Report CSF Ab42 and tau in relation to amyloid in humans The paper explained PROBLEM: AD pathology is estimated to develop many years before detectable cognitive decline. Once symptoms are apparent, the brain has already experienced substantial neuronal and synaptic loss. Thus there is a great need to develop biomarkers that can identify people in the very earliest symptomatic and even ‘preclinical’ stages, prior to any cognitive impairment, when potential treatments will have the best opportunity to preserve cognitive function. RESULTS: We analysed CSF samples and in parallel determined the amount of cortical amyloid as evidenced by retention of the in vivo amyloid binding agent, PIB, in 189 cognitively normal research participants (age 43–89 years). We observed a positive linear relationship between the levels of CSF tau and ptau181 (primary the two batteries. Selective Reminding test (sum of three free recall trials) measures episodic memory (Grober et al, 1988). Animal Naming (name as many animals as possible in 1 min) assesses semantic memory (Goodglass & Kaplan, 1983). The final test is a speeded visuospatial test with two parts: Trailmakings A and B (number of connections per second for each part) (Armitage, 1946). CSF and plasma collection and processing CSF (20–30 ml) was collected at 8:00 am after overnight fasting as previously described (Fagan et al, 2006). Samples were gently inverted to avoid possible gradient effects, briefly centrifuged at low speed to pellet any cellular debris and aliquoted into polypropylene tubes prior to freezing at 84 8C (Fagan et al, 2006). Fasted blood was obtained at the time of LP and collected into polypropylene tubes containing EDTA. Plasma was prepared by standard centrifugation methods prior to aliquoting and freezing at 84 8C. CSF and plasma biomarker assessment CSF samples were analysed for Ab42, total tau and phospho-tau181 (ptau181) by enzyme-linked immunosorbant assay (ELISA) (InnotestTM, Innogenetics, Ghent, Belgium). CSF Ab40 was assayed by ELISA (Cirrito et al, 2003), and plasma Ab1–40, Abx–40 and Ab1–42 and Abx–42 were analysed by xMAP (bead-based) ELISA (Inno-Bia Plasma Ab Forms Multiplex AssayTM; Innogenetics, Ghent, Belgium). CSF Ab38 was analysed by an antibody-based electrochemiluminescence assay according to the manufacturer’s recommendations (MSD 96 well Multi-Array Human (6E10) Abeta 38 Ultra-Sensitive KitTM, Meso Scale Discovery, Gaithersburg, MD). For all biomarker measures, samples were continuously kept on ice, and assays were performed on sample aliquots after a single thaw following initial freezing. In vivo amyloid imaging with PIB All participants underwent in vivo amyloid imaging via PET PIB as previously described (Mintun et al, 2006) within 2 years of LP. The constituents of neurofibrillary tangles) with the amount of cortical amyloid. We also observed a strong inverse relationship between cortical PIB binding and CSF Ab42 (the primary constituent of amyloid plaques), but not plasma Ab species, demonstrating that a low level of CSF Ab42 is an excellent marker of brain amyloid even in the absence of cognitive symptoms. IMPACT: The data obtained shed light on the potential utility of PIB amyloid imaging and CSF Ab42, tau and ptau181 as antecedent (‘preclinical’) biomarkers of AD and also provide insight into the normal time course of the pathophysiology of the disease as reflected in the CSF. These findings have important implications for preclinical AD diagnosis and treatment, and should aid in the design and evaluation of secondary prevention trials in AD. cerebellum was chosen as a region with very low specific PIB binding for use as a reference region, and Logan graphical analyses were performed to calculate the mean cortical PIB distribution volume (Logan et al, 1996) for each participant. The value for MCBP was defined by averaging the distribution volumes for the prefrontal cortex, precuneus, lateral temporal cortex and gyrus rectus. Statistical analyses All statistical analyses were carried out using SAS (SAS Institute, Inc., Cary, NC). The relationships between MCBP and the fluid measures and between the biomarker measures and psychometric test scores were determined by Pearson correlations, adjusting for age. Chi-square was used to determine whether APOE e4 status differed across the PIB/ CSF Ab42 quadrants (low PIB/high CSF Ab42 vs. low PIB/low CSF Ab42 vs. high PIB/low CSF Ab42). Due to small expected frequencies in some cells, Fisher’s exact test was used to test whether there were associations between various comorbidities and quadrant member- ship. One-way ANOVA was used to test whether the interval between PIB scan and LP differed across the quadrants. General linear models were used to examine whether quadrant membership was significantly associated with scores on the four psychometric tests, after adjusting for age. Statistical significance was defined as p < 0.05. Author contributions A. M. F. oversaw the CSF studies, performed some of the CSF analyses, created the final dataset and graphs, evaluated and interpreted the data and wrote the paper. M. A. M. oversaw the PIB studies, assisted in data evaluation and interpretation, provided critical comments during the writing of the paper and obtained funding. A. R. S. processed the CSF samples, per- formed some of the CSF analyses and managed the CSF database. P. A. contributed to the analysis of the PIB data. 378 2009 EMBO Molecular Medicine EMBO Mol Med 1, 371–380 www.embomolmed.org