How well do plasma Alzheimer's disease biomarkers reflect the CSF amyloid status?

Background: Can plasma biomarkers as well as cerebrospinal fluid (CSF) perform in the separation of amyloid-beta-positive (Aβ+) vs amyloid-beta-negative (Aβ-) groups across an age range seen in an NHS cognitive disorder clinic?

Methods: As part of the routine diagnostic investigation of 111 clinic patients who had contemporaneous blood and CSF samples taken, patients were categorised into Aβ+ and Aβ- groups based on their CSF in an Aβ42/40 ratio. We then evaluated four single molecule array (Simoa) Quanterix assays, quantifying single plasma analytes and ratios (p-tau217, p-tau217/Aβ42 ratio, p-tau181, p-tau181/Aβ42 ratio and Aβ42/40 ratio) in their ability to distinguish between these groups and the effect of age.

Results: The median (range) age of participants was 66 (55-79) years with 48 females (43.2%). The areas under the curve (AUC), not accounting for age, for the ability to discriminate Aβ+ from Aβ- groups were plasma p-tau217 AUC=0.94, Aβ42/40 AUC=0.78 and p-tau181 AUC=0.77. Combining p-tau217/Aβ42 increased the AUC to 0.97. The difference between the groups was influenced by age with less separation in older individuals: a significant negative interaction term between age and group for plasma p-tau217 concentrations (-0.037, p=0.013) and p-tau217/Aβ42 ratio (-0.007, p=0.008).

Conclusions: There was variable performance of plasma biomarkers to recapitulate the CSF assay. Both p-tau217 and p-tau217/Aβ42 showed excellent promise as surrogates of CSF amyloid status, although with slightly reduced performance in older individuals. There was poorer discriminatory ability for p-tau181 and Aβ42/40. Further research is needed to address potential age-related confounds.