The technique, described in a small study in the journal Cell, relies on the immune system's ability to recognize foreign material.
Proteins on viruses and bacteria are recognized as alien, so the body produces antibodies, and the same is true for Alzheimer's.
So, in theory, if you can test for the antibody, you can test for the disease.
There is still no cure for Alzheimer's, but an early diagnose means getting patients into treatment earlier.
Researchers also hope to use this method to diagnose other disease earlier, including lung and pancreatic cancers.