing Alzheimer's disease is challenging, partly due to the closely related pathological features shared with other neurodegenerative diseases. Presently, a definite diagnosis of Alzheimer's disease can only be established by post mortem pathological examination focusing on two main pathological hallmarks: (i) amyloid plaques consisting of aggregated amyloid beta (Aβ) peptides, and (ii) neurofibrillary tangles made of abnormally phosphorylated tau protein.In living individuals, Alzheimer's disease diagnosis relies on two main approaches: (i) imaging of the accumulation of tau tangles and Aβ plaques in the brain using positron emission tomography (PET), and (ii) measuring brain-specific biochemical changes in CSF reflecting tau and Aβ pathophysiology. However, tau PET is expensive and only available in specialised medical centres. In 1995, our group developed two immunoassays for quantifying tau in CSF, one for measuring pathological tau phosphorylated at threonine-181 (p-tau181) and the other for the neuronal injury marker "total tau." These assays, targeting mid-region tau species, were subsequently developed into commercial kit assays, and have recently been approved by the United States Food and Drugs Administration to support diagnosis and candidate drug testing.The assays have been used in hundreds of published independent clinical studies. In reviewing

The quantification of phosphorylated tau in biofluids, either cerebrospinal fluid (CSF) or plasma, has shown great promise in detecting Alzheimer’s disease (AD) pathophysiology. Tau phosphorylated at threonine 231 (p-tau231) is one such biomarker in CSF but its usefulness as a blood biomarker is currently unknown. Here, we developed an ultrasensitive Single molecule array (Simoa) for the quantification of plasma p-tau231 which was validated in four independent cohorts (n = 588) in different settings, including the full AD continuum and non-AD neurodegenerative disorders. Plasma p-tau231 was able to identify patients with AD and differentiate them from amyloid-β negative cognitively unimpaired (CU) older adults with high accuracy (AUC = 0.92–0.94). Plasma p-tau231 also distinguished AD patients from patients with non-AD neurodegenerative disorders (AUC = 0.93), as well as from amyloid-β negative MCI patients (AUC = 0.89). In a neuropathology cohort, plasma p-tau231 in samples taken on avergae 4.2 years prior to post-mortem very accurately identified AD neuropathology in comparison to non-AD neurodegenerative disorders (AUC = 0.99), this is despite all patients being given an AD dementia diagnosis during life. Plasma p-tau231 was highly correlated with CSF p-tau231, tau pathology as assessed by [18F]MK-6240 positron emission tomography (PET), and brain amyloidosis by [18F]AZD469 PET. Remarkably, the inflection point of plasma p-tau231, increasing as a function of continuous [18F]AZD469 amyloid-β PET standardized uptake value ratio, was shown to be earlier than standard thresholds of amyloid-β PET positivity and the increase of plasma p-tau181. Furthermore, plasma p-tau231 was significantly increased in amyloid-β PET quartiles 2–4, whereas CSF p-tau217 and plasma p-tau181 increased only at quartiles 3–4 and 4, respectively. Finally, plasma p-tau231 differentiated individuals across the entire Braak stage spectrum, including Braak staging from Braak 0 through Braak I–II, which was not observed for plasma p-tau181. To conclude, this novel plasma p-tau231 assay identifies the clinical stages of AD and neuropathology equally well as plasma p-tau181, but increases earlier, already with subtle amyloid-β deposition, prior to the threshold for amyloid-β PET positivity has been attained, and also in response to early brain tau deposition. Thus, plasma p-tau231 is a promising novel biomarker of emerging AD pathology with the potential to facilitate clinical trials to identify vulnerable populations below PET threshold of amyloid-β positivity or apparent entorhinal tau deposition.

Objective:To test the hypothesis that COVID-19 has an impact on the CNS by measuring plasma biomarkers of CNS injury.Methods:We recruited 47 patients with mild (n=20), moderate (n=9) or severe (n=18) COVID-19 and measured two plasma biomarkers of CNS injury by Single molecule array (Simoa): neurofilament light chain protein (NfL) (a marker of intra-axonal neuronal injury) and glial fibrillary acidic protein (GFAp) (a marker of astrocytic activation/injury) in samples collected at presentation and again in a subset after a mean of 11.4 days. Cross-sectional results were compared with 33 age-matched controls derived from an independent cohort.Results:The patients with severe COVID-19 had higher plasma concentrations of GFAp (p=0.001) and NfL (p<0.001) than controls, while GFAp was also increased in patients with moderate disease (p=0.03). In severe patients an early peak in plasma GFAp decreased upon follow-up (p<0.01) while NfL showed a sustained increase from first to last follow-up (p<0.01), perhaps reflecting a sequence of early astrocytic response and more delayed axonal injury.Conclusion:We show neurochemical evidence of neuronal injury and glial activation in patients with moderate and severe COVID-19. Further studies are needed to clarify the frequency and nature of COVID-19-related CNS damage, and its relation to both clinically-defined CNS events such as hypoxic and ischemic events and to mechanisms more closely linked to systemic SARS-CoV-2 infection and consequent immune activation, and also to evaluate the clinical utility of monitoring plasma NfL and GFAp in management of this group of patients.