Correct diagnosis and prognosis of Lewy body diseases remain challenging. Two new immunoassays enable the quantification of the DOPA decarboxylase biomarker in multiple fluid samples. In cerebrospinal fluid, elevated concentrations of DOPA decarboxylase show great potential to support the diagnosis of Lewy body disorders. This biomarker is highly accurate in identifying patients with Lewy body disorders and is associated with impaired cognitive function. Elevated levels of DOPA decarboxylase are also seen in preclinical subjects with a positive alpha-synuclein seed amplification test, where there were 35 such cases versus 310 without abnormal aggregation. Higher levels predict progression to clinical Lewy body disease with a hazard ratio of 3.7 per standard deviation over a mean follow-up of 2.53 years. Levels are also increased in atypical parkinsonian disorders such as multisystem atrophy. The combination with the alpha-synuclein test can improve the diagnosis of parkinsonian disorders in practice.[1][2]