BackgroundThe Centiloid scale has been developed to standardize measurements of amyloid PET imaging. Reference cut-off values of this continuous measurement enable the consistent operationalization of decision-making for multicentre research studies and clinical trials. In this study, we aimed at deriving reference Centiloid thresholds that maximize the agreement against core Alzheimer’s disease (AD) cerebrospinal fluid (CSF) biomarkers in two large independent cohorts.MethodsA total of 516 participants of the ALFA+ Study (N = 205) and ADNI (N = 311) underwent amyloid PET imaging ([18F]flutemetamol and [18F]florbetapir, respectively) and core AD CSF biomarker determination using Elecsys® tests. Tracer uptake was quantified in Centiloid units (CL). Optimal Centiloid cut-offs were sought that maximize the agreement between PET and dichotomous determinations based on CSF levels of Aβ42, tTau, pTau, and their ratios, using pre-established reference cut-off values. To this end, a receiver operating characteristic analysis (ROC) was conducted, and Centiloid cut-offs were calculated as those that maximized the Youden’s J Index or the overall percentage agreement recorded.ResultsAll Centiloid cut-offs fell within the range of 25–35, except for CSF Aβ42 that rendered an optimal cut-off value of 12 CL. As expected, the agreement of tau/Aβ42 ratios was higher than that of CSF Aβ42. Centiloid cut-off robustness was confirmed even when established in an independent cohort and against variations of CSF cut-offs.ConclusionsA cut-off of 12 CL matches previously reported values derived against postmortem measures of AD neuropathology. Together with these previous findings, our results flag two relevant inflection points that would serve as boundary of different stages of amyloid pathology: one around 12 CL that marks the transition from the absence of pathology to subtle pathology and another one around 30 CL indicating the presence of established pathology. The derivation of robust and generalizable cut-offs for core AD biomarkers requires cohorts with adequate representation of intermediate levels.Trial registrationALFA+ Study, NCT02485730ALFA PET Sub-study, NCT02685969Electronic supplementary materialThe online version of this article (10.1186/s13195-019-0478-z) contains supplementary material, which is available to authorized users.

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Objective:To determine whether CSF synaptic biomarkers are altered in the early preclinical stage of the Alzheimer’s continuum and associated with Alzheimer’s disease (AD) risk factors, primary pathology, and neurodegeneration markers.Methods:Cross-sectional study in the ALFA+ cohort, comprising middle-aged cognitively unimpaired participants. CSF neurogranin and GAP-43 were measured using immunoassays and SNAP-25 and synaptotagmin-1 using immunoprecipitation mass spectrometry. AD CSF biomarkers Aβ42/40, p-tau and t-tau, and the neurodegeneration biomarker NfL were also measured. Participants underwent structural MRI, and fluorodeoxyglucose and Aβ PET imaging. General linear modeling was used to test the associations between CSF synaptic biomarkers and risk factors, Aβ pathology, tau pathology, and neurodegeneration markers.Results:All CSF synaptic biomarkers increased with age. CSF neurogranin was higher in females, while CSF SNAP-25 was higher in APOE-ε4 carriers. All CSF synaptic biomarkers increased with higher Aβ load (as measured by CSF Aβ42/40 and Aβ PET Centiloid values) and, importantly, the synaptic biomarkers were increased even in individuals in the earliest stages of Aβ deposition. Higher CSF synaptic biomarkers were also associated with higher CSF p-tau and NfL. Higher CSF neurogranin and GAP-43 were significantly associated with higher brain metabolism, but lower cortical thickness in AD-related brain regions.Conclusion:CSF synaptic biomarkers increase in early preclinical stages of the Alzheimer’s continuum even when a low burden of Aβ pathology is present, and they differ in their association with age, sex, APOE-ε4, and markers of neurodegeneration.