Plasma p-tau181/Aβ ratio predicts Aβ-PET status and correlates with CSF-p-tau181/Aβ and future cognitive decline.

Background: In Alzheimer's disease (AD), plasma amyloid beta (Aβ) and phosphorylated tau (p-tau) predict high amyloid status from Aβ positron emission tomography (PET); however, the extent to which combination of these plasma assays can predict remains unknown.

Methods: Prototype Simoa assays were used to measure plasma samples from participants who were either cognitively normal (CN) or had mild cognitive impairment (MCI)/AD in the Australian Imaging, Biomarkers and Lifestyle (AIBL) study.

Results: The p-tau181/Aβ ratio showed the best prediction of Aβ-PET across all participants (area under the curve [AUC] = 0.905, 95% confidence interval [CI]: 0.86-0.95) and in CN (AUC = 0.873; 0.80-0.94), and symptomatic (AUC = 0.908; 0.82-1.00) adults. Plasma p-tau181/Aβ ratio correlated with cerebrospinal fluid (CSF) p-tau181 (Elecsys, Spearman's ρ = 0.74, < 0.0001) and predicted abnormal CSF Aβ (AUC = 0.816; 0.74-0.89). The p-tau181/Aβ ratio also predicted future rates of cognitive decline assessed by AIBL Preclinical Alzheimer Cognitive Composite or Clinical Dementia Rating Sum of Boxes ( < 0.0001).

Discussion: Plasma p-tau181/Aβ ratio predicted both Aβ-PET status and cognitive decline, demonstrating potential as both a diagnostic aid and as a screening and prognostic assay for preclinical AD trials.