Project Summary Developing minimally invasive and low-cost AD and AD-related dementia (ADRD) tests with high sensitivity and specificity is an urgent and unmet need for early AD diagnosis, differential diagnosis with other tauopathies, monitoring disease severity, predicting disease courses, and assessing effectiveness of drug treatment. In this project we will develop phosphorylated tau (p- tau) new biomarkers for early AD diagnosis and for differentiation of AD from other tauopathies. Through a systemic site-specific p-tau antibody screening, we have identified several new and highly selective p-tau AD biomarkers from postmortem brain tissues. They are not only able to differentiate AD from non-AD tauopathies, but also able to diagnose mild cognitive impairment (MCI), an early disease stage of AD, from cognitively normal subjects with better diagnostic performance than those of existing p-tau biomarkers, p-tau181 and p-tau217. We intend to validate our newly discovered p-tau biomarkers in accessible biofluid plasma and cerebrospinal fluid (CSF) by developing ultrasensitive single molecular array (Simoa) based tests for detection and quantification. Two Specific Aims will be pursued. In Aim 1, we will develop novel biofluid p- tau biomarkers for early AD diagnosis. In Aim 2, we will develop new p-tau biomarkers for differentiating AD from other tauopathies. In both Aims, we will test our new biomarkers with significant number of subjects, extend to premortem patient plasma and CSF samples, and develop ultrasensitive Simoa biomarker tests. We anticipated our work has significant clinical and translational values that may lead to AD and ADRD diagnostic tests with improved sensitivity and specificity.