Parkinson’s disease is the second most common neurodegenerative disease affecting seven million people worldwide with a projected doubling in cases by 2040. Parkinson’s disease starts more than ten years before patients come to the clinic with symptoms because their brain cells fail to handle a small protein called alpha-synuclein. Overall, the test could distinguish an individual with high risk of developing Parkinson’s from a healthy control with 90% probability. These findings indicate that the blood test, together with a limited clinical assessment, could be used to screen and identify people who are at high risk of getting the disease. This pathway may also direct alpha-synuclein outside cells in extracellular vesicles, when intracellular protein turnover is inefficient in conditions such as ageing and Parkinson’s disease.