Introduction Studies using different assays and technologies showed highly promising diagnostic value of plasma phosphorylated (P-)tau levels for Alzheimer’s disease (AD). We aimed to compare six P-tau Simoa assays, including three P-tau181 (Eli Lilly, ADx, Quanterix), one P-tau217 (Eli Lilly), and two P-tau231 (ADx, Gothenburg). Methods We studied the analytical (sensitivity, precision, parallelism, dilution linearity, and recovery) and clinical (40 AD dementia patients, age 66±8years, 50%F; 40 age- and sex-matched controls) performance of the assays. Results All assays showed robust analytical performance, and particularly P-tau217 Eli Lilly; P-tau231 Gothenburg and all P-tau181 assays showed robust clinical performance to differentiate AD from controls, with AUCs 0.936–0.995 (P-tau231 ADx: AUC = 0.719). Results obtained with all P-tau181 assays, P-tau217 Eli Lilly assay, and P-tau231 Gothenburg assay strongly correlated (Spearman’s rho > 0.86), while correlations with P-tau231 ADx results were moderate (rho < 0.65). Discussion P-tau isoforms can be measured robustly by several novel high-sensitive Simoa assays.

Blood-based biomarkers could prove useful to predict Alzheimer’s Disease core pathologies in advance of clinical symptoms. Implementation of such biomarkers requires a solid understanding of their long-term dynamics, and the contribution of confounding to their association with Alzheimer's disease pathology. Here we assess the value of plasma amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein to detect early Alzheimer's disease pathology, accounting for confounding by genetic and early environmental factors. Participants were 200 monozygotic twins, aged ≥60 years with normal cognition from the EMIF-AD PreclinAD study. All twins had amyloid-β status and plasma samples available at study enrollment. For 80 twins, additional plasma samples were available that had been collected approximately 10 years prior to amyloid-β status assessment. Single Molecule Array assays were applied to measure amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein. Predictive value of, and longitudinal change in these biomarkers were assessed using Receiver Operating Characteristic curve analysis and linear mixed models. Amyloid pathology could be predicted using blood-based biomarkers obtained at time of amyloid status assessment (amyloid-β1-42/1-40: Area Under Curve = 0·65, p = 0·01; phosphorylated-tau181: Area Under Curve = 0·84, p &lt; 0·001; glial fibrillary acidic protein: Area Under Curve = 0·74, p &lt; 0·001), as well as, using those obtained 10 years prior to amyloid status assessment (amyloid-β1-42/1-40: Area Under Curve = 0·69, p = 0·03; phosphorylated-tau181: Area Under Curve = 0·92, p &lt; 0·001; glial fibrillary acidic protein: Area Under Curve = 0·84, p &lt; 0·001). Longitudinally, amyloid-β1-42/1-40 levels decreased (β(SE)=-0·12(0·01), p &lt; 0·001) and phosphorylated-tau181 levels increased (β(SE) = 0·02(0·01), p = 0·004). Amyloid-β+ individuals showed a steeper increase in phosphorylated-tau181 compared to amyloid-β- individuals (β(SE) = 0·06(0·02), p = 0·004). Also amyloid-β+ individuals tended to show a steeper increase in glial fibrillary acidic protein (β(SE) = 0·04(0·02), p = 0·07). Within monozygotic twin-pairs, those with higher plasma phosphorylated-tau181 and lower amyloid-β1-42/1-40 levels were more likely to be amyloid-β+ (β(SE) = 0·95(0·26), p &lt; 0·001; β(SE)=-0·28(0·14), p &lt; 0·05) indicating minimal contribution of confounding by genetic and early environmental factors. Our data support the use of amyloid-β1-42/1-40, phosphorylated-tau181 and glial fibrillary acidic protein as screening tools for Alzheimer's disease pathology in the normal aging population, which is of importance for enrollment of high-risk subjects in secondary, or even primary, prevention trials. Furthermore, these markers show potential as low-invasive monitoring tool of disease progression and possibly treatment effects in clinical trials.