Plasma A beta levels are affected by age and by systemic and CNS vascular risk factors. After controlling for these conditions, A beta-40 and A beta 1-42 are weak predictors of conversion to Alzheimer disease (AD) in normal subjects and are only weakly associated with AD in cross-sectional analysis. Consequently, plasma levels of A beta do not seem to be useful biomarkers for AD.

Plasma homocysteine is directly related to Abeta40. The association with Abeta42 is not significant. These results seem to indicate that homocysteine is related to aging but not specifically to AD.