This disclosure relates to the use of anti-&alpha-synuclein antibody to diagnose an elevated level of &alpha-synuclein in the brain. Specifically, the disclosure relates to the method of assessing the levels of &alpha-synuclein in a blood plasma or CSF following administration to the test subject of an anti-&alpha-synuclein antibody or antigen-binding fragment thereof, which can bind &alpha-synuclein with sufficient activity to alter the net efflux of &alpha-synuclein from brain to blood, or from brain to CSF.
