The idea is straightforward: Take a blood test now, even without symptoms, and learn if you could some day develop Alzheimer’s disease. Whether you should get this test is a more complicated matter. Most Alzheimer’s blood tests work by measuring levels of amyloid or tau , proteins that build up in the brain and are thought to play key roles in the disease. Both can begin gathering in the brain decades before any symptoms appear. But it’s still an ongoing debate how well the tests can predict