Clinical performance of an antibody-free assay for plasma A[beta]42/A[beta]40 to detect early alterations of Alzheimer's disease in individuals with subjective cognitive decline

Background Accessible and cost-effective diagnostic tools are urgently needed to accurately quantify blood biomarkers to support early diagnosis of Alzheimer's disease (AD). In this study, we investigated the ability of plasma amyloid-beta (A[beta])42/A[beta]40 ratio measured by an antibody-free mass-spectrometric (MS) method, ABtest-MS, to detect early pathological changes of AD. Methods This cohort study included data from the baseline and 2-year follow-up visits from the Fundació ACE Healthy Brain Initiative (FACEHBI) study. Plasma A[beta]42/A[beta]40 was measured with ABtest-MS and compared to .sup.18F-Florbetaben PET as the reference standard (cutoff for early amyloid deposition of 13.5 centiloids). Cross-validation was performed in an independent DPUK-Korean cohort. Additionally, associations of plasma A[beta]42/A[beta]40 with episodic memory performance and brain atrophy were assessed. Results The FACEHBI cohort at baseline included 200 healthy individuals with subjective cognitive decline (SCD), of which 36 (18%) were A[beta]-PET positive. Plasma A[beta]42/A[beta]40 levels were significantly lower in A[beta]-PET positive individuals (median [interquartile range, IQR], 0.215 [0.203-0.236]) versus A[beta]-PET negative subjects (median [IQR], 0.261 [0.244-0.279]) (P < .001). Plasma A[beta]42/A[beta]40 was significantly correlated with A[beta]-PET levels (rho = -0.390; P < .001) and identified A[beta]-PET status with an area under the receiver operating characteristic curve (AUC) of 0.87 (95% confidence interval [CI], 0.80-0.93). A cutoff for the A[beta]42/A[beta]40 ratio of 0.241 (maximum Youden index) yielded a sensitivity of 86.1% and a specificity of 80.5%. These findings were cross-validated in an independent DPUK-Korean cohort (AUC 0.86 [95% CI 0.77-0.95]). Lower plasma A[beta]42/A[beta]40 ratio was associated with worse episodic memory performance and increased brain atrophy. Plasma A[beta]42/A[beta]40 at baseline predicted clinical conversion to mild cognitive impairment and longitudinal changes in amyloid deposition and brain atrophy at 2-year follow-up. Conclusions This study suggests that plasma A[beta]42/A[beta]40, as determined by this MS-based assay, has potential value as an accurate and cost-effective tool to identify individuals in the earliest stages of AD, supporting its implementation in clinical trials, preventative strategies and clinical practice. Keywords: Alzheimer's disease, Amyloid, A[beta]42/A[beta]40, Ratio, Biomarkers, Pla