Researchers have developed two immunoassays that measure a protein called DOPA decarboxylase (DDC) in cerebrospinal fluid and blood, and report that elevated CSF levels help separate dementia with Lewy bodies and Parkinson's disease from Alzheimer's disease and from cognitively normal people, according to a study published in Nature Medicine on February 16, 2026. The team, led by Katharina Bolsewig and colleagues at Amsterdam UMC, tested the assays across three clinical cohorts totaling 740 people, then checked the results against three further groups: one defined by underlying biology (253 people), one with detailed dopamine transporter brain scans (102 people), and one confirmed by autopsy (78 people).
DDC is an enzyme involved in making dopamine and is already known to leak from damaged brain cells in Parkinson's disease and Lewy body dementia. In this diagnostic accuracy study, CSF DDC levels were up to 2.5 times higher in people with Lewy body dementia or Parkinson's disease than in controls, and 1.9 times higher than in people with Alzheimer's disease, with area under the curve values above 0.9, a statistical measure indicating strong separation between the groups. Elevated CSF DDC was linked to the presence of visible motor symptoms such as tremor or stiffness, but not to how severe those symptoms were, the researchers reported.
The distinction the test aims to sharpen matters clinically: misdiagnosis of Lewy body disorders as Alzheimer's disease can lead to harmful treatment decisions, the study authors wrote, though the paper itself does not specify which treatments are affected. The autopsy confirmed and brain scan cohorts were used to check that the fluid measurements lined up with more established, but costlier or invasive, ways of confirming Lewy body pathology.