Abstract: The present Invention provides a method for diagnosing and/or prognosing Parkinson's disease dementia (PDD) comprising the step of detecting O-glycosylation. in a protein comprising a Ser/Thr motive, in particular Serpin A1, and/or the level of sialic acid on a protein comprising a Ser/Thr motive, in particular Serpin A1. Further, the present invention relates to a molecule for detecting O-linked glycomoieties in a protein comprising a Ser/Thr motive, in particular Serpin A1, and/or glycosylated i so forms of a Ser/Thr motive comprising protein, in particular Serpin A1, for use in the diagnosis and/or prognosis of Parkinson's disease dementia (PDD), Furthermore, the present invention relates to means for diagnosing and/or prognosing Parkinson's disease dementia (PDD) and a kit for diagnosing and/or prognosing Parkinson's disease dementia (PDD).