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

Plasma phosphorylated tau181 (P-tau181) might be increased in Alzheimer's disease (AD), but its usefulness for differential diagnosis and prognosis is unclear. We studied plasma P-tau181 in three cohorts, with a total of 589 individuals, including cognitively unimpaired participants and patients with mild cognitive impairment (MCI), AD dementia and non-AD neurodegenerative diseases. Plasma P-tau181 was increased in preclinical AD and further increased at the MCI and dementia stages. It correlated with CSF P-tau181 and predicted positive Tau positron emission tomography (PET) scans (area under the curve (AUC) = 0.87-0.91 for different brain regions). Plasma P-tau181 differentiated AD dementia from non-AD neurodegenerative diseases with an accuracy similar to that of Tau PET and CSF P-tau181 (AUC = 0.94-0.98), and detected AD neuropathology in an autopsy-confirmed cohort. High plasma P-tau181 was associated with subsequent development of AD dementia in cognitively unimpaired and MCI subjects. In conclusion, plasma P-tau181 is a noninvasive diagnostic and prognostic biomarker of AD, which may be useful in clinical practice and trials.

Context Small single-center studies have shown that cerebrospinal fluid (CSF) biomarkers may be useful to identify incipient Alzheimer disease (AD) in patients with mild cognitive impairment (MCI), but large-scale multicenter studies have not been conducted. Objective To determine the diagnostic accuracy of CSF ␤-amyloid 1-42 (A␤42), total tau protein (T-tau), and tau phosphorylated at position threonine 181 (P-tau) for predicting incipient AD in patients with MCI. Design, Setting, and Participants The study had 2 parts: a cross-sectional study involving patients with AD and controls to identify cut points, followed by a prospective cohort study involving patients with MCI, conducted 1990-2007. A total of 750 individuals with MCI, 529 with AD, and 304 controls were recruited by 12 centers in Europe and the United States. Individuals with MCI were followed up for at least 2 years or until symptoms had progressed to clinical dementia. Main Outcome Measures Sensitivity, specificity, positive and negative likelihood ratios (LRs) of CSF A␤42, T-tau, and P-tau for identifying incipient AD. Results During follow-up, 271 participants with MCI were diagnosed with AD and 59 with other dementias. The A␤42 assay in particular had considerable intersite variability. Patients who developed AD had lower median A␤42 (356; range, 96-1075 ng/L) and higher P-tau (81; range, 15-183 ng/L) and T-tau (582; range, 83-2174 ng/L) levels than MCI patients who did not develop AD during follow-up (579; range, 121-1420 ng/L for A␤42; 53; range, 15-163 ng/L for P-tau; and 294; range, 31-2483 ng/L for T-tau, PϽ.001). The area under the receiver operating characteristic curve was 0.78 (95% confidence interval [CI], 0.75-0.82) for A␤42, 0.76 (95% CI, 0.72-0.80) for P-tau, and 0.79 (95% CI, 0.76-0.83) for T-tau. Cutoffs with sensitivity set to 85% were defined in the AD and control groups and tested in the MCI group, where the combination of A␤42/P-tau ratio and T-tau identified incipient AD with a sensitivity of 83% (95% CI, 78%-88%), specificity 72% (95% CI, 68%-76%), positive LR, 3.0 (95% CI, 2.5-3.4), and negative LR, 0.24 (95% CI, 0.21-0.28). The positive predictive value was 62% and the negative predictive value was 88%. Conclusions This multicenter study found that CSF A␤42, T-tau, and P-tau identify incipient AD with good accuracy, but less accurately than reported from single-center studies. Intersite assay variability highlights a need for standardization of analytical techniques and clinical procedures.

It is not known exactly where amyloid-β (Aβ) fibrils begin to accumulate in individuals with Alzheimer’s disease (AD). Recently, we showed that abnormal levels of Aβ42 in cerebrospinal fluid (CSF) can be detected before abnormal amyloid can be detected using PET in individuals with preclinical AD. Using these approaches, here we identify the earliest preclinical AD stage in subjects from the ADNI and BioFINDER cohorts. We show that Aβ accumulation preferentially starts in the precuneus, medial orbitofrontal, and posterior cingulate cortices, i.e., several of the core regions of the default mode network (DMN). This early pattern of Aβ accumulation is already evident in individuals with normal Aβ42 in the CSF and normal amyloid PET who subsequently convert to having abnormal CSF Aβ42. The earliest Aβ accumulation is further associated with hypoconnectivity within the DMN and between the DMN and the frontoparietal network, but not with brain atrophy or glucose hypometabolism. Our results suggest that Aβ fibrils start to accumulate predominantly within certain parts of the DMN in preclinical AD and already then affect brain connectivity.