Excellent. This is a long debated question. I suggest looking at the recently published paper (attached) by Hanon et al, 2018. It is helpful to take a look at their supplementary table 2 which is very promising. This shows that there is a significant correlation between AB42 CSF marker and AB1-42 plasma marker in Alzhiemer's disease. However, Tau are P-Tau correlations are not significant. I believe this is a start. Whether this correlation can be inferred as causative is a different story. We will still need more studies to tell us more. Thank you for asking this question.
In case of Alzheimer's disease, amyloid clearing pathway is affected which results in increased amyloid deposition in brain, whereas decrease excretion in blood/plama.