Background: Research has focussed upon the identification of a blood-based biomarker to inform clinicians on the likelihood of Alzheimer’s disease pathology prior to ordering the CSF and PET testing. Objective: To explore if the plasma total Aβ42/Aβ40 ratio is a reliable predictor of the amyloid-PET status we have explored the association between these two variables in a subset of the Australian Imaging, Biomarkers and Lifestyle (AIBL) study of ageing cohort. Methodology: Taking plasma samples at three separate time points, we assessed the total Aβ42/Aβ40 ratio in plasma (TP42/40) with regard to neocortical amyloid beta (Aβ) burden via PET SUVR, and investigated both association with Aβ-PET status and correlation (and agreement) with SUVR. Results: The TP42/40 plasma ratio was associated with PET status at all time points (p<0.013). Correlations with SUVR were similar across each time point, with Spearman’s Rho reaching -0.64 (p<0.0001). Area under the curve (AUC) values were highly reproducible over time points, with values ranging from 0.880 at 36 months to 0.913 at 54 months. Assessing the healthy control (HC) group only, the same relationships were found. Conclusions: The current study demonstrates reproducibility of the plasma assay to discriminate between amyloid-PET positive and negative over three time points, which can help to substantially reducing the screening rate of failure (SRF) for clinical trials targeting pre-clinical or prodromal disease. Classification of evidence: This study provides Class II evidence that plasma total AB42/AB40 ratio is associated with neocortical amyloid burden as measured by PET SUVR.