You need to check out this article in Nature

A team of international scientists think they have found a way to identify patients at risk for Alzheimers Disease-and all through a simple blood test. We have known for years that AD is the result of a build-up of amyloid proteins in the brain. These proteins destroy neurons. For healthy people, these proteins are broken down and released into the blood stream for elimination. For people with AD, they stay put. Taking amyloid levels in healthy people versus those with AD should something startling…The amyloid blood levels are higher in healthy people…So measuring amyloid serum levels may provide insight into those with higher levels of amyloid in their brain (AD). Nature