DDC and Parkinson disease: Numerous studies have explored the use of plasma biomarkers in neurodegenerative diseases.8–13 Plasma p-tau217 has demonstrated high accuracy in detecting AD pathophysiology within the context of routine clinical practice in a memory clinic.8 In addition, plasma biomarkers such as the ratio of amyloid beta (Aβ)42 to Aβ40, neurofilament light (NEFL/NfL), and glial fibrillary acidic protein (GFAP) hold significant potential for AD diagnosis and monitoring.9,10 Similarly, plasma proteomic analysis in PD has revealed multiple novel biomarkers, such as DOPA decarboxylase (DDC).